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MODULE – II
AC MACHINES
ROTATING MAGNETIC FIELDS
A rotating magnetic field is that field which is constant in magnitude but whose axis of
direction rotates in space as field system of a dc machine. The magnetic field produced
by single phase alternating current is an alternating magnetic field, ie, a field acting along
a fixed axis, varying in magnitude periodically and changing its direction alternately
positive and negative.
THE FERRARIS PRINCIPLE
Consider two rotating magnetic fields OA1 and OA2 each having magnitude of H units
and travelling in opposite directions with angular velocity w.let both the fields start
travelling from axis OX at time t = 0.
After time t seconds the angle through which fields, OA1and OA2 have rotated, θ=wt
radians. Ie, field represented by vector OA1 has moved in counter clockwise direction
through an angle θ=wt from axis OX and field represented by vector OA2 has moved in
clockwise direction through some angle θ, because both fields are travelling with same
angular velocity w. Resolving the magnetic fields represented by vector OA1 and OA2
along X and Y axis we get,
X component, Hx = OA1 Cos θ + OA2 Cos θ = H Cos θ + H Cos θ = 2Hcos θ.
Y Component, Hy = OA1 Sin θ – OA2 Sin θ = H Sin θ – H Sin θ = 0.
Y
w
A1
θ =WT
X
θ =WT
H A2
1
Hence resultant magnetic field is 2H Cos θ along X-axis. Therefore it is obvious that two
rotating magnetic fields, travelling in opposite directions with the same angular velocity,
result in an alternating field of twice their amplitude (OR) An alternating field can be
replaced exactly by two rotating fields of half its amplitude travelling in opposite
directions at synchronous speed.
PRODUCTION OF 3 PHASE ROTATING MAGNETIC FIELD:c
c1
C
c2
B1
O
B2
B
A1
A1
B1
A2
B
C1
A2
B2
C2
A
A
Consider 3 coils which are similar, A, B and C Displaced in space by 120 0, and
connected to 3 phase AC supply. Let each of alternating fields due to currents in coil A B
and C be resolved into two components OA1 and OA2, OB1 and OB2 and OC1and OC2
respectively travelling with angular velocity w in the directions as shown in fig (a) where
2
w = 2πf, f being supply frequency consider the instant when the current in coil A is
maximum. At this instant both the components OA1 and OA2 of the alternating field
produced by the current in coil A are along the axis of the coil A ie, along OA. The
current in coil B is 1200 behind the instant of its maximum value, so each of its
components OB1and OB2 will have to rotate through 1200 in order to reach the axis of the
coil B. Hence components OB1and OB2 are along OC and OA respectively. The current
in coil is 2400 behind the instant of its maximum value to each of components OC1and
OC2 of the field produced by current in coil C will have to rotate through 2400 in order to
reach the axis of coil c. Hence at this instant components OC1 and OC2 are along OB and
OA respectively.
From Fig (b) it is clear that components OA1,OB1 and OC1 are rotating in counter
clockwise direction with same angular velocity w but are 1200 apart, therefore their
resultant is always zero components OA2, OB2 and OC2 are rotating in clock wise
direction with same angular velocity w and also points in same direction, therefore
resultant gives a pure rotating field rotating at synchronous speed.
CLASSIFICATION OF ac MACHINES
1. According to type of current
single phase
Three phase
2. According to speed
constant
Variable
Adjustable
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3. According to principle of operation
Synchronous
Plain
Asynchronous
Super
Induction
Squirrel cage
slipring
Commutator
Series
Compensated
Single
double
Single universal
Cage
cage
phase
Conductively Inductively
CLASSIFICATION OF AC MOTORS
1. According to type of current : 1. Single phase
2. Three Phase
2. According to speed : 1. Constant speed
2. Variable speed
3. Adjustable speed.
3. According to principle of operations : 1. Synchonous motors
(a). Plain (b) Super
2. Asynchronous motors : 4
(a) Induction motors
squirrel cage
Single cage
Double cage
Ship ring
(b) Commutator motors
series
Single phase
Universal
Compensated
Conductively
Inductively
Out of all the above, induction motor is the most commonly used, for industrial
application due to the following advantages :
1. Simple design and rugged construction
2. Reliable operation and simple maintenance
3. Low initial cost and high efficiency.
INDUCTION MOTORS
In general conversion of electric power to mechanical power takes place in the rotating
part of an electric motor. In dc motors, the electric power is conducted directly to
armature hence they are called conduction motor. But in induction motors rotor receives
power by induction as transformer receives its power from primary. That is why such
motors are known as induction motor in fact such an induction motor can be treated as
rotating transformer. (Primary – stationary – secondary – rotating)
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CONSTRUCTIONAL FEATURES ;
An induction motor consists of a stator, wound for a definite no. of poles and for 3
phase. No. of poles are decided according to the speed requirements, greater the no: of
poles, the lesser the speed.
Synchronous speed : Ns = 120f
P
(if N is the speed in rpm, then revolutions per second is N/60 in one revolution it will
generate p/2 cycles. The total cycles generated per record f is N/60 x P/2 = PN/120
f = PN/120 x N=120f/P)
The stator winding when supplied with 3 phase currents produce a magnetic flux which is
of constant magnitude and revolves at synchronous speed.
Rotor :
1. Squirrel cage type and (2) phase wound or slip ring type.
SQUIRREL CAGE ROTOR
Almost 90% of induction motors are of this type. Rotor consists of a cylindrical
laminated core with parallel stots for carrying rotor conductors which consists of base
bars of copper, aluminium or alloys. The rotor bars are brazed or short circuited with end
rings, thus giving a squirrel cage type construction. The rotor stots are not quite parallel
to the shaft, but slightly skewed, which helps
1. To run the motor more quietly.
2. To reduce locking tendency of the rotor.
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In small motors, rotor conductors with end rings are moulded as a single piece. Another
form of rotor consists of a solid cylinder of steel. Motor works on the principle of eddy
current.
ii. PHASE WOUND ROTOR.
This type of rotor is provided with a 3φ double layer distributed winding. The rotor is
wound for as many poles as the stator. The three phases are starred internally other ends
are connected to 3 slip rings mounted on the shaft and insulated from each other and the
shaft. This facilitates an addition of a resistance in the rotor circuit during starting. When
running at normal conditions the slip rings are short circuited and then the brushes are
lifted out to reduce frictional losses.
PRINCIPLE FO OPERATION OF A 3 PHASE INDUCTION MOTOR
When the 3 phase stator windings are fed from a 3 φ supply, a rotating magnetic field is
produced. This sweeps past the rotor surface and cuts the stationary rotor conductor. Due
to the relative speed between the rotating flux and rotor conductors an emf is induced in
the rotor winding. Since the winding is in the form of a closed circuit a current flows, the
direction of which is such as to oppose the cause producing it. Now the cause is the
relative motion between the rotor and the rotating magnetic field. So that to oppose this
the rotor starts rotating in the same direction as the field and attempts to catch up with it.
The torque for the rotation is produced by the current in the rotor conductors which are
perpendicular to the magnetic field created by the stator.
When the motor shaft is not loaded the speed is very close to the synchronous speed. But
the rotor speed cannot be equal to synchronous speed, since in that case, the emf induced
in the rotor conductors will become zero and there will be no torque and the motor will
stop.
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When the motor shaft is loaded the rotor will slow down, and the relative speed of he
rotor with respect to stator field will increase, and emf induced and rotor current will in
crease. A higher torque will be the result. The motor speed at full load will be less than
the the no load speed.
SLIP
The difference between the rotor speed and the synchronous speed is known as slip. It is
usually expressed as a fraction of synchronous speed.
Slip : s= Ns – N
Ns
Or N = Ns (1 – s)
Where : Ns
N
Syn. speed
Speed of rotor
The value of slip is very small. No load slip will be about 3%. Induction motor is a motor
with substantially constant speed like a dc shunt motor.
FREQUENCY OF ROTOR CURRENT
When the rotor is stationary rotor frequency is same as supply frequency. But when the
rotor starts rotating, when the frequency depends upon the relative speed between rotor
and rtator field or on the slip speed. Let at any slip speed the frequency of the rotor
current be f then
Ns – N = 120f '
P
P
Ns – N = f '
Ns
Ns = 120f
But : Ns – N =s
f
Ns
f '= sf
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The following relations are also to be noted
At stand still
Rotor voltage per phase = E2
Rotor resistance / phase = R2
Rotor reactance
= X2
Rotor impedance Z2 =
R22 + X22
At Slip speed S
Rotor voltage = s E2
Rotor resistance = R2 (No change with frequency)
Rotor reactance = s x 2
Rotor impedance = R22 + (sx2)2
TORQUE EQUATION
In the case of dc m/cs we have seen that the torque is proportional to the product of
armature current and flux per pole.
Ie. T α φ I a. But in the case of induction motor, being alternating current, another factor,
ie the phase angle between flux and the rotor current also has to be considered, then
T α φ I2 Cos φ 2
…… (1)
I2= rotor current, φ = flux
Φ2 is the power factor angle
Flux φ is proportional to the stator voltage E1 which is again proportional to E2.
Since E1/E2=N1/N2
(N1 and N2 being the no: of turns / phase in the stator and rotor windings)
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So we may write φ α E2
I2 = Rotor current = Rotor voltage
Rotor impedance
I2 at slip s =
sE2
R22 + (S X 2) 2
Cos φ2 at slip s =
R2
R22 + (S X 2) 2
Substituting the values for φ, I2 and Cos φ2 in equation (1)
Torque T α
E2
x
sE2
x
R22 + (S X 2) 2
=
KsR2 E22
R2
R22 + (S X 2) 2
…………………(2)
(Where K is a constant of value 3/2πNs)
R22 + (S x 2) 2
This is the torque equation for an induction motor.
Starting torque :
At starting slip = 1.1f we substitute s=1 in the above formula we will get starting torque
as Tst = K R2 x E22 ……………..(3)
R22 + X22
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CONDITION FOR MAXIMUM TORQUE : From equation (2) we will device the condition for maximum torque as
Rs – X2 s = O (or) s = R2 ( S is the slip for max torque for a given R2 and X2)
√S
X2
Putting value of S as R2/X2 in eqn (2)
Max. Torque T Max = KE22
…………(4)
2 X2
Max. torque is independent of rotor circuit resistance.
Max. torque varies with rotor stand still reactance. Hence for increased max torque, the
reactance should be small.
The slip at which maximum torque occurs depends on the resistance of the rotor.
Condition for maximum torque at starting can be obtained by putting s = 1 in the
condition for Max. torque ie, R2/X2 = s = 1 or R2 = x2.
STARTING TORQUE OF SQUIRREL CAGE MOTOR.
Its rotor resistance is fixed and is low compared to reactance which is very high
especially at start (S = 1). Hence the rotor current I2 though very high lags behind the
voltage (power factor Cos θ2 is small). Hence starting torque per ampere is very poor.
Starting torque is roughly 1.5 times for load torque even though he starting current is 5 to
7 times. Hence such motors are not suitable when high starting torque is required.
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STARTING TORQUE OF SLIPRING MOTOR
In slip ring induction motor starting torque is improved by improving the power factor of
rotor current, by adding an external resistance in the rotor circuit. As the motor gains
speed the resistance is gradually cut out. This additional resistance increases the rotor
impedance and reduces rotor current. So optimum value of resistance is chosen for
enhanced starting torque.
TORQUE SLIP AND TORQUE SPEED CURVES :
Expression for torque as follows :
T = KsR2E22
R22 + (s X 2)2
From the above it is evident that
Torque is zero when s = 0 (ie, when speed is syn. Speed
When S is very small s X 2 is negligible compared to R2 and then
Torque α sE22
Since R2 and E2 are constant.
R2
Torque α s. (This portion of torque slip curve is a straightline)
When slip increases (speed decreases with load) torque increases and reaches its
maximum value when s = R2 . The maximum torque is also known as pullout torque or
breakdown torque.
X2
When the slip is further increased, torque decreases, the result is that the motor slows
down and eventually stops. The motor operates only for value of slip between zero and
that corresponding to max. torque with higher values of slip s, R2 is negligible when
compared to sX2 and now T α 1/s and this portion of the curve will be a rectangular
hyperbola.
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TORQUE SLIP CURVES
Pull out torque
Max torque line
Torque
Very large resistance
Medium resistance
low rotor resistance
Starting torque
0.25
0.5
0.75
Slip
TORQUE SPEED CURVES
Starting torque
Maximum torque or pull out
torque
Torque
25%
50%
75%
Ns.
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CONDITION FOR MAXIMUM STARTING TORQUE
It can be proved that starting torque will be maximum when rotor resistance / phase is
equal to stand still rotor reactance / phase
T = K E22R2
R22 + X22
TS =
K1 R2
K1 being another constant,
R22 + X22
dTs
=0
K,
1
-
R2 (2 R2)
R22 + X22
dR2
=0
(R22 + X22)2
R22 + X22 = 2R22
R2 = X2
Hence starting torque will be maximum when rotor resistance / phase = stand still rotor
reactance / phase
R2 = X2
Torque
Rotor Resistance
The curve shows the variation of starting torque with rotor resistance. As rotor resistance
is increased from a relatively low value, the starting torque increases until it becomes
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maximum when R2 = X2. If rotor resistance is increased is beyond his optimum value ,the
starting torque will decrease.
STARTING TORQUE OF 3 φ INDUCTION MOTORS
The rotor circuit of an induction motor has low resistance and high inductance. At
starting, the rotor frequency in equal to stator frequency so that rotor reactance is large
compared to rotor resistance. So the rotor current lags the rotor emf by a large angle ,the
power factor is low and consequently starting torque is low. When additional resistance is
installed in rotor circuit power factor is improved which results in improved starting
torque.
SQUIRREL CAGE MOTORS
Since the ends of the rotor conductors are permanently short circuited, no additional
resistance can be installed so starting torque of such motors are low.
SLIP RING MOTORS
The resistance of rotor circuit of such motors can increased by the addition of external
resistance. By inserting proper value of external resistance (R2 = X2) maximum starting
torque can be obtained. As the motor accelerates the external resistance is gradually cut
out until the rotor circuit is short circuited on itself for running conditions.
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METHODS OF STARTING SQUIRREL CAGE MOTORS
Small capacity motor upto 2kw may be directly switched on to mains. But those of higher
capacity must use some type of starting device to restrict the initial rush of current which
is about 5 to 7 times the rated full load current. This high current has 2 draw backs.
1. Large starting current of higher capacity motor will cause large voltage drop in the
distribution supply system which may cause stoppage of motor already running.
2. Initial high current may damage the motor it self. The aim of the starting devices is
to provide a low voltage to the stator wdgs at the time of starting or to include an
external resistance in the rotor circuit in the case of slip ring motor. The different
methods of starting are :
DIRECT ON LINE STARTING :
1) This method is just what the name implies the motor is started by connecting it
directly it 3 φ supply. The impendence of motor at standstill is relatively low and
when it is directly connected to the supply system, the current at starting will be
high (4 to 10 times the full load current) and at a low power factor. Consequently
this method in suitable for relatively small machines.
STATOR RESISTANCE STARTING
In this method external resistances are connected in series with each phase of stator wdg
during starting. This causes voltage drop across the resistances so that the voltage
available across motor terminals is reduced and hence the starting current. The starting
resistances are gradually cut out in steps from stator circuit as the motor picks up speed
and when rated speed is attained resistances are completely cut out and full line voltage is
applied to the motor
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DRAWBACKS.
The reduced voltage applied to the motor during the starting period lowers the starting
torque and hence increases the accelerating time.
Lot of power is wasted in the starting resistances.
AUTO TRANSFORMER STARTING
This method aims at connecting the induction motor to a reduced supply at starting and
then connecting it to the full voltage as the motor picks up sufficient speed. The tapping
on the auto transformer is so set that when it is in the circuit,65% to 80% of line voltage
is applied to the motor.
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Let the motor be started by an auto transformer having transformation ratio K. if Isc is the
starting current when normal voltage is applied, and applied voltage to stator wdg at
starting be KV.
Then motor input current Ist= K Isc
Supply current =Primary current of auto transformer
= K x secondary current of auto transformer
= K x Ist = K x K Isc = K2Isc
Starting torque = K2 x original starting torque
ADVANTAGES
1. No power is wasted as in stator resistance starting
2. High torque per ampere of starting current
3. Adjustment of stator voltage by selection of proper tap
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4. Suitable for long starting period, since no heat production
DISADVANTAGES : 1. Low power factor
2.Higher cost
STAR DELTA STARTER.
The stator winding of the motor is designed for delta operation and is connected is star
during the starting period. When the m/c is upto speed, the connection are changed to
delta. The circuit is shown. The six leads of the stator winding are connected to the
changeover switch. At the instant of starting the change over switch is thrown to star
position. Now each stator phase gets V/√3 volts where V is the line voltage this reduces
the starting current also. When motor picks up speed the change over switch is thrown to
delta position now each stator phase gets full line voltage V.
The disadvantages are : (1) With star connection during starting, stator phase voltage is
1/√3 times the line voltage so starting torque is (1/√3)2 or 1/3 times the value it would
have with delta connection. This is a large reduction is starting torque.
(a) The reduction is voltage is fixed .Used is medium type machines 25 HP.
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STARTING OF SLIP RING INDUCTION MOTORS
Sliping induction motor are started with full line voltage applied across the stator wdgs.
The value of starting currant is adjusted by introducing a variable resistance in the rotor
circuit. The controlling resistance is in the form of a rheostat connected in star, the
resistance being gradually cut out as the motor gains speed. By increasing the rotor
resistance not only the rotor current is reduced, but also the starting torque is increased
due to improvement of power factor, Hence such motor can be started under load when
the motor run under normal conditions, slip rings are short circuited and the brushes are
lifted to reduce frictional losses.
SPEED CONTROL OF INDUCTION MOTORS :The relation between motor speed (N). synchronous speed ( Ns) and slip (s) is given by,
N = (1-s) Ns
N = (1-s) 120 f/p………. (1)
From eqn (1) the speed of an induction motor can be changed by the following methods
1. By changing number of stator poles (p)
2. By changing line frequency (f)
3. By changing the slip (s) for a given load. The slip can be changed
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1.By changing the applied voltage
2.By changing resistance in the rotor circuit
3.By inserting foreign voltage of appropriate frequency in the rotor circuit.
Speed control by changing number of stator poles
Synchronous speed Ns = 120f/p. By changing the no: of stator poles (p), The
synchronous speed and hence the rotor speed (n) can be changed. This method is
applicable to squirrel cage motors but not for wound rotor motors.
Speed control by changing line frequency
Ns = 120f/p By changing line frequency synchronous speed Ns of the motor and hence
the running speed (N) can be changed. When employing line frequency control, the
applied line collage should be changed in direct proportion to the frequency ie. If
frequency is increased the supply voltage must also be increased and if frequency is
deceased the supply voltage must also be decreased proportionately. This is necessary to
maintain an approximately constant flux in the air gap of the machine otherwise the
motor performance will not be satisfactory.
SPEED CONTROL BY CHANGING APPLIED VOLTAGE
The torque developed by an induction motor is directly proportional to the square of
applied voltage (T αV2). By changing the applied voltage the torque and hence speed of
the motor can be changed.
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LIMITATIONS
The stator voltage control method is the cheapest and the easiest method of speed control
of induction motors it is rarely used because of the following drawbacks.
1. A large change in voltage is required for a relatively small change in speed.
2. The large change in voltage results in large change in the flux density this affects
the magnetic conditions and hence performance of the motor.
SPEED CONTROL BY CHANGING ROTOR CIRCUIT RESISTANCE
This method of speed control is suitable only for slip ring motors. The speed of the motor
can be decreased by adding external resistance to the rotor.
3φ
Introduction motor
R
Under normal running conditions, the relation b/w torque (T) and slip (s) of an induction
motor T ∞ s/R2.
Here R2 is the rotor resistance / phase. It is clear from the above relation that for a given
toque, S α R2 : slip can be increased by increasing the rotor resistance.
DRAW BACKS
1. There is an increase is the rotor cu losses due to the increased rotor circuit
resistance
2. Due to increased rotor cu loss, the efficiency of the motor is decreased.
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3. There is an increase in temperature of the motor .Applicable for short periods
only.
NO LOAD TEST :
This test is performed to determine no load current I0 load power factor cos φ0 windage
and friction losses Pwf, no load core loss Pi, no load power input P0 and no load resistance
R0 and reactance Xo. More over it reveals any mechanical fault, noise etc.
The no load test is performed with different values of applied voltage below and above
rated voltage while the motor is running under no load.
The power input for each phases P0, voltage V0 (line to line) and current I0 are measured
by wattmeter W, voltmeter V and an meter A connected in the circuit since motor is
running on no load, the power factor will be less than 0.5 at no load power input is equal
to the core loss Pi, stator core loss Pc, and windage and friction losses Pwf. The rotor
circuit is practically open at no load.
To determine windage and friction losses, the curve drawn b/w input power in watt and
applied voltage in volts is extended to intersect vertical axis at any point B. Point B
corresponds to power input when applied voltage is zero. Since when applied voltage is
zero, the stator cu losses and core losses become zero, therefore power input at no load
and zero voltage given by OB represents the windage and friction losses. (Pwf.)
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Input
Power
In watts
B
Windage
And friction
Losses.
o
Applied voltage in V
If Pwf so determined and stator cu losses (3I20R1) are subtracted from power input at no
load Po, core losses Pi can be determined. Knowing the total core losses Pi no load
current I0, Applied voltage V0,(line to line value) the values of magnetizing component I
and active component Iw of no load current I0, no load resistance R0 and no load reactance
X0, can be determined as follows.
Cos φ0 = Pi
√3 V0 I0
Iw = Io cos θ0
Im = Io Sin θ0
Z0 = Vo/√3
I0
R0 = v0 / √3
Iw
X0 = V0/√3
Im
or
Z02 – R02
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BLOCKED ROTOR TEST
This test is performed to determine the short circuit current Isc with normal applied
voltage to the stator, power factor on short circuit , total equivalent resistance and
reactance of the motor as referred to stator (R01 and X01)
In this test rotor windings are held firmly (rotor wdgs are short circuited) and stator is
connected across supply of variable voltage. This test is equivalent to short circuit test on
transformer. The connections are done as in diagram. Starting with zero voltage across
the stator, the applied voltage is gradually increased in steps till the full load current
flows in the stator. The readings are noted the wattmeter readings are additive as the pf is
much grater than 0.5. The windage and friction losses are nil as the rotor is at stand still,
the core losses are ignored. Thus power input during this test can be considered to be
entirely stator and rotor copper losses.
If V2 in the applied voltage and causes current Is in the stator wdg and Ps is the total
input power at short circuit then,
Cos φS = Pi
√3 Vs Is
Input on sc meets with stator and rotor cu losses Ps = 3I2s R01
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Or motor equipment resistance / phase as referred to stator
R01 = Ps
3I2s
Motor equivalent impedance /phase as referred to stator
Z01 = Vs/√3
Is
Motor equivalent reactance / phase as referred to stator
X01 = Z012 = R012
+
IS
R01
X01
Vs/√3
-
SINGLE PHASE INDUCTION MOTORS
The performance of 3φ induction motors are good but it is not always convenient to have
a 3φ supply available. The most common situation is in the home where almost
universally we have a single phase supply. So arose the need for motors operating on
single phase.
We know that a single phase supply does not produce a rotating magnetic field. It
produces an alternating or pulsating field.
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If we would start the rotor by some means it continue to rotate in much the same as in the
case of 3φ induction motor. So here the problem is the single phase induction rotor is not
self starting. The explanation of this action in given below based on double field
revolving theory based on Ferraris principle.
According to ferraris principle, the effect of an alternating field can also be explained in
term of the effect of 2 rotating fields equal in magnitude, but opposite in direction.
If the rotor starts rotating in any direction, it has to move in the direction of one of these
fields and in the opposite direction with respect to the other field. When related to the
field moving in the same direction, we have a motor action with a slip value between 0
and 1 ie, Ns-N = s in the
Ns
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case of rotor rotating in the direction of the forward flux. However if related to the field
moving in the opposite direction we have a braking action with value of slip between 2
and 1 ie, Ns – (-N) = s
Ns
= 2 N s – Ns + N
Ns
Ns + N
Ns
= 2 N s – Ns – N
Ns
Ns
= 2-s
(in the case of rotor rotation with
respect to backward flux.)
The total action is the sum of these two. Fortunately the rotor action is stronger than the
braking action. So that m/c acts as a motor.
The torque slip characteristics are shown in the figure. When the slip is 1, the two torques
are equal and opposite and resultant torque is zero (at stand still s=1) and m/c has no
starting torque.
As soon as the movement commences a torque is developed which will continue to
accelerate the motor up to operating speed. All we require is a small torque sufficient to
over come the load torque to start the motor and once it is started it will accelerate to full
speed. So the problem is how to start motor.
To make single phase induction rotor self starting we have to produce a revolving stator
magnetic field. This may be achieved by converting a single phase supply into two phase
supply through the use of an additional winding when rotor attain sufficient speed, the
starting means may be removed. The single phase induction motors are classified and
named according to the method employed to make them self starting.
1. Split phase motors – started by two phase motor action through the use of an
auxiliary or starting winding with a main winding.
2. Capacitor motors – started by two phase motor action through the use of an
auxiliary winding and a capacitor.
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3. Shaded pole motors : started by the motion of the of the magnetic field produced
by means of a shading coil around a pole portion.
Split phase induction rotor.
In this type of motor the auxiliary wdg is made in such a way that its resistance is
emphasized, then again phase difference would occur. Hence the main wdg has a thick
wire and has high inductance while auxiliary wdgs is relatively thin giving high
resistances and low inductance.
When the 2 stator wdgs are energized from a single phase supply, the main wdg carries Im
while auxiliary wdg caries Is. Since main wdg is made highly inductive while the starting
wdg is highly resistive, the current Im and Is have reasonable phase angle α Hence a weak
revolving field approximating to that of a 2φ m/s is produced which starts the motor.
These are usually contant speed motors. They are used where moderate starting torque is
required (a) fans (b) washing machines. The power rating of such motors generally lies
b/w 60W and 250W.
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3.SHADED POLE MOTOR
Shaded pole motor is very popular for ratings below 40w because it is extremely simple
in construction. It is has salient poles on stator and squirrel cage type rotor. A portion of
each pole is surrounded by a short circuited turn of copper strip called shading will.
The operation can be explained by referring to the figure:
During the portion OA of alternating current cycle, the flux begins to increase and an emf
is induced in the shading coil. The resulting current in shading coil will be is such a
direction so as to oppose the change in flux. Thus the flux in the shaded pole portion is
weakened while that in the unshaded portion is strengthened.
During the portion AB of the alternating current cycle, the flux has reached almost
maximum value and is not changing. Consequently flux distribution across the pole is
uniform since no current is flowing in the shading coil.
30
As flux decreases, BC of alternating current cycle, current is reduced in the shading coil
so to oppose the decrease in current. Thus the flux in the shaded portion of the pole is
stre ngthened while that in unshaded portion is weakened. The effect of shading coil is to
shift the field flux across pole face from shaded to unshaded portion. The shifting flux is
the rotating flux suitable for low power applications since starting torque efficiency and
pf are very low.
(1) Small fans (2) toys (3) hair dries etc. The power rating o such motors is upto about
30W.
(2) Capacitor start motor
Capacitor start motor is identical to a split phase motor except that the starting wdg has
an many turns as the main winding. More over a capacitor C is connected in series with
the winding (starting wdg). The value of C is so chosen that Is leads Im by about 800. So
starting torque is much more than that of split phase motor. The starting wdg is opened
by the centrifugal switch when the motor attains 75% of synchronous speed.
31
Capacitor start motor are used where high starting torque is required and where starting
period may be long.
(a) Compressors (b) large fans (c) pumps.
The power rating of such motor the b/w 120w and 7.5kw.
ALTERNATOR
AC generator or alternator operate on the same fundamental principle of electromagnetic
induction. In dc machines the voltage induced in the armature winding is alternating and
is taken out as DC using commutator and brushes. But if this supply is taken out through
slip sings we would get AC. This type of alternators are called revolving armature type.
But in case of large capacity machines, high current at high voltage are to be handled
through rotating slip rings and brush system. To avoid this difficulty, armature wdg is
transferred to the stationary stator and the field wdg transferred to rotating rotor. This
type of generator are called revolving field type alternator.
ADVANTAGES
1. Armature wdgs are easily braced since mechanical stresses and centrifugal forces
are less.
2. Easy to insulate the stator wdg since it is stationary
3. Less power is handled by slip rings
4. Armature wdg is cooled more easily.
32
OPERATING PRINCIPLE
When the rotor rotates, the stator conductor being stationary are cut by the magnetic flux.
Hence they have induced emf produced in them since the magnetic poles are alternately
N and S, the induced emf and the current in each conductor is alternating. The frequency
of the alternating current depends upon the two of poles and the speed of rotation of the
rotor (speed of the prime mover)
The relation is F = NP/120
All synchronous AC generation and motor require direct current for excitation (DC
supply to the pole system). This is supplied from a DC generator called the exciter or
from static exciters by rectifying the AC output from the alternator.
CONSTRUCTIONAL FEATURES
STATOR
Stator consists of a cast iron or welded steel frame supporting the cylindrical stator core
made up of laminated varnished silicon steel punchings. The armature conductors are
placed in the slots. Open slots are used for easy installation and removal of stator
windings. A fractional rather than integral number of slots per pole is often used in order
to eliminate harmonics in the output ware form.
33
ROTOR :
Rotor are of two types
i) Salient pole type rotor
ii) Smooth cylindrical rotor
Salient pole type : This type of rotor is used in low speed mc/cs have large diameter and
small axial length.
SALIENT POLE TYPE ROTOR
34
In this case the poles are projecting out and are made of thick steel laminations riveted
together and attached to a rotor by a dove tail joint. Overhang of the poles gives
mechanical strength to the windings. In order to reduce hunting damper windings may be
provided on the pole faces.
SMOOTH CYLINDRICAL ROTOR
This type of rotor is used for steam turbine alternators, where the speed is 3000 rpm or
1500 rpm. Diameter of the rotor is reduced and axial length increased. The no of poles is
2or4. Rotor is made up of solid steel forging. the outer periphery has slots in which the
field wdgs are placed. About 2/3rd of the pole pitch is slotted. Balance is left as pole
centre. Heavy wedge of phosphorus bronze are forced in the grooves in the teeth of slots
to keep the coil in position. Bearings, slip rings etc are the other parts of an alternator.
EMF equation
Let Zp = No : of conductors or coil sides / phase
= 2Tph = where Tph is turns per phase
P = No. of poles
35
φ= useful flux / pole in wb.
N = speed in rpm
F = frequency in Hertz
Kd = Distribution factor
Kp = Pitch factor.
Consider a conductor on the stator of the alternator. In one revolution the conductor will
cut P x φ webers of magnetic lines
dφ) = pφ
Time taken for one revolution dt = 60/N. Average emf induced in one conductor is
Eav = dφ = Pφ
=PφN Volts
dt
60/N
60
But we know that N = 120f/p substituting the value of N in eqn we have
Eav = Pφ x 120 f = 2φf volts / conductor
60 P
Average emf in all the Zph conductor is
Eav / phase = 2φf x Zph (But Zph = 2 Tph)
= 2 Zph x φf = 4 Tph φf
To get rms value we multiply the average value with 1.11
Erms / phase = 1.11 x 4 x φ x f x Tph = 4.44φf Tph
The above eqn is true only if the conductors / pole/ phase is concentrated in one slot and
also if the wdg pitch is same as pole pitch. Since it is not so, we have to multiply the
above V by pitch factor Kp and distribution factor kd.
36
Erms / phase = 4.44kdkpfφTph
ALTERNATOR ON LOAD
When the load on the alternator is increased (Ia is increased) the field excitation and
speed being constant, the terminal voltage V (per phase value) of the alternator decreases.
This is due to :
1. Voltage drop IaR a where Ra in the armature resistance per phase
2. Voltage drop IaXLwhere XL is the armature leakage reactance / phase
3. Voltage drop due to armature reaction.
EQUIVALENT CIRCUIT
For one phase, the equivalent circuit of a loaded alternator is shown
E0 = No load emf, E = load induced emf. Induced emf after allowing for armature
reaction. It is equal to phasor difference of E0 and IaXAR
V = Terminal voltage : V<E by voltage drops in XL and Ra
E0 = V + Ia (Ra + jXL + jXAR)
XAR Ia XL
E0
E
Ra
V
ZL
37
SYNCHRONOUS REACTANCE (XS)
XS = XL + XAR The equivalent circuit can be simplified as: synchronous reactance is a
fictitious reactance employed to account for the voltage effects in the armature circuit
produced by the armature resistance, the actual armature leakage reactance and the
change in air gap flux caused by armature reaction.
Synchronous impedance Zs = Ra + jXs
E0 = V + IaZs = V + Ia (Ra + jXs)
VOLTAGE REGULATION
The voltage regulation of an alternator is defined as the change in terminal voltage from
no-load to full load (the speed and field excitation being constant) divided by full load
voltage.
% voltage regulation = No load voltage – full load voltage x 100
Full load voltage
= E0 – V
x 100
V
Here E0 – V is the arithmetic difference and not the phasor difference. The factor
affecting the voltage regulation of an alternation are :
i) Ia Ra drop in armature winding
ii) Ia X L drop in armature winding
iii) Voltage change due to armature reaction.
38
For leading load pf, the no load voltage is less than the full load voltage. Hence voltage
regulation is negative in this case. Since regulation of an alternator depends on the load
and the load power factor, it is necessary to mention power factor while expressing
regulation.
DETERMINATION OF VOLTAGE REGULATION
Since the ratings of commercial alternators are very high, voltage regulation cannot be
found by direct loading. The indirect methods of determining he voltage regulation of an
alternator are : 1. Synchronous impedance or EMF method.
3. Ampereturn or MMF method.
These methods require small amount of power compared to power required for direct
loading methods.
For either method, the following data are required
i. Armature resistance
ii. Open circuit characteristics (OCC)
iii. Short circuit characterizes (SCC)
1) ARMATURE RESISTANCE : The armature resistance Ra per phase is
determined by using direct current and the voltmeter – ammeter method. This is
the dc value. The effective armature resistance (ac resistance) is greater than this
value due to skin effect it is a usual practice to take the effective resistance 1.5
times the dc value.
2) OPEN CIRCUIT CHARACTERISTICS : The OC of an alternator is the curve
between armature terminal voltage (phase value) on open circuit and the field
current when the alternator is running at rated speed.
39
The alternator is run on no load at the rated speed. The field current If is gradually
increased from zero until open circuit voltage E0 (phase value) is about 50%
greater than the rated phase voltage. The graph is drawn as shown between the
open circuit voltage values and the corresponding values of If.
EMF
OCC
Phase
Value
Field current (If)
3.SHORT CIRCUIT CHARACTERSTICS (SCC)
In a short circuit test the alternator is run at rated speed and armature terminals are short
circuited. The field current If is gradually increased from zero until short circuit am
mature current Isc has the rated value. The graph between short circuit armature current
and field current gives the SCC as shown below. The SCC is a straight line passing
through the origin since armature resistance is much smaller than the synchronous
reactance, the short circuit armature current lags the induced voltage by very nearly 900.
The armature flux and field flux are in direct opposition and the resultant flux is small. So
saturation effect will be negligible and the short circuit armature current α field current
over the range from zero to well above rated armature current.
SC
Current
SCC
Field current If
40
SYNCHRONOUS IMPEDANCE METHOD (EMF METHOD OR PESSIMISTIC
METHOD)
Here synchronous impedance Zs (and hence synchronous reactive Xs) of the alternator are
found from the OCC and SCC For this reason, it is called synchronous impedance
method. The method involves the following steps.
SCC
EMF (OC)
OCC
SC current
E1
I1
If
Field current
1) Plot the OCC and SCC on the same field current base.
2) Consider a field current If, the open circuit voltage corresponding to this field
current is E1. The Sc armature current corresponding to field current If, is I1. On
short circuit to circulate the short circuit armature current I1 against the
synchronous impedance Zs.
E1 = I1Zs. Or Zs = E1/I1 = Open circuit voltage E1
Short circuit current I1
3) The armature resistance can be found as explained ealier.
Synchronous reactance Xs = Zs2 – Ra2
41
4) Once Xs and Ra are known the phasor diagram for any load and any pf can be
drawn. The phasor diagram for the usual case of an inductive load is shown here.
Here current Ia is the reference phasor Ia Ra is in phase with Ia, Ia Xs drop leads Ia
by 900. The phasor sum of V1, Ia Ra, Ia Xs gives no load emf E0.
E0 =
OB2 + BC2
OB = V Cos φ + Ia Ra and BC = V Sin φ + Ia Xs.
E0 = (V Cos φ + Ia Ra)2 + (V Sin φ + Ia Xs)2
% voltage regulation = E0 – V x 100
V
LEADING PF LOAD
The phasor diagram of a leading pf load is drawn here and the voltage regulation is
found. Here V leads Ia by angle φ
E0 = (OA)2 + (AB)2
OA = V Cos φ + Ia Ra
42
AB = V Sin φ – Ia Xs
E0 = (V Cos φ + Ia Ra)2 + (V Sin θ – Ia Xs)2
% regulation = E0 – V x 100
V
Unity pf load. V and Ia are in phase : Cos φ = 1
B
E0
OA = V + Ia Ra
I a Xs
O
V, Ia , Ia Ra
E0 = OA2 + AB2
A
AB = Ia Xs
E0 =
(V + Ia Ra)2 + (Ia Xs) 2
% regulation = E0 – V x 100
V
This method gives values higher than the actual load test. So it is called pessimistic
method.
AMPERE – TURN METHOD (MMF METHOD OR OPTIMISTIC METHOD)
INTRODUCTION
This method of finding voltage regulation considers the opposite view to the synchronous
impedance method. It assumes the armature leakage reactance to be additional armature
reaction. Neglecting armature resistance, this method assumes that change in terminal p.d
on load is due entirely to armature reaction. Under short circuit, the current lags by 900
43
and power factor is 0. Hence the armature reaction is entirely demagnetizing. Since
terminal p.d is zero, all the field AT are neutralized by armature AT produced by short
circuit armature current.
Since current α AT, it is more convenient to work in terms of field current. The diagram
shows the current diagram for the usual case of lagging power factor. Here AO represents
the field current required to produce normal voltage V (or V+Ia Ra Cosφ) on no load. The
phasor OB represents the field current required for producing full load current on short
circuit. The resultant field current is AB and is the phasor sum of AO and OB. Phasor AB
represents the field current required for demagnetizing and to produce voltage V and Ia Ra
Cosφ drop (if Ra is considered)
φ
Total field current
A
B
SC field current
O
NLfield current
PROCEDURE FOR AT METHOD
Suppose the alternator is supplying full load current Ia at operating voltage V and Pf Cosφ
lagging. The Ia at operating Voltage V and pf Cosθ lagging. The procedure to find
voltage regulation by AT method is given below.
1. From the OCC, field current OA required to produce the operating load voltage,
V1 (V+Ia Ra Cosφ) is determined. This field current is laid off horizontally.
44
2. From SCC, the field current OC required for producing full load current Ia on short
circuit is determined. The phasor AB (OC) is drawn at an angle of (90 + φ) ie,
<OAB = 90 + φ
3. The phasor sum of OA and AB gives the total field current OB required. The OC
voltage E0 corresponding to field current OB on OCC is the no load voltage E0.
Voltage regulation = E0 - V x 100
V
This method gives a regulation lower than the actual performance of the machine. So it is
called optimistic method.
At unity pf load. Since Cosφ = 1, V1 = V + Ia Ra
From OCC the field current If, required to produce V1, is found. From SCC the field
current If2 required to produce full load current Ia on short circuit is found. The phasor
diagram for upf load is drawn as shown below.
Total filed current OB, If in this case is
B
If
If2
If =
O
If1
OA2 + AB2
=
If12 + If22
A
45
Then the voltage corresponding to If from OCC gives the E0.
% Voltage regulation = E0 – V x 100
V
II)AT ANY PF LAGGING LOAD
EMF at full load current V1 = V + Ia Ra Cosφ.
From OCC the field current If1 required to produce V1, is found and from SCC If2 is
found. The phasor sum of If1 and If2 in this case is
B
If
90+ φ
O
If1
If2
A
OB = If
If = If12 + If 22 + 2If1 If2 Cos (180 – (90+φ))
= If12 + If22 – 2If1 If2 Cos (90 + φ)
The E0 corresponding to If is found from OCC and % voltage regulation is found.
% voltage regulation = E0 – V x 100
V
46
AT ANY PF LEADING LOAD :
EMF AT FULL LOAD CURRENT V1 = V + Ia Ra Cosφ
If1 required to produce V1 is found from OCC. The If2 is found from SCC. The phasor
sum of If1 and IF2 is If. The E0 corresponding to If is found from OCC to calculate the %
voltage regulation finally.
B
If12 + If22 + 2If1 If2 Cos (180 – (90-φ)
If
=
=
If12 + If22 – 2If1 If2 Cos (90 – φ)
% voltage regulator = E0 – V x 100
V
If
If2
φ
O
(90-φ) A
If1
SYNCHRONOUS MOTORS
Synchronous Motor is a m/c which converts electrical energy into mechanical energy and
operates at synchronous speed. It is fundamentally an alternator operated as a motor.
the main parts are :
i) A stator which houses 3φ armature winding in the slots of the stator core and
receives power from a 3φ supply.
47
ii) A rotor that has a set of salient poles excited by direct current to form alternate N
and S poles. The exciting coils are connected in series to two slip rings and dc
fed into the wdg from an external exciter mounted on the rotor shaft.
The stator is wound for same number of poles as the rotor poles. As in the induction
motor.
Synchronous speed Ns = 120f
P
Where F = frequency of supply in Hz.
P = No. of poles.
An important draw back is that it is not self starting and auxiliary means has to be used to
start it.
SALIENT FEATURES OF SYNCHRONOUS MOTORS ARE :
1. Run at synchronous speed or not at all. Speed constant at all loads. The only way
to alter the speed is change the supply frequency.
2. The outstanding characteristic of a synchronous motor is that it can be made to
operate over a wide range of power factor (lagging, unity or leading) by
adjustment of its field excitation. So a synchronous motor can be made to carry the
mechanical load at constant speed and at the same time improve the power factor
of the system.
3. They are of salient pole type.
4. A synchronous motor is not self- starting and an auxiliary means has to be used for
starting it. We use either induction motor principle or a separate starting motor for
this purpose.
48
OPERATING PRINCIPLES
1) Consider a 3φ synchronous motor having 2 rotor poles NR and SR. Then the stator
which is also wound for 2 poles has Ns and SS. The motor has direct voltage
applied to rotor winding and 3φ voltage applied to stator wdg. The stator field
produces a rotating magnetic field which revolves round the stator at synchronous
speed Ns = 120f/P. The dc sets up a 2 pole field which is stationary so long as the
rotor is not rotating. Now there is a revolving armature field (Ns – Ss) and a
stationary rotor field (NR – SR). Suppose at any instant, the stator poles are at
positions A and B. It is clear that poles Ns and NR repel each other and so do the
poles Ss and SR. Therefore R starts rotating in anticlockwise direction. After a
period of half cycle, the polarities of the stator poles are reversed but the polarities
of motor pole remains same. Now Ss and NR attract each other and so do Ns and
SR. Therefore the rotor tends move in the clockwise direction. Since the stator
poles damage their polarity rapidly, they tends to pull the rotor first in one
direction and then after a period of half cycle in the other. Due to high inertia of
the rotor, the motor fails to start.So the motor is not self starting.
HOW TO GET CONTINOUS UNIDIRECTIONAL TORQUE.
If the rotor poles are rotated by some external means at such a speed that they interchange
their positions along with the stator poles, the rotor will experience a continues uni
directional torque.
49
i) Suppose the stator field is rotating in the clock wise direction and the rotor is also
rotated clockwise by some external means at such a speed that the rotor poles
interchange their positions along with the stator poles.
ii) Suppose at any instant the stator and rotor poles are in the position such that
torque on the rotor will be clockwise. After half cycle, the stator poles reverse
their polarity, at same time rotor poles also interchange their positions. Now
again the torque on the rotor is clockwise. Hence continuous unidirectional
torque acts on the rotor and moves it in the clockwise direction. Under this
condition poles on the rotor always face poles of opposite polarities on the
stator and strong magnetic attraction is set up between them. This mutual
attraction locks the rotor and stator together and the rotor is virtually pulled
into step with the speed revolving flux ie synchronous speed.
STARTING OF SYNCHRONOUS MOTORS
USING DAMPER WINDING : The starting is as explained above. In this case the
motor is started as an induction motor. It may be remembered that it is not possible to
start with its dc field energised. The DC field winding is short circuited during starting. it
may aid starting torque. In large motor field sectionisng switches are provided to avoid
cumulative addition of voltage induced.
USING DC MOTOR COUPLED TO THE SHAFT.
This method is used when synchronous motor are not equipped with damper windings. If
the synchronous motor is used as a prime mover for a dc generator, the same DC
generator can be run as a motor during starting. The DC motor is run and the speed of the
synchronous motor is raised to synchronous speed and at this stage, the field is energised
from DC supply. Now the synchronous motor is running as alternator developing 3φ
voltage. This is now synchronized to 3φ external supply and now it will work as a motor
the excitation of DC motor can be adjusted so that it works as a generator.
50
USING THE FIELD EXCITER GENERATOR AS MOTOR
Some of the synchronous motors will have an exciter coupled to the shaft to generate DC
voltage for excitation. This exaite can be worked as a DC motor at the time of starting.
The starting method is explained for 2 above.
USING A SMALL INDUCTION MOTOR.
The induction motor is coupled to the same shaft and it should have one pair of poles less
than the synchronous motor to compensate for the speed reduction caused to the
induction motor, when loaded. The starting procedure is the same as ‘3’ above.
In the last three methods the following conditions must be met with.
1. There should be little or no load on the synchronous motor during starting.
2. The capacity of the starting motor should be 5% to 10% of the rating of
synchronous motor.
EFFECT OF LOAD ON SYNCHRONOUS MOTOR
When mechanical load on AC or DC motor is increased, the speed decreases, this in turn
decreases the counter emf Eb so that the source is able to supply more current. However
this action cannot take place is a synchronous motor since the motor cannot run at a speed
below synchronous speed.
Fig above shows the relative position of rotor and stator poles on no load and rated speed.
There is a shift to the rotor poles in a direction opposite to the direction of rotation stator
field. This angular displacement is called Torque angle.
SYNCHRONOUS MOTOR RATINGS
Synchronous motor are rated specifying the power factor. When it is working at lesser
power factor, the power output will be much less.
51
HUNTING
When the mechanical load is constant the rotor settles down to constant speed with a
torque angle depending on the load. If the load increase the rotor slip backwards for an
increased torque angle. But due to inertia, the rotor overshoots the required position. To
correct this, the rotor moves forward a little which also overshoots. This continues until
the rotor settles down to the new torque angle, this rapid forward and backward motion
of the rotor, while it revolves at the average constant speed is called hunting. Hunting
produces severe mechanical stress as well as great variation in current and power taken.
The pole face damper winding help to reduce hunting.
SYNCHRONOUS CONDENSER
When a synchronous motor runs at no load with over excitation, it takes large amount of
current at leading pf. Normally system power factor is lagging due to large no. of
induction motors working .Lagging currents of the system is compensated by the leading
current taken by the synchronous motor and hence power factor is improved. Since its
function is that of a static capacitor, such synchronous motors are called synchronous
condensers.
COMPARISON BETWEEN SYNCHRONOUS MOTOR AND INDUCTION
MOTOR.
Sl.No. Synchronous motor
Induction motor
1
Inherenthy no starting torque
Self starting
2
Requires Dc excitation
No Dc excitation required.
3
Speed control not possible
Speed can be controlled
52
4
Speed is constant and independent of Speed falls on loading
load
5
It can be operated over a wide range Operates at lagging power factors
of power factors
6
Torque is less sensitive to supply More sensitive
voltage
7
Break down torque is proportional to Proportional to square of supply voltage
supply voltage
8
Complicated and sensitive
Simpler and economical
9
Used for mechanical loading as well Mechanical loading only.
as power factor improvement
MODULE – 3
Industrial device – Electric drives – advantages – individual drive and group drive factors
affecting choice of motor mechanical characteristics of A.C and D.C motor motors for
particular application like textile mill, steel mill, paper mill, mine, hoists, crane etc. Size
and rating of motor – motor selection of intermittent loads. Electric traction different
systems of traction – comparison – track electrification – different systems – traction
motor characteristics – electric braking – plugging – dynamic and regenerative braking.
53
ELECTRICAL DRIVE
An electric drive is defined as a form of machine equipment designed to convert electric
energy into mechanical energy and provide electrical control of these processes.
BLOCK DIAGRAM OF AN ELECTRIC DRIVE IS AS SHOWN HERE
Source
Power
Motor
Load
Modulator
Feedback circuit
Control
Sensing
Unit
Unit
Input command
(a) Source : It is either type of electrical power dc or ac supply
(b) Power Modulator : it performs the following functions.
i. If converts electrical energy received from the source in the form suitable to the
motor ie, if the source is dc and an induction motor is to be employed then it
converts d.c into a variable frequency a.c
ii. During transient operations, such as starting, braking an speed reversal it restricts
source and motor currents with in permissible limits.
iii. It selects the mode of operation of motor, ie, motoring or braking
C) Load : is usually a machinery designed to perform a given operation ie, fans machine
tools, domestic appliances, trains, pumps etc.
54
d) Motor : Motor commonly used in electrical drives are DC motors – shunt, series,
compound and permanent magnet, induction motor – squirrel cage, wound rotor and
linear synchronous motor – cofound field and permanent magnet, Brushless dc motor,
stepper motor and switched reluctance motors.
e) Sensing unit : It is employed for sensing the drive parameters, such as speed, motor
current etc. which may be required either for protection purpose or for closed loop
operations. These signals are fed to control limit.
(f) Control unit : It control the power modulator besides generating commands for the
protection of the motor and power modulator. It usually operates at much lower voltage
and power levels. Input command signal, which adjusts the operation of drive forms an
input o the control unit apart from signals from the sensing unit.
TYPE OF ELECTRIC DRIVES
Electric drives used in industry may be divided into three types
(a) Group drive (b) individual drive (c) Multimotor drive)
GROUP DRIVE.
In this, one motor is used as a drive for two or more than two machines. The motor is
connected to a long shaft, on which belt and pulleys are connected to run other m/cs it is
also called line shaft drive. This type of drive is economical as a single motor of large
capacity costs less than the costs of a number of small motors of the same total capacity.
55
The use of this type of drive is restricted due to following reasons
1. In case of fault in the motor, all the connected m/cs to this motor will cease to
operate.
2. If at certain instance all the m/cs are not in operation then the motor will be
working at low capacity.
3. It is not possible to install a new m/c at a far away distance.
4. Speed control of different m/cs using belts and pulley is cumbersome.
INDIVIDUAL DRIVE
In this type of electric drive a single electric motor is used to drive one individual m/c.
Through it costs more than group drive but each operator has complete control on his
machine, which enables him to either increase the speed of motor or to stop it while not
in operation. Machines can be located at convenient places. If there is a fault in one
motor this will not effect the production of the industry appreciably.
No a days electric drive is being extensively used in the industry because of its certain in
herent advantages over the other types of drives.
Some of the advantages are
Advantages of Electric drive.
1. Cost is too low. 2) The system is more simple and clean 3) The control is very
easy and smooth 4) Flexible in layout 5) Facility for remote control. 6)
Transmission of power from one part to other can be done with the help of cables
instead of long shafts etc. 7) Maintenance cost is quite low. 8) Can be started at
any time with out delay in time.
56
DISADVANTAGE
1) Electric drive system is tied only up to the electrified area. 2) The condition
arising under the short circuits, leakage from conductor and breakdown of over
head conductor may lead of fatal accident. 3) Failure in supply for a few minutes
may paralyze the whole system.
ADVANTAGES OF GROUP DRIVE.
1) Initial cost : Initial cost of group drive is less as compared to that of the individual
drive
2) Sequence of operation : Group drive system is useful because all the operations
are stopped simultaneously
3) Space requirement : Less space is squired in group drive as compared to individual
drive
4) Low maintenance cost: small maintenance cost as compared to individual drive.
DISADVANTAGES
1) Power factor : Group drive has low pf.
2) Efficiency : If all the motor are not working together the main motor shall work at
reduced efficiency.
3) Reliability if main motor fails, the whole industry will come to stand still.
4) Flexibility – The system is not flexible
5) Speed: no constant speed
6) Types of machines : Not suitable for driving heavy machines such as cranes, lifts,
hoists etc.
ADVANTAGES OF INDIVIDUAL DRIVES
1. Individual drive give desired operation as each m/c is drive by its own individual
motor
2. Works at good power factor
57
3. Efficiency of the system is high
4. More reliable
5. May be fitted where ever suitable
6. More useful where constant speed is required
7. Suitable for driving huge machines like canes, lifts, hoist etc.
COMPARISON BETWEEN TWO DRIVE
Points
Individual drive
Group drive
1. Initial cost
More
Less
2. Power factor
Works at good pf.
Low pf.
3. Efficiency
High
Low when worker at light loads.
4. Reliability
More reliable
Not reliable
5. Flexibility
May
be
fitted
where’re Not possible for group drive.
convenient
6. Speed
More useful where constant speed Does not give constant speed
is required
7. Sequence
operation
of Useless
where
sequence
operation is required
of Use full, since all operation are
stopped simultaneously.
58
FOUR QUADRANT OPERATION OF MOTOR DRIVING A HOIST LOAD
In the first quadrant the load torque acts in a direction opposite to that of rotation. Hence
to drive the loaded hoist up, the developed torque in the motor M must act in the same
direction as the speed of rotation ie, TM should be of +ve sign since the speed is also
positive being an upward motion, the power will also have a positive sign, ie the drive is
said to be motoring. Quadrant I is arbitrarily and conventionally thus designated as
forward motoring quadrant.
The hoisting up of the unloaded cage is represented in the second quadrant. Since the
counter weight is heavier than the empty cage, the speed at which the hoist is moved
upwards may attain a dangerously high value. In order to avoid thin, the motor torque
must act in a direction opposite to that of rotation. ie, motor should switch over to a
59
braking or generating region. Here TM will have a negative sign and speed still has a
positive sign, being forwards, upwards and counter clockwise, giving power a negative
sign, corresponding to the generating or braking operation.
The third quadrant represents the downward motion of the empty cage. The down ward
journey of the cage opposed by the torque due to the counter weight and friction at the
transmitting parts. So for moving the cage downwards, the motor torque must act in the
same direction as the motion of the cage. The electrical m/c acts as a motor as in the fist
quadrant but in the reverse direction. This quadrant III becomes reverse motoring. The
motor torque has a –ve sign as it causes an increase in speed in the negative sense and the
speed is –ve being a downward motion. Power has a +ve sign. The downward motion of
the loaded cage is in IVth quadrant. The motion can take place under the action of load
itself, with out the use of any motor. But in order to limit the speed of the down ward
motion of the hoist, the electrical m/c must act as a brake. The motor torque has a
positive sign as it causes a decrease in speed in the down ward motion. The speed has a
-ve sign being a down ward journey. The power has –ve sign corresponding to braking
operation of the motor.
THE TRACTION MOTORS
The traction motor incorporates the following features
a) Electrical features
1) High starting torque
2) Series spaced torque characteristics (T α 1/N)
3) Simple speed control.
4) Possibility of dynamic or regenerative braking
5) Better communication
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b) Mechanical features
1) Light in weight and small space requirements.
2) Totally enclosed especially if mounted beneath the locomotive
3) Robust and should be able to with stand vibrations
The motor which are used for this purposes are ) series and compound motors operate
satisfaction on dc system.
2) Ac series motor on single phase system
3) Induction motor on 3φ ac system
The speed torque characteristics of dc shut and series motor are given below.
THE SPEED TOQUE CHARACTERISTICS OF DC SHUNT AND SERIES
MOTOR ARE GIVEN BELOW.
T
shunt
N
Shunt
Series
N
Series
Ia
Shnut
Series
Ia
T
SPEED TORQUE CHARA. OF AC AND DC SERIES MOTORS.
N
ac
dc
T
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ELECTRIC TRACTION
Traction system can be classified as non – electric and electric traction. Non electric
traction does not use electricity at any stage such as steam engine drive and internal
combustion drive whereas electric traction involves the use of electrical energy at some
stage or the other such as battery electric drive, diesel electric drive and straight electric
drive.
STEAM ENGINE DRIVE HAS THE FOLLOWING ADVANTAGES
1) Simplicity in design 2) Ease of speed control 3) No interference with
communication n/w 4) Low capital cost.
But because of the disadvantages like 1) Low thermal efficiency 2) considerable wear on
the track 3) corrosion of the steel structures due to smoke emitted by the engine 4) air
pollution straight electric systems are preferred.
ELECTRIC TRACTION IS THE MOST EFFICIENT OF ALL OTHER
SYSTEMS BECAUSE OF THE FOLLOWING REASONS.
1. Since electric motion are used as the drive, the system in clean and pollution free.
2. Speed control and braking is simple
3. In fact by using regenerative braking energy can be pumped back into the system
especially during periods of descents.
4. Electric traction is more suitable especially for suburban and urban railway where
frequent starting and stopping and high schedule speeds and required.
5. The coefficient of adhesion in high
6. Over loading of electricmotors is possible for sometimes
7. Centre of gravity of eclectic locomotive in lower than steam locomotive hence
electric locomotives run faster at curved surfaces or routes.
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REQUIREMENTS OF AN IDEAL TRACTION SYSTEM
The following are the important requirements of the driving equipment used for traction
purpose
1) The coefficient of adhesion should be light so that high tractive effort at start is
possible and rapid acceleration of the train can be obtained
2) It should be possible to over load the equipment for short periods
3) The wear caused on the brake shoes, wheel tyres and the track should be
minimum.
4) It should be possible to use reganeative braking so that on descents it should be
possible to generate energy and feed back to the supply system.
5) The loco motive or train unit should self contained so that it can run on any route.
6) It should be pollution free.
SYSTEM OF RAILWAY TRACK ELECTRIFICATION
PRESENTLY, FOLLOWING FOUR TYPES OF TRACK ELECTRIFICATION
ARE AVAILABLE.
1. Direct current system – 600V, 750V, 1500V, 3000V.
2. Single phase ac system – 15 – 25kv, 16 2/3 , 25 and 50 hz.
3. Three phase ac system – 3000 – 3500 V at 16 2/3
4. Composite system – involving conversion of single phase ac into 3 phase ac or dc.
DIRECT CURRENT SYSTEM
Direct current at 600 – 750 V is universally employed for tramways in urban areas and
for many suburban railways while 1500 – 3000 V dc is used for main line railways. The
current collection is from third rail upto 750V, where large currents are involved and
from over head wire for 1500V and 3000 V, where small currents are involved since in
63
majority cases, track rails are used as the return conductor, only one conductor rail is
required. Both of these contact systems are fed from substations which are spaced 3 to 5
km for heavy suburban traffic and 40-50 km for mainlines operating at higher voltages of
1500 V to 300 V. The required voltages are obtained in these substations using rectifiers
or inverters (conversion from ac to dc). These substations are usually automatic and are
remote controlled. The dc supply so obtained is fed via suitable contact system, to the
traction motors which are either dc series motors for electric locomotives or compound
motors for tramways and folly buses.
For heavy suburban service, low voltage dc system is undoubtely superior to 1φ ac
system.
SINGLE PHASE LOW FREQUENCY AC SYSTEM
In this system, ac voltages from 11 to 15 KV at 16 2/3 or 25 Hz are used. If supply is
from a generating station exclusively meant for the traction system, there is no difficulty
in getting the electric supply of 16 2/3 or 25 Hz. If electric supply is taken from the high
voltage transmission lines at 50 Hz then is addition to step down transformer, the
substation is provided with a frequency converter. The frequency converter equipment
consists of a 3φ synchronous motor which drives a 1φ alternator having 25Hz frequency.
The 15KV 16 2/3 or 25 Hz supply is fed to the electric locomotive via a single over head
wire.
A step down transformer carried by the locomotive reduces the 15 KV voltage to 300 –
400 V for feeding the ac series motor. Speed regulation of ac series motors is achieved by
applying variable voltage from the tapped secondary of the above transformer. Low
frequency ac supply is used because apart from improving the commutation properties of
ac motors it increases their efficiency and pf.
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Three phase low frequency AC system.
It lines 3 θ induction motors which work on a 3.3kv, 16 2/3 Hz supply. Substation receive
power at a very high voltage from 3 φ transmission lines at the usual industrial frequency
of 50 Hz. This high voltage is stepped down to 3.3Kv by transformer whereas frequency
is reduced from 50 Hz to 16 2/3 Hz by frequency converters installed at the sub station.
This system employs two over head contact wires, the track rail forming the third phase.
Induction motors used in the system are quite simple and robost and give trouble free
operation.
Composite system.
Such a system incorporate good points of two system while ignoring their bad points.
Two such composite system presently in use are :
1) 1 φ to 3 φ system also called kando system
2) 1 φ to dc system
KANDO SYSTEM
In this system single phase 16-KV, 50 HZ supply from the sub station is picked up by the
locomotive through the single over head contact wire. It is then converted into 3φ ac
supply at the same frequency by means of phase converter equipment carried on the
locomotives. The 3φ supply is then fed to the 3 θ induction motors.
SINGLE φ AC TO DC SYSTEM
This system combines the advantages of
high voltage ac distribution at industrial
frequency with the dc series motor traction. It employs overhead 25kv, 50 Hz supply
which is stepped down by the transformer installed in the locomotive itself.The low
65
voltage ac supply is then converted into dc supply by the rectifier which is also carried on
the locomotive. This dc supply is finally fed to dc series traction motor.
The motors commonly used for particular services :
Domestic uses
Small universal motors of the series type are used in domestic appliances like
vacuum cleaners, refrigerator, washing machines, fans etc.
Grinding and milling machine
Up to 50 HP the motors may be dc shunt or induction with sliprings and arrangements for
pole changing with cage rotors.
PLANNERS
There is cutting stroke and a quick return stroke. Arrangements for revering the speed
have to be incorporated. A dc compound motor may be used.
PUNCHING AND SHEARS
On account of the heavy fluctuation of laod, a flywheel is provided. The motor may be dc
shunt or compound or slip ring induction type.
CRANES AND HOIST WORK
For cranes dc motor of the series of compound wound type are preferred as they have a
high starting torque and the speed control is smooth.
Induction motor are also used for hoisting.
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LIFTS
Lift duty involves high acceleration and high retardation. The motor armature must
therefore be light and it should rum at moderate speeds. DC compound, slip ring
induction, induction- repulsion and a-c commutators are used.
TEXTILE INDUSTRY
Motors must be of the totally enclosed type of prevent particles of the material being
manufactured from getting into them. They should also be moisture proof on account of
damp atmosphere inside a textile plant.
Three phase motor are used since their speed is fixed by the supply frequency. DC motor
cannot be used as their speed varies with voltage.
PRINTING MACHINERY
Most presses require a variable speed. Induction motors using rotor resistance may be
used. Where large speed variations are required d-c compound or a-c commutator motors
may be used.
PAPER INDUSTRY
For constant speed, the synchronous motor is used. Constant speed is essential to
maintain uniform thickness of the paper. Where speed is not required to be kept constant,
squirrel cage induction motors or dc motors may be used.
IRON AND STEEL INDUSTRY
Motors in the mill shop are dc shunt with flywheel. Induction motors with speed control
are also used.
67
MINING WORK
Flame proof motors are essential. Cage motors are used upto 10hp and for large output
slip ring or dc motor.
FACTORS GOVERNING SELECTION OF ELECTRIC MOTORS
The conditions under which an electric motor has to operate and the type of load it has to
handle determine its selection. The various factors that are to be considered in the
selection of an electric motor for a particular service are :
1. Nature of electric supply (Explanation in next page)
2. Type of drive
3. Nature of load
4. Electrical characteristics
a. Operating or running characteristics
b. Staring characteristics
c. Speed control
d. Braking characteristics
5. Mechanical characteristics
a. Type of Enclosures
b. Type of Bearings
c. Type of transmission for drive
d. Noise level.
e. Heating and cooling time constants
6. Service capacity and rating
a. Requirement for continues, intermittent or variable load cycle
b. Pull out torque and overload capacity
7. Appearance
8. Cost
68
a. Capital or initial cost
b. Running cost – power factor, losses, maintenance and depreciation etc.
SIZE AND RATING
The factors which govern the size and rating of motor for any particular service are its
maximum temperature rise under given load conditions and the maximum torque
required. A motor which is satisfactory from the point of view of maximum temperature
rise usually satisfies the requirement of maximum torque as well. For class A insulation,
maximum permissible temperature size is 400C where as for class – C insulation, it is
500C. This temperature rise depends on whether the motor has to sum continously,
intermittently or on variable load.
DIFFERENT RATINGS FOR ELECTRICAL MOTOR ARE :
1. CONTINOUS RATING
It is based on the maximum load which a motor can deliver for an indefinite
period with out its temperature exceeding the specified limits and also possessing
the ability to take 25% over load for a period of time not exceeding 2 hrs under the
same conditions.
Eg: If a motor is rated continuous 10kw, it means that it is capable of giving an output of
10kw continuously for an indefinite period of time and 12.5 kw for a period of 2 hrs with
out its temperature exceeding he specified limits.
CONTINOUS MAXIMUM RATING
It is the load capacity as given above but with out overload capacity. Hence these motors
are a little bit inferior to the continuous rated motors.
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INTERMITTENT RATING
It is based on the output which a motor can deliver for a specified period, say one hour as
½ hour or ¼ hr with out exceeding the temperature rise.
This rating indicates the maximum load of the motor for the specified time followed by a
no load period during which the machine cools down to its original temperature.
DIFFERENT TYPES OF INDUSTRIAL LOADS
The three different types of industrial loads under which electric motors are required to
work are as under.
1) Continuous load 2) Intermittent load and 3) variable or fluctuating load.
The size of the motor depends on two factors. Firstly on the temperature rise which is
turn, will depend on whether the motor is to operate on continuous intermitted or
variable load, seconds it will depend on the maximum torque to be developed by the
motor. Keeping in mind the load torque requirements, the rating of the motor will be
decided by the load contains as described below.
1) CONTINOUS LOAD
In such cases, the calculation of motor size is simpler because the loads like pumps
and fans require a constant power into keep them operating.
INTERMITTENT LOADS
Such loads can be of the following two types.
70
a) In this type of load, motor is loaded for a short time and then shut of for a
sufficient long time, allowing the motor to cool down to room temperature. In
such cases, a motor with a short time rating is used as in a kitchen mixie.
Load
Load
Temperature
Temperature
Time
b)in this type of load, motor is loaded for a short time and then it is shut off for a short
time. The shut off time is so short that the motor cannot cool dorm to the room
temperature. In such cases, a suitable continuous or short time rated motor is chosen
which, when operating on a given load cycle will not exceed the specified temperature
limit.
Variable loads : In the case of such load, the most accurate method of selecting a suitable
motor is to draw the heating and cooling curves as per the load fluctuation for a number
of motors.
1.NATURE OF ELECTRIC SUPPLY
Electric supply available may be A.C single phase, three phase or D.C. The motor chosen
should suit to the supply available. In case of 3 phase AC supply for small capacity and
slip induction motor for higher capacity can be used. But if speed variation is required
pole changing motor or motors with stepped pulley are to be used. If precise speed
control is required charge motor can be used. Even if DC supply is not available AC can
be rectified to DC and DC motors can be used.
71
2.TYPE OF DRIVE
The type of drive may be group drive or individual drive. The selection of motor is
dependent on the type of drive chosen
3. NATURE OF LOAD
Loads which are usually met with may be classified according to the speed torque
characteristics as follows.
i) Constant load torque : Torque is constant at all speeds. Eg: cranes, hoists etc.
ii) Load torque α speed : - The torque increases linearly with speed and occurs is case
of fluid friction where lubricants is used.
Constant torque
Load
TαN2
Torque
TαN
Tα1/N
Speed
Load – torque characteristics of different type of loads.
iii)Load torque αN2:- The torque increase as square of speed such as in fans, centrifugal
pumps etc.
iv)Load torque α 1/N : The torque varies inversely as the speed and it occurs in case of
grinding boring machines etc.
72
All electric motor have a dropping speed torque characteristics. But the drop in speed
with increase in torque varies for different type of motor for a given load, the motor
selected should have such a speed torque characteristics so that it interacts the load –
torque characteristics of the load at about 900.
ELECTRICIAL CHARACTERISTICS
i) Starting characteristics
Some motor have high starting torque. Some of them are having moderate starting torque.
Some are not self starting. Eg: Synchronous motor. The starting torque should be capable
of starting and accelerating the motor to rated speed in a reasonable time. When the
motor has to start against full load, this factor is more important. Starting torque is
required to overcome the initial static friction and to accelerate the motor can load to the
full speed. Static friction may be much more than the full load torque, especially when
the machine was idling for a long time. Torque for accelerating depends on load torque.
The time taken to attain the full speed affects the heating of the motor and the control
equipments. DC shut motor has moderate starting torque. DC series motor is having very
high starting torque. But series motor cannot be run with out load. Hence when high
starting torque and a definite no load speed is required, d.c compound motor can be used
AC squirrel cage motors are having moderate starting torque. If we require higher starting
torque slip ring motor can be used. Its starting torque can be increased by connecting a
resistance to its rotor at the time of starting.
Except is case of fractional H.P motors a state in used for starting of electric
motor. A stator is used i) to reduce the starting current. Ii) to obtain the required starting
torque. Different motor have different type of starter. In most cases the technique is to
impress a reduced voltage at the time for tarring and to apply the full voltage when motor
picks up speed.
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ii)RUNNING CHARACTERISTICS
Running characteristics such as speed torque characteristics, speed current characteristics,
losses, efficiency and powerfactor are to be considered. Other features such as
temperature rise, insulation strength are taken case of by Indian standards specifications.
DC MOTORS
i) shunt motor
For a DC motor Nα (V-IaRa)/φ. For a shunt motor φ is almost constant. Hence N α (V – Ia
Ra) since Ia Ra, the armature resistance drop is small compared to V, speed is almost
constant. But depending upon the load current Ia Ra increase and hence there is drop in
speed from no load to full load to full load even though not very appreciable. Shunt
motor can be used for loads which may be suddenly thrown off, with out resulting in
excessive speed increase. Hence shunt motor are used for line shafting, machine lathes,
milling machines.
Conveyors, fans etc. But shunt motor is not suitable for fluctuating loads or parallel
operation.
Speed
Torque in Nm. (Load)
74
DC SERIES MOTOR
In series motor flux varies with armature current (load) Hence T α φIa α Ia2. Hence
Torque increases as the square of current. Hence the starting torque will be very high. To
provide two times starting torque, the armature current need be increased to 1.414 times,
where as for shunt motor, it should be 2 times. N α V-IaRa Since Ia Ra is small compared
to V, Nα1/φ and is 1/Ia. Hence as the load increase flux increases and speed suddenly
drops. Hence the mechanical characteristics (Torque /speed) of a series motor is as
shown below.
From the characteristics, it may be seen that i) series motor is not having a definite no
load speed. Hence series motor cannot be started with no load. (2) when the motor is
loaded speed drops heavily. This saves the motor from over loading. These two qualities
makes dc series motor the most suitable for electric traction.
Speed
Torque
DC COMPOUND MOTORS
This type of motor has both series field and shunt field winding if series field aids shunt
field, the motor is called cumulative compounded motor. If the series field flux is
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opposite to shunt field flux the motor is called differentially compounded motor.
Compound motor have a definite no load speed and can be started on no load. In case of
cumulatively compounded motor speed drops with load and saves the motor from over
loading. Hence these motors can be used for machines which are subjected to sudden
heavy loads, such as rolling mills, shears and punches.
In case of differentially compounded motor series field weakness shunt field and hence
full speed may be equal or more than no load speed.
In heavy overloads, the series field may become stronger and the motors may reverse
direction. This type of motors are rarely used.
A.C. MOTORS
Among A.C motor three phase induction motor is the most commonly used motor it is
simple in design, rugged in construction, reliable is service, low initial cost, easy
operation, low maintenance cost, high efficiency and requires simple control gear for
starting and speed control.
There are mainly two types of induction motors 1) cage induction motors 2) slip sing
induction motor. Both types are having a shunt characteristics, ie speed drops with load.
Squirrel cage motor is having moderate starting torque. But in case of slip ring motor, the
starting torque can be increased by inserting a resistance is its rotor circuit at the time of
starting and cut of circuit when normally running. This rotor resistance can also be used
for speed control. For same rating slip ring motor are larger in size and cost is load high.
Speed torque characteristics of induction motor is as shown here.
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T
Stating torque
Maximum
Normal operating region.
Torque
Speed N
Ns
3 phase squirrel cage motors are used for application which require moderate starting
torque and almost constant speed and capacities up to 50kw. Above 50kw, slip ring
motors are insisted by supply company in order to restrict starting current. Slip spring
motors are preferred for high starting torque as in lifts etc. for lower capacities also.
SYNCHRONOUS MOTORS
They are constant speed motors. Speed can be varied by varying the supply frequency.
They are used for constant speed application such as in paper with etc. They are
expensive and require a dc source for excitation and hence maintenance cost also will be
more.
Other A.C motors used for industrial drives are , scharge motor (for wide speed control)
single phase and 3 phase series motors, universal motor, stator fed commutator motor
single phase induction motor repulsion motor etc.
SPEED CONTROL
The speed can be controlled electrically and mechanically. Electrical method is by
controlling the input to the drive motor. Mechanical methods are by means of stepped
pulleys or gears. Some drives may require a continuous speed variation from zero to full
speed, other esquires one or more fixed speed.
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DC MOTORS : The speed control of dc motor are simple and easier and hence when
wide range of speed control is essential dc motors are still used.
A.C. MOTORS : speed of induction motor can be varied by changing the frequency, no.
of poles or by injection an voltage to its rotor circuit. In case of slip ring induction motor,
rotor resistance can be varied for speed control. The range of speed control required
determines the choice of the drive motor to a great extent.
MECHANICAL FEATURES
i) Type of enclosure : Type of enclosure to be used depends on the type of work and
the place of installation. Different types of enclosures are open type, protected
type, drip proof, totally enclosed, pipe ventilated, frame proof etc. When the
motor is open type, cooling will be better, but there are more chances of
entering of foreign material. When totally enclosed cooling will be difficult.
ii) Bearings : Upto 100 Hp, Ball or roller bearings are used. For larger motors journal
type bearings are used.
iii) Noise : It is important to keep the noise level with in limits.
SIZE AND RATING OF MOTORS
Size means the kw capacity of the motor and rating means the loading nature specified
for that capacity. The size of motor selected should match with the load.
Depending upon the type of loading such as continuous loading or intermittent loading
size of motor will vary.
COST : Cost is a major consideration in the selection of a drive motor. When we
compromise with quality, initial cost may be less. But efficiency of such motor may be
low or power factor poor and repairs more. So the cost as energy and maintenance will be
more and running cost will be high. Hence the motor selection has to be done for the
lower total cost.
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CHARACTERISTICS OF TRACTION MOTORS
A. D. C SERIES MOTORS
The torque eqn of a dc motor is T α φ Ia and in case of series motor flux φ α Ia and
hence T ∞ Ia2. Again N αV-Ia Ra.
Φ
Since Ia Ra is small compared with V, N α 1, and φ α Ia. Hence N α 1/ θa means
φ
N ∞ 1/Ia Hence as load current increase speed drops considerably.
Shunt motor
T
Shunt
N
Series motor
Series
T
Torque- speed chara. Of shunt and series motors.
Armature current
Torque – current chara. Of shunt
and series motors.
The series motor is ideally suited for traction as
i)
It is capable of exerting high starting torque
ii)
It possess high free running speed and
iii)
The speed decreases with increase in torque thereby protecting the
motor from over loads.
iv)
They are best suited for series and parallel operation.
79
A.C.SERIES MOTORS.
Single phase compensated series motor are used for traction work. They have low pf at
start and therefore the starting torque is low. A.C series motor is not well suited for
suburban work where stops are frequent. It is mainly employed for main line work.
Series field
Series field
Eb
Compensating wdg
Conductively compensated
Inductively compensated.
Eb.
V
I (Xa – Xse)
IXse
IRse
I
If an ordinary series motor is used for AC, due to the alternating flux, there will be heavy
iron loss. There will be heavy sparking at the brushes. To reduce iron loss the entire yoke
is laminated. In order to reduce the sparking a compensating wdg is used either in series
(conductively compensated) or short circuited (inductively compensated)
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The effect of the compensative wdg is to reduce the effect of armature reactance. The
compensating edge provides an mmf opposite to that of armature wdg and their by
reduces armature reactance drop. Hence the value of Eb is increased and also power
factor is improved.
ELECTRIC BRAKING
If any electric motor is to be stopped, electric supply to it is disconnected. In this way, the
motor shall take a long time to stop as the motor shall continue to rotate due to inertia. If
the kinetic energy is more it shall take longer time to stop and if it is less, it shall take
smaller time to stop. Sometimes motor is to be stopped at once to avoid some accident or
in other cases motor is to be stopped quickly. So some braking system must be used so
that it stops with in the predetermined short time.
A good braking system must possess the following important features
i) It should be fast and reliable
ii) The braking force must be capable of being controlled
iii) Kinetic energy of the rotating parts of the motor and its driven machine must be
suitably dissipated and suitable means must be provided.
Following are the two types of braking system normally employed.
i) Mechanical braking : The stored energy of the rotating parts is dissipated in the
form of heat by a brake shoe of brake lining which rubs on a wheel or brake
drum.
ii) Electric braking or electro dynamic braking : In this type of braking system, the
kinetic energy of the moving part of the system is converted into electrical
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energy which in turn is dissipated as heat in a resistance or in certain cases may
be returned to the supply
Advantages of electric braking over mechanical braking
i) Electric braking is quite fast
ii) It is quite cheap as far as maintenance part is concerned due to the fact that no
replacement of brake shoes or liming is needed as in the case of mechanical
braking system
iii) Since electric braking is quire fast, higher speeds can be maintained. This results
in higher capacity of the system
iv) In some cases of electric braking such as when eh electric train, in moving down
gradient or the heavy weight is lowered by the crane, a part of electric energy
produced driving the braking period, can be fed back to the supply system.
v) Heat produced is in no way harmful but heat produced on the lining etc. During
the application of mechanical brakes way result in the failure of brakes
vi) Electric braking is free from fires and is more smooth than mechanical braking.
DISAD VANTAGES
i) Electric braking can stop the motor but it cannot hold it stationary
ii) This cannot be applied to all types of electric motors
iii) Its initial cost is very high.
THE FOLLOWING TYPES OF ELECTRIC BRAKING ARE EMPLOYED
i) Plugging or reverse current braking
Reconnection of motor to supply is done in such a way that motor develops a torque
in opposite direction to the movement of the rotor. The system speed decelerates to
zero speed and than it will accelerate in opposite direction. It is necessary to
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disconnect the supply of the system as soon as it comes to rest. This method is
applicable to dc, induction and synchronous motors.
In this method of electric braking system, the connection of the supply to the armature
are reversed to that the motor tends to revolve in the opposite direction and in doing
so it comes to rest. As soon as it comes to rest, the supply gets automatically cut off
and the motor does not start revolving in the opposite direction. At that time
mechanical brakes may also be applied. This method of braking can be applied in
both DC as well as AC motors.
a) Plugging as applied to DC shunt motors
The direction of the torque developed in DC shunt motor can be reversed either by
interchanging the field connection or the armature. In shunt motors usually armature
current is reversed. With reversed armature connections, the motor develops a torque
in opposite direction. When speed reduces to zero, motor will accelerate in the
opposite direction. Hence the arrangement is made to disconnect the motor from the
supply as soon as it comes to rest. The circuit shows the running and reversed
connections for shunt motors and drives motors.
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Since with reversed connection, V and Eb are in the same direction, voltage across the
armature is almost double of its normal value. In order to this excessive current
through the armature, additional resistance R is connected in series with armature.
This method of braking is wasteful because in addition to wasting kinetic energy of
the moving parts, it draws additional energy from the supply during braking.
Plugging of Induction Motors
This method of braking is applied to an induction motor by transposing any of its two
line leads as shown.
84
Slipping motors are more suitable for plugging as external resistance can be added to
get the desired braking torque.
It reverse the direction of rotation of the synchronously rotating magnetic field which
produces a torque in the reverse direction, thus applying braking on the motor. Hence
at the first instant after plugging the rotor is running in a direction opposite to that of
the stator field.
RHEOSTATIC BRAKING
In this method of electric braking motor is disconnected from the supply through its
field continues to be energized in the same direction. The motor starts working as a
generator and all the kinetic energy of the equipment to be braked is converted into
electrical energy and is further dissipated in the variable external resistance R
connected across the motor during the braking period. This external resistance must
be less than the critical resistance otherwise there will not be enough current for
generator excitation.
DC and synchronous motors can be braked this way but induction motor require
separate dc source for field excitation.
In this method no power is drawn from the supply during braking.
85
Rheostat braking of dc motors.
The fig shows connections for dc shunt motor. For applying she static braking
armature is disconnected from the supply and connected to a available external
resistance R while the field remain on the supply. The motor starts working as a
generator whose induced and Eb depends upon its speed. At the start of braking when
speed in high, Eb is large hence Ia is large. As speed decrease, Eb decreases, hence Ia
decreases. Since Tb α φIa, it will be high at high speeds but low at low speeds. By
gradually cutting out R, Ia and hence Tb can be kept constant through out where Ia =
Eb(R+Ra)
Fig. Shows running and braking conditions for a dc series motor. In this case also for
rheostatic braking, the armature is disconnected from the supply and at the same time,
is connected across R. Connection are so made that current keeps flowing through the
series field in the same direction other wise no braking torque would be produced.
86
The motor starts working as a series generator provided R is less than the critical
resistance.
Rheostatic braking of Induction Motors.
If an induction motor is disconnected from the supply for rheostatic braking, there
would be no magnetic flux and hence, no generated emf and in the rotor and no
braking torque. After disconnection, direct current is passed through the rotor, steady
flux would be set up in the gap which will induce current, in the short circuited rotor.
This current which is promotional to the rotor speed, will produce the required
braking torque where value can be regulated by either controlling dc excitation or
varying the rotor resistance.
Regenerative braking
In this method of braking motor is not disconnected from the supply but is made to
run as a generator by utilizing the KE of the moving train. Electrical energy is fed
back to the supply. The magnetic drag produced on account of generator action offers
the braking torque. It is the most efficient method of braking. Take the case of a shunt
motor. It will run as a generator when ever its Eb becomes grater than V. Now Eb can
exceed V in two ways.
1. By increasing field excitation
2. By increasing motor speed beyond its normal value field current remaining same.
Regenerative braking can be easily applied to dc shunt motors though not down to
very low speeds because it is not possible to increase field current sufficiently. In
dc series motor reversal of current of the field and hence of Eb. Regenerative
braking cannot be used for stopping a motor. The main advantages are :
87
i) Reduced energy consumption particularly on main – line railways having long
gradients and mountain railways.
ii) Reduced wear of brake shoes and wheel types and
iii) Lower maintenance cost for these items.
88
MODULE – 4
Basic principles of transitory amplifier – R.C coupled amplifier F.B amplifier – Basic
principles. Oscillator – basic principle typical R.C and L.C oscillator circuits (no
analysis) Actable multivibrator pulse circuits – wave shaping circuits like simple
clipping, clamping R.C differentiating, integrating circuits – simple sweep generator.
CRO – basic principle of cathode say tube deflation methods – block schematic of CRO –
measurement of current voltage and frequency.
MODULE – IV
TRANSISTOR
A semi conductor device consisting of two pn junctions formed by sandwitching either ptype or n-type semi conductor between a pair of opposite types in known as a transistor.
Accordingly there are two types of transistors
1) npn transistor 2) pnp transistor
1) npn transistor
A transistor in which two blocks of n – type semi conductors are separated by a
thin layer of p-type semi conductor is know as npn transistor.
Base
n
p
n
collector
Emitter
A npn transistor is show here :
89
2) pnp transitor
A transistor in which two blocks of p-type semi conductors are separated by thin layer of
n-type semi conductor is know as pnp transistor.
E
P
n
p
C
B
A pnp transistor is shown here :
The transistor has three terminals called emitter base and collector
Emitter E: The reaction which replies large no. of majority carriers.
The emitter is always for ward biased w.r.t base so that it can supply a large number of
majority carriers to its junction with the base. The biasing of emitter – base junction
of npn transitory and pnp transistor show here. Since emitter is to supply or inject a
large amount of majority carriers into the base, it is heavily doped but moderate in size.
E
n
Forward
biasing
B
C
p
n
Reverse
p
F.B
n
p
R.B
biased
Biasing of npn transistor.
Biasing of pnp transistor.
90
Collector : The section on the other side of he transitory that collects the major portion of
the
majority
carriers
suppliers
by
the
emitted
is
called
collector.
The collector base jns is always reverse biased. Its main fns is to remove majority carriers
from its junction with base. The biasing is shown here. The collector is moderately doped
but larger in size so that it can collect most of the majority carriers supplied by the
emitter.
Base – The middle section which forms two pn junctions between emitter and collector is
called base. The base forms two circuits, one input circuit with emitter and other output
circuit with collector. The base emitter juns is reverse biased, offering high resistance
path to the collector circuit. The base is lightly doped and very thin so that it can pass on
most of the majority carries supplied by the emitter to the collector.
p-n junction and depletion region.
Both p-type and n-type region are electrically neutral in the beginning but as the junction
is formed the charge carriers diffuse across the junction. Free electron from the N region
cross the junction and join with holes in the p region. Holes from the p region cross the
junction through valence band and join with electron in the N region.
Immobile
Acceptor
P
Donor
N
Depletion Region
91
When the electron crosses the junction and join with a hole in p region that atom which
receives the electron become –ve iron and atom which losss an electron become +ve ion.
As this diffusion occurs the region becomes positively charged (as it has lost some
electrons) and p region becomes -vely charged.
The accumulation of +ve immobile chargers (ions) at N side of the junction and –ve
immobile charges (ions) at p side of the junction constitute a potential difference across
the junction. These potential difference is called junction potential because these potential
prevent the further movement of charge carriers across the junction. (But this potential
barrier aid the transfer of minority carriers (thermally generated) from one side of the
junction to the other side) The barrier potential is 0.7B for silicon pn juns at 250C. 0.3V
for germanium pn juns at 250C.
Since the region near the junction formed by ptype and ntype semi conductor is depleted
of mobile charges it is know as depletion region or transition region.
To cross the junction or to over come the potential barrier electron or holes are to be
given external energy. When a pn junction to connected across an electric supply the
junction is said to be under biasing. Application of external electric supply aids charge
carrier movement aiding conduction.
P.N JUNCTION WITH FORWARD BIASING
When the positive terminal of a dc source or battery is connected to p-type and negative
terminal is connected to n-type semi conductor of a pn junction, the junction is said to be
forward biased. Here the applied forward potential acts in such a way that it establishes
92
an electric field which reduces the field due to potential barrier. Thus the potential barrier
is reduced. Since the potential barrier voltage is very small (0.3 for ge and 0.7 for silicon)
a small forward voltage is sufficient to completely eliminate the barrier. Once the
potential barrier is eliminated by the forward voltage, a conducting path is established for
the flow of current. Thus a large current called the forward current flows through the
junction. Thus the junction offers low resistance and the magnitude of flow of current
though circuit depending upon the applied forward voltage. The external voltage applied
to pn jns that cancels potential barrier and constitutes easy flow of current is called
forward biasing.
Potential barrier.
P
N
REVERSE BIASING
When the positive terminal of a dc source or battery in connected to n type and negative
terminal is connected to p-type semi conductor of a pn juns, the jns is said to be reserve
biased.
Barrier potential
P
N
93
Here the applied reverse potential acts in such a way that it establishers an electric field
which increases the field due to potential barrier. Thus the potential barrier is
strengthened. The increased potential barrier prevent the flow of majority charge carriers
across the junction. Thus a high resistive path is established by the junction and
practically no current flows through the circuit. It may be noted that at reverse biasing,
practically no current flows through the junction but a little current of µa flows through
the junction because of majority carrier available in the semi conductor even at room
temperature. This current is always neglected.
Unbiased transistors
When a transistor is unbiased, two pn junctions are formed. The free electrons diffuse
across the junction forming two depletion layers. The width of two depletion layer will be
different as the three regions are doped at different level. Depletion layer at the emitter
junction is small and the at the collector junction in large.
Working of npn transistor :
An npn transistor circuit in shown the emitter base junction is forward biased while
collector base junction is reverse biased. The forward biased voltage VEB is quite small,
where as reverse biased voltage VCB is considerably high.
n
p
n
IE
IC
IB
VEB
VCB
94
As the emitter base junction is forward – biased a large no. of electrons in the emitter are
pushed towards the base. This constitutes the emitter current Ie. When these electron
enter the p type material (base) they tend to combine with holes. Since the base is lightly
doped and very thin, only a few electrons combine with the holes to constitute the base
current IB. The remaining electron diffuse across the thin base region and reach the
collector space layer. These electron come under the influence of the positively biased n
region and are attracted by the collector. This constituted the collector current Ic. Thus
almost the entire emitter current flows into the collector circuit. This the emitter current is
the sum of collector current and base current IE = IC + IB.
Working of pnp transistor
A pnp transistor circuit is shown. The emitter – base junction for ward biased, while
collector base junction is reverse biased. The forward biased voltage Veb is quite small,
when even the revenue biased voltage Vcb is high.
P
n
IE
p
Ic
Veb
Vcb
95
Different operating conditions of a transitor
When the emitter junction of a transitory is forward biased and the collector junction is
revenue biased it is said to be operated in its active region. A transitory has 2 junction
which can be biased in different ways. Accordingly it work in different conditions as
listed below.
Conditions
Emitter junction
Collector junction
Region of operation
I FR
Forward biased
Reverse biased
Active
II FF
Forward biased
Forward biased
Situration
III RR
Reverse biased
Reverse biased
Cut off
IV RF
Reverse biased
Forward biased
Inverted
1.FORWARD REVERSE CONDITIONS (FR)
In this conditions, emitter junction is forward biased where as the collector junction is
reverse biased. The transistor is in the active region and collector current depends upon
the emitter current. Generally transistor is operated in this region for amplification
II. FORWARD – FORWARD CONDITIONS (FF)
In this condition, both emitter and collector junction are forward biased. The transistor is
in saturation and the collector current becomes independent of the base current. The
transistor acts like a closed switch.
96
III. RESERSE – RESERSE CONDITIONS (RR)
In this conditions both the junction are revene biased. The emitter does not emit carrier
into the base and no carrier are collected by the collector except little thermally generated
minority carriers) Thus the transistor acts like an open switch.
IV. RF (REVERSE – FORWARD CONDITIONS)
In this condition the emitter junction in reverse biased where as the collector junction is
forward biased. As the collector is not doped to the extent as they emitter is doped, it
cannot emit the majority carrier to the base. Hence very poor transistor action is achieved.
Thus the emitter – collector terminals of a transistor are not interchangeable.
TRANSISTOR SYMBOLS
(E) Emitter
(E) Emitter
Collector (C)
Collector C
Base (B)
Base (B)
Symbol for npn transistor
Symbol or pnp transistor.
An arrow shows the emitter which indicates the conventional current flow direction in
emitter with forward biasing applied to emitter base junction. The only difference b/w
npn and pnp transistor is in the arrow head placed on the emitter.
97
TRANSISTOR AS AN AMPLIFIER
A transistor is a device which raises the strength of a weak signal and thus acts as an
amplifier. The basic transistor amplifier circuit is show.
The input (weak signal) is applied across emitter base and the output amplified signal is
obtained across the load resistor Re connected in the collector circuitry. It may be noted
that a dc voltage VEE is applied in the input circuit in addition to the signal to achieve
faith full amplification. This dc voltage VEE keeps the emitter base junction under
forward biased condition regardless of the polarity of the single and in know as bias
voltage.
Transistor connections
A transistor can be connected in the following
i)
Common base connection (CB configuration)
ii)
Common emitter connection (CE configuration)
98
Common base connection
Current amplification factor is the ratio of out put current to input current. In a common
base configuration out put current is Ic where as input current is IE. α =
. The ratio of
change in collector current to change in emitter current at constant collector base voltage
VCB is know as current amplification factor.
IE = I C + I B
Ic = α IE + ICBO. {IcBo – leakage current from collector to base with emitter circuit open)
Common Emitter configuration
Base current amplification factor
B=
. ie the ratio of change is collector current to the change in base current.
B = α/(1-α) where α
=
Where α =
99
Common collector configuration : The collector is common to both i/p and o/p circuits
and hence the name common collector connection.
Current amplification
Factor r =
ie, the ratio of change in emitter current to the change is base current is
know as current amplification factor.
Commonly used transistor connection
Out of the three transistor configuration the common emitter connection are used in about
90 to 95% of all the transistor amplification because of the following reasons.
1.High current gain
Since the value of o/p current Ic is much more than i/p current Ib, the current gain in CE
configuration is very high.
100
2..high voltage and power gain
As the output resistance is nearly 50 times to that of the i/p resistance on CE circuit and
because of high current gain this type of transistor circuit has highest voltage and power
gain.
3.Moderate output to input impedance ratio
The ratio of output impedance to input impedance is quite small in a common emitter
circuit. So this circuit arrangement is ideal for coupling between various transtor
amplifier stages.
Transistor as an amplifier – (CE configuration)
A common emitter upon transistor amplifier circuit is shown. A battery VBB is connected
in the input circuit is addition to the signal. This battery provides forward bias voltage to
the emitter – base junction of the transitor. The magnitude of bias voltage should be such
that it keeps the emitter base junction always forward biased regardless of the polarity of
the signal source.
101
OPERATION
When a signal is applied in the emitter base junction during positive half cycle, the
forward bias across this junction increases. This increases the flow of electrons from
emitter to collector via base and this increases the collector current. The increased
collector current produces more voltage drop across the collector load resistor Rc.
However, during negative half cycle of the signal, the forward bias voltage across emitter
base junction decreases. This decreases the collector current which consequently
decreases the voltage drop across appears across the collector a load resistor Rc.
Collector current analysis
The graphical representation of collector current is shown. When no signal is applied the
input circuit ie the emitter base junction is forward biased by the battery VBB. There fore,
a dc collector current Ic flows in the collector circuit.
This current is called zero signal collectors current. When the signal voltage is applied,
during positive half cycle the forward bias on emitter – base junction increase, carrying
increase in total collector current ic. Whereas, during negative half, cycle the forward
bias on emitter base junction decreases which camas decrease in total collector current Ic.
Thus total collector current consists of two components, namely.
102
ic
iCS
IC
O
t
i) The dc collector current Ic when no signal is applied. This is due to forward bias
created at the emitter – base junction by the bias battery VBB.
ii) The ac collector current iCS due to signal applied in the emitter base junction.
Total collector – current ic = ics + Ic.
The useful output is the voltage drop across collector load resistor Rc due to the AC
component of current ics which flows through it because of applied signal. The purpose
of zero signal collector current ie, dc component I is only to ensure that emitter – base
junction is forward biased at all times.
TRANSISTOR BIASING
The process by which the strength of a weak signal is raised with out changing its general
shape is know as faithfull amplification.
In order to achieve faithfull amplification, emitter base junction is kept in forward biased
and collector base junction is kept in reverse biased condition during all part of the signal.
This is known as transistor biasing.
103
For faithfully amplification a transistor amplifier must satisfy 3 basic condition.
1) Minimum zero signal collector current
2) Minimum base emitter voltage at any instant
3) Minimum collector emitter voltage at any instant
Fulfillment of these condition is know as transistor biasing
The value of zero signal collector current Ic must be set equal to or more than the peak of
collector current due to signal only.
Zero signal collector current is the collector current of amplifier circuit with out any input
signal. For faithfull amplification proper zero signal collector current must flow.
104
Minimum base emitter voltage
In order to achieve faithfully amplification base emitter voltage VBE should not full
below 0.7V for si and 0.3v for Ge transistor.
IB
Ge
Si
0.3V 0.7V
VBE
Minimum collector emitter voltage
For faithfully amplification collector emitter voltage VCE should not fall below IV to si
transistor and 0.5V for Ge transistor called know voltage.
Ic
Ic
V knee
V knee Si
Ge
VCF
0.5V
VCF
1.0 V
If the value of VCE falls below V knee during any part of the signal that part will be less
amplified due to reduced This a distorted out put signal is obtained.
Thus the process by which the required condition such as proper flow of zero signal
collector current and the maintenance of proper collector emitter voltage during the
105
passage of signal are obtained are know as transistor biasing. The circuitry which
provides the necessary conditions of transistor biasing is known as bearing circuit.. A
transitor biaring sets up the quisant point θ (or operating point) on the dc load line.
OPERATING POINT
The point obtained by the values of IC and VCE when no signal is applied at the input is
known as operating point.
It is called an operating point since variation of Ic and VCE take place around this point
when signal is applied at the input. This point is also called quisent or Q point. Because
it is a point on Ic – Vce Characteristics when transistor is silent ie, no signal at the input.
The operating point for a particular base current Ib, can be obtained by the intersection
point of output characteristics and dc load line.
Stablisation
The inherent variation of transistor parameter may change the operation point resulting in
unfaithful amplification (ie distorted o/p signal) Therefore while designing the biasing
circuit,. This property of transistor is taken into consideration. The biasing circuit is
designed which can work with all the transistors of one type whatever may be value of B
or VBE.
The operating point (Q point) is shifted due to
1. Temperature changes (it affect Ic)
2. The transistor is replaced by another one of the same type. This is due to inherent
variation of transistor parameters (ie θ)
106
The process of making operating point independent of temperature changes or inherent
variation in transistor parameter is called stabilization.
Stability factors
Stability of the operating point means the value of Ic remains constant even though there
is variation in IcBo (or Ico) because of rise in temperature.
The rate of change of collector current Ic w.r.t the rate of change of collector leakage
current ICBO (ICo) at constant Ib and β is called stability factor.
Stability factor S – dfc/dIw at constant Ib and β.
The lower the value of stability factor, greater is the thermal stability of transistor. The
ideal value of s is 1but it is never achieved in practice. However the value of s below 25
results in satisfactory performance.
Methods of transistor biasing
The following are the most commonly used biasing arrangement employed single supply
sources.
i) Base resistor biasing ii) Feed back resistor biasing
ii) Emitter resistor biasing iv) Voltage divider biasing.
107
R.C. COUPLED AMPLIFIER
SINGLE STAGE
When only one transistor with its associated circuitry is used to increase the strength of a
weak signal, the complete circuit or network is known as single stage transistor amplifier.
When a weak ac signal is applied to the base of transistor a small current (iB which is ac)
flows through it. This causes a much larger ac current through the collector ie, Ic = β IB,
As the value of Rc is quite high (5 to 10k Ω) a large voltage (Vout = Ic Rc) appears
across it. Thus a large output (Vout is obtained across Rc. This is how a transistor
amplifiers a weak signal.
Figure shows the practical circuit of a single stage transistor amplifier. In this circuit
various elements are used with the transistor to accomplish faithful amplification. The
function of these elements are described below :
108
Biasing and stabilization circuit (R1, R2 and RE) : Resistors, R1, and R2 are employed to provide necessary biasing. These resistor set up the
Q point in the middle of the dc load line. Their values are so selected that I1 ≥ or 10 IB at
zero signal conditions. Resistor RE provides perfect stabilization ie, it, does not allow the
Q point to shift its position due to rise in temperature or due to change to transistor
parameters while replacing it in the circuit. Usually the value RE = 500 to 1000 Ω.
INPUT CAPACITOR CIN :
An ac single is supplied to the base of the transistor through an electrolytic capacitor
called input capacitor. This capacitor allows only ac signal to enter the base but isolates
the signal source from Rc. If this condition is not used, the signal source resistance will
come across the resistor R2 and thus changes the biasing conditions. The value of input
capacitors is nearly 10µF.
COUPLING CAPACITOR CC.
It is an electrolytic capacitor used to couple one stage of amplification to next stage or
load. It allows only amplified ac signal to pass to the other side but blocks the dc voltage.
There fore, it is also called blocking capacitor.
If this capacitor is into used, the biasing conditions of the next stage will be drastically
changed due to the shunting effect of Rc. The value of this capacitor is nearly 10µF.
Emitter Bypass capacitor CE.
A capacitor connected across the resistor RE is called emitter bypass capacitor CE. Since
it provides an easy path to the ac emitter current and allows it to pass on instead of
flowing through emitter resistor RE. Hence the name emitter bypass capacitor.
109
An emitter bypass capacitor CE (≈100µF) is used in parallel with RE to provide a low
reactance path to the amplified ac signal. If it is used then amplified ac signal flowing
through RE will came a voltage drop across it, these by reducing the output came a
voltage drop across it, there by reducing the output voltage XCE ≤ RE/10.
VARIOUS CIRCUIT CURRENTS
The various circuit currents in the amplified circuits are :
i)BASE CURRENT : (IB) when signal is applied at the input, the total current fed to the
base (iB) is the sun of dc base current IB which flows due to biasing circuit and ac base
current ib which flows due to applied signal. Hence Total base current
iB = IB + ib
ii) COLLECTOR CURRENT iC – Due to transistor action, much larger (B times the base
current) current iC flows through the collector. This total current iC is the sum dc
collector current IC which flows due to biasing circuit and ac collector current ic which
flows due to signal. Hence Total collector current : iC = IC + iC.
Where Ic = βIB – zero signal collector current .
Ic = βib – Collector current due to signal
ii) Emitter current iE. Total emitter current iE is the sum of dc emitter current and ac
emitter current.
iE = IE + ie
Where IE = Ic + IB ≈Ia.
And ie = ic + ib ≈ ic.
110
Hence iE ≈ ic.
Phasor relation between input and output.
In the CE amplifier circuit an ac signal is fed at the input terminals (ie, b/w base and
emitter). The o/p is taken either across the resistor connected in the collector circuit Re (ie,
Ic Rc ) or across collector and emitter and of supply ie, VCE. Usually, the output is taken
from collector and emitter and of the supply as number if stages are required to be
coupled for large amplification. The o/p voltage is given by VCE = Vcc – Ic Rc.
When ac signal voltage increase in the positive half cycle, this increase the bias potential
for emitter junction which in turn increases the base current. This results in increase in
collector current (ic = β ib) and hence the voltage drop ic Rc also increases. Since Vc is
content, the output voltage (VCE = VCC – Ic Rc) decreases.
In other words, it can be
stated that when signal voltage Vin increases in the positive half cycle, the output voltage
(v out) increases in the negative half cycle as shown. This clearly shows that the output
voltage is 1800 out of phase total of input voltage. This is called phase reversal.
This the phase difference of 1800 between the i/p signal voltage and the out put voltage in
a common emitter amplifier is known as phase reversal.
111
MULTI STAGE TRANSISTOR AMPLIFIER.
A transistor circuit in which a number of amplifier stages are used in succession is called
a multistage or cascaded amplifier.
In fact, in a multistage amplifier, the output of one stage is connected to the input of the
next stage through a suitable coupling device and so on. The function of a coupling
device is
i) To transfer only ac output of one stage to the input of the next stage.
ii) To block dc components and isolate the dc conditions of one stage from the other
The block diagram of a 3 stage amplifier is shown
The name of a multistage amplifier is usually given after the type of coupling used
Coupling used
name of amplifier
RC coupling
RC coupled amplifier
Transformer coupling
Transformer coupled amplifier
Direct coupling
Direct coupled amplifier coupling
In put
Single
First
Coupling Second coupling
Third out put
Stage
stage
stage
IMPORTANT TERMS
Gain – The ratio of output to the input of an amplifier is called its gain. Usually it is
represented by letter ‘G’ or ‘A’
Gain for multistage amplifier : Let vin be input to first stage and G1, G2, G3 – Gn be the
gain of Ist, 2nd, 3rd 5th stage of the amplifier.
112
Of amplifier respectively then.
Output of the 1st stage = G1 vin
Out put of the IInd stage = G2 x G1 Vin = G1 x G2 x Vin
Output of the IIIRd stage = G3 (G1 G2 Vin) = G1 x G2 x G3 x Vin
Out put of the 5th stage = Vout = Gn x (G1 x G2 x G3 ….. X Vin)
= (G1 x G2 x G3 x …… x Gn) Vin
Over all gain of a multi stage amplifier,
G = vont
=
(G1 x G2 x G3 x ……….. Gn) Vin
Vin
Vin
G = G1 x G2 x G3 x …… Gn.
Thus the overall gain of a multi stage amplifier is equal to the product of gains of
individual stages.
FREQUENCY RESPONSE
The curve drawn between voltage gain and signal frequency of an amplifier is know as its
frequency response.
The voltage gain of an amplifier is not constant rather it varies with the signal frequency.
It is so, because the reactance’s of the capacitors connected in the circuit change with
signal frequency and here effect the output voltage or gain. It may be noted that voltage
113
gain increases with the increase in frequency and becomes maximum at resonant
frequency for if the frequency of signal increases beyond for, the gain decreases rapidly.
BAND WIDTH
The range of frequency over which the gain of an amplifier is equal to or greater than
70.7% of its maximum gain is know as band width.
Here gain is 70.7% or more with in the frequency range F1 and F2. This range of
frequencies F1 to F2 is called band width. The lower value of frequency F2 where gain is
0.707 Gm is called lower cut off frequency and higher value F2 when gain 0.707 is called
higher cut off frequency. For distrotionless amplification, the signal frequency must be
within the band width of the amplifier. The band width of an amplifier can also be
defined in terms of d B as under :
Let maximum voltage gain from maximum gain
= 20 log 10 Gm/ 0.707 Gm d B
= 20 log 10 1.4142 dB = 3dB.
Hence, the range of frequency with in which the voltage gain of an amplifier falls by 3dB
from its maximum gain is called band width of that amplifier.
114
R-C (Resistance – capacitance) coupled transistor amplifier) (Two stage)
Fig shows the circuit diagram of a two stage R-C coupled transistor amplifier is shown is
fig. The output of first stage appears across the collector resistance Rc. This output is
coupled to the base of the next stage through the coupling capacitor Cc. As the coupling
from one stage to the next stage is achieved by the coupling capacitor followed by a
connection to a shunt resistor hence the name R-C coupled transistor amplifier.
The two transistors used are identical and we a common power supply Vcc. The resistor
R1, R2 and RE from the biasing and stabilization network. Input capacitor Cin allows
only the ac signal the enter the base (flow into the i/p ckt.)
The emitter by pass capacitor CE offers low reactance path to the signal. If it is not
present, then the voltage drop across Re will reduce the effective voltage available across
the base emitter terminals and then reduces the gain with out CE, the voltage gain of each
stage would be lost.
115
The coupling capacitor Cc allows amplified ac signal of one stage to pass on to the next
stage but blocks dc. This prevents dc interface b/w stages and shifting of operating point.
OPERATION
When ac signal vin is applied to the base of Ist transistor Q1, its amplified form appears
across the collector resistor Rc. This amplified signal developed across Rc is fed to the
base of the next transistor Q2 through coupling capacitor cc. The second stage further
amplifies the signal. This is low, the cascaded stages amplify the signal and the overall
gain (G = G2 x G2 x …. Gn) is considerably increased.)
It may be mentioned here that the over all gain is always less than the product of gains of
individual stages. It is so because the effective load resistance of each stage decreases due
to the shunting effect of the input resistance of the next stage. This loading effect of next
stage reduces the gain of each stage except the last one.
FREQUENCY RESONSE CHARACTERISTICS
The curve between voltage gain and signal frequency of an amplifier is called frequency
response. Fig. shows frequency response of an R-C coupled transistor amplifier. The
frequency response characteristics can be divided into 2 sections. In first band the gain
decreases as frequency is decreased and second band is the band where gain is constant
and it is not varying with frequency and thin band is know as mid frequency band. In
high frequency band as frequency increases again the gain decreases. These 3 bands can
be explained briefly.
116
i)At low frequency : (less than 50 Hz) the gain in small it is because of the following
reasons.
a) A part of ac output of first stage is lost in Cc which offers very high reactance (Xc
= 1/2πfc) at low frequencies.
b) At low frequencies Ce offers high reactance and could not shunt the emitter
resistance RE effectively.
iii) At mid frequency range (50Hz to 20 kHz) also called audio gain almost constant.
With the increase is frequency in this range the reactance of coupling capacitor cc
reduces there by increasing.
Advantages and disadvantage of R C coupled amplifier.
Advantages
1. The cost of RC coupled amplifier in low because of low cost of coupling capacitor
and resistors.
2. They occupy less space because of small size of resistor and capacitors
3. They are light in weight
117
4. They have better frequency response since the gain is the mid frequency region is
nearly constant
DISADVANTAGES
1. Comparatively they have smaller gain. It is became of the loading effect of next
stage. This loading effect decreases the effective load (Rac) and hence the gain
(due to low resistance presented by the i/p of each stage to proceeding stage)
2. For impedance matching due to difference in impedance of the RC coupled
amplifier is several hundred ohms where as speaker is of only few ohms. Because
of this effect a very little power can be transferred.
3. They have the tendency to become noisy with age particularly is moist climates
the gain but at the same time lower capacities reactance causes higher loading
resulting in lower voltage gain. Thus the two effect cancel each other and uniform
gain is obtained in mid frequency range.
iii)At high frequencies (>20 KHz) voltage gain drop off because of the following reasons.
(a) Al high frequencies the reactance of cc is very small it behaves as a short circuit.
This increases the loading effect of next stage and reduces the voltage gain.
(b) The capacities reactance of base emitter junction at these frequencies is low which
increases the base current. This reduces the current amplification factor (B). Due
to these reasons the voltage gain drops off at higher frequencies.
118
In case of transistors, there exist some capacitance due to the formation of depletion
region at the junction. These inter electrode capacitance is shown. These are not
physically present in the circuit, but inherently present with the device.
FEED BACK AMPLIFIER
The process by which a fraction of output energy of a device (amplifier) is injected back
to its input is know as feedback.
The fraction of output energy of an amplifier when injected back at the input may aid
oppose the input signal. Accordingly feed back are name das positive feed back and
negative feed back respectively.
POSITIVE FEEDBACK.
When the feed back energy is in phase with the input signal and thus aids it. It is know as
positive feed back. The positive feed back is also know as regenerative or direct feed
back. Although positive feed back increases the gain of the amplifier but it is associated
with increase in noise, distortion and poor stability.
There fore positive feed back is seldom employed in amplifiers however its importance is
in oscillators.
119
NEGATIVE FEED BACK.
When the feed back energy is 1800 out of phase with the input signal and thus opposes it,
it is known as negative feed back. It is also know as degenerative or reverse feed back.
Although negative feed back decreases the gain of amplifier but it has the following
merits.
It improves the stability in gain decreases noise level and distortion, improves input and
output impedances etc. because of these merits negative feed back is frequently employed
in amplifiers.
The feed back can also be classified on the basis of electrical quantity (ie, voltage or
current) to be feed back.
VOLTAGE FEED BACK
In this case the signal feed back is proportional to the output voltage respective to load
impedance
CURRENT FEED BACK.
In this case, the signal feed back is proportional to the output current irrespective to the
load impedance.
They can be again classified as
1. Series voltage feed back
2. Series current feed back
3. Shunt voltage feed back
120
4. Shunt cement feed back
Principle of Negative feed back in amplifiers
Consider an open loop system or non feed back system.
Vin
+
A
-
+
Gain of the amplifier (open loop system ) A = V0/V1
In this circuit, the input remains unaffected even if the output changes due to any reasons.
Consider Negative feed back system
121
The circuit arrangement has two main parts viz an amplifier and a feed back circuit. The
out put V0 of the amplifier is obtained across the load resistor RL. A fraction of output
voltage (mV0) is fed back by the feed back circuit to the input circuit. Therefore the
actual input voltage Vi applied across the input terminals of the amplifiers in not the
signal voltage Vs but it is Vs – imV0 since the feed back is negative Thus,
Fraction of voltage fed back to the input = mV
Input to the amplifier Vi = Vs – mV0
Gain of amplifier with out feed back A = V0 =
VI
Gain of amplifier with feed back = Afb = Vo =
Vs
Vo
Vs- mV0
V0
Vi + mV0
Fraction of output feed back = mVo/ Vo = m
Gain of amplifier without feed back A = V0 / Vi = Vo/Vs – mV0
(Vs – m Vo) A = Vo
AVs = V0 + mVo A
AVs = (1 + ma) V0
Vo/Vs = A/1+mA = Afb
122
The ratio of o/p voltage to signal voltage with feed back circuit is called voltage gain with
negative feed back. Afb.
Afb = A/1+mA (With negative feed back)
This expression shows that gain of amplifier with negative feed back is reduced by a
fraction 1+mA
Transistor amplifier circuit with negative voltage feed back.
The new components introduced in this circuit are R3 and R4 they act as feed back circuit
and provide the necessary feed back voltage (mVo) to the input. Let the voltage fed back
is vF (mV0)
Voltage across R3, Vf =
R3
V0
R3 + R4
mVo =
R2 V0
R3 + R4
m=
R3
R3 + R4
123
Feed back factor m depends upon R3 and R4. By adjusting the values of these resistor we
can vary the feed back voltage and also output voltage.
The sign marked in the circuit clearly show that feed back is negative. The negative
voltage fedback reduce the output intendance and also decreases the gain of the amplifier.
However it reduces destortion and increases band width.
Advantages of negative feed back amplifiers
1. Improves gain stability
2. Reduces distortion and noise
3. Increases input impedance
4. Decreases output impendence
5. Increases band width or improve frequency response.
124
OSCILLATORS
An oscillator is an electric circuit which converts dc energy into ac energy of required
frequency. These are used in radio and television receivers to generate high frequency
waves. Industry, oscillators are used with induction and dielectric heating.
CLASSIFICATION OF OSCILLATORS
Sinusoidal oscillators – generate sinusoidal voltage or current
Relaxation oscillators – generate non sinusoidal wave form such as square, rectangle.
Gain of +ve feed back Af = A / 1-AB
When AB = 1, Af = α
The mathematical meaning of Af = α is that, there is an output, with out any input,
amplifier becomes an oscillator which supplies its own input.
BARKHAUSEN CITERIA
1) Feedback must be +ve (Total phase shift around the closed top is zero or 3600)
2) βA = 1
Signal i/p
Amplifier
o/p signal
Dc power input
oscillator
signal o/p
Dc power input
Oscillator does not require an external signal.
125
OSCILLATORY CIRCUIT
A circuit that produces electrical oscillations of a desired frequency is known as an
oscillatory circuit or tank circuit.
TYPE OF ELECTRICAL OSCILLATIONS
The sinusoidal electrical oscillations are of two type ie, damped oscillations and
undamped oscillations
(a) damped oscillations
The electrical oscillations in which amplitude decreases with line are know as damped
oscillations.
(b) Undamped oscillations
The electrical oscillations in which amplitude does not change with time are known as
undamped oscillations.
E or i
e or i
t
(a)
t
(b)
The figure below shows a simple oscillatory circuit which contains a capacitor and an
inductor (or coil) L connected in parallel. The frequency of oscillation produced by this
oscillatory circuit is determined by the value of C and l.
The circuit arrangement to charge the capacitor of an oscillatory circuit.
126
When switch S is thrown to stud 1 as show in the figure below the capacitor C is charged
at the battery potential in the direction shown. Now throw the switch to stud 2, the
capacitor will discharged through inductor L and sets up the electrical oscillations as
explained below. When capacitor discharged through inductor L, the excess of electron
on plate B travel through L and reach at plate A as show in (a). This flow of current sets
up magnetic field around the coil as in (b). Due to inductive effect, the current builds up
slowly and attains its maximum value when the capacitor is fully discharged. Thus the
electrostatic field energy at the capacitor is converted into magnetic field energy around
the coil.
Now the magnetic field set up around the coil begins to collapse and produces a counter
emf. This content will continue the flow of current in the same direction. The result is
that the capacitor is now charged with opposite polarity.
127
Once the magnetic field is totally collapsed, the capacitor is fully charged. Then the
capacitor starts discharging and electron start moving from plate A to B through inductor
L (e). Again the magnetic field is set up around the coil but in opposite direction when
the capacitor is fully discharged as when (f). The magnetic field collapses which keep the
flow of electron to continue in the same direction (g). When the magnetic field is
completely collapsed, the capacitor C is fully charged (b) and regain its first position.
Thus a sequence of charging and discharging of capacitor results in alternating motion
of electrons and produces oscillations. Actually, in this L-C circuit, the energy stored in
electrostatic field of capacitor is supplied to inductor and is stored in magnetic field. Then
the energy stored in magnetic field is transferred to the capacitor and is stored in the
electro static field. Thus inter change of energy between capacitor (c) and inductor (L) is
repeated over and again resulting in the production of oscillation. Wave form in the given
L-C circuit, during each cycle there are resistive and radiation losses in the capacitor
there fore the amplitude of oscillating current decreases gradually and eventually
becomes zero. Thus damped oscillations are produced by this circuit.
+Im
t
-Em
In order to make the oscillations is the tank circuit undammed,
i) The amount of energy supplied should be such so as to meet the losses in the tank
circuit
ii) The applied energy should be in phase with the oscillations setup in the tank
circuit.
iii) The frequency of the energy supplied to the tank should be the same as that of the
oscillations produced.
128
The block diagram of a transistor oscillator
It has the following essential parts
OSCILLATORY CIRUIT
An oscillatory circuit contains inductive coil L and capacitor C connected in parallel with
each other. The frequency of oscillation depends upon the value of inductance of the coil
and capacitance of the capacitor.
Ie fr = 1/ 2π√LC
Transistor amplifiers
To compensate losses that occur in the oscillatory circuit, a source of energy is required.
That source of energy is combination of battery and transistor working as amplifier. The
transistor amplifier amplifies these oscillations and an amplified output is obtained. This
amplified output of oscillations is due to the dc power supplied by the battery. The out
put of the transistor then obtained can now be supplied back to the oscillatory circuit to
compensate the losses.
129
FEED BACK CIRCUITS : This circuit is used to feedback a fraction of amplifier
output to the oscillatory circuit in correct phase so that it aid the oscillations and
compensate the losses. It provide +ve feedback.
Different type of Lc oscillators
1. Tuned collector oscillator 2) Hartley oscillator 3) Colpitts oscillator
Tuned collector oscillator
This oscillator employs an oscillatory circuit L1 C1 and frequency of oscillation depends
as the value of L1 and C1 F = 1/ 2π√L1C1.
The will L2 in the base circuit is magnetically coupled to L1. In fact L1 and L2 form the
10 and 20 of a transformer. R1, R2, RE and CE are the components which provide biasing
and stabilization to the circuit capacitor C provides a low reactance path to oscillations.
OPERATION
As long as switch is open, there is no collector current when switch is closed collector
current starts rising thus charging the capacitor.
130
After sometimes capacitorC, will discharge through L1, setting up oscillation of
frequency
F = 1/ 2π√2c1
These oscillation which set up in the oscillatory will induce an emt in the coil L2. (similar
to transformer action) and frequency of induced emf is same as that of oscillations F = 1/
2π√L1C1. This induced emf is applied b/w emitter and base. This will increase the
magnitude if collector cement due to amplifying properties to transistor. The freq. of the
amplified out put is same.
The amplified output is applied to the oscillatory. Power supplied to the oscillatory circuit
is based to over come the losses curing in it. Hence we get imdamped oscillations in the
output.
Here a total phase shift around the closed loop is 3600 degree. Here 1800 phase difference
between emf of L1 and L2 is obtained by the transformer action b/w L1 of L3. Further
phase shift of 1800 is obtained b /w base voltage and output voltage due to the properties
of the transistor (CE configuration).
Principles of phase shift oscillator.
In phase shift oscillator a phase shift of 1800 is obtained with a suitable phase shift circuit
instead of transformer action. Further 1800 phase shift is obtained due to transistor
properties.
In an oscillator energy supplied to the oscillator circuit will be 180+180 = 3600 out of
phase ie in phase.
131
RC PHASE SHIFT CIRCUIT
Consider an RC circuit in this circuit there is a phase difference φ between applied
voltage and resulting current. This current produce a voltage drop is resistor applied
voltage and voltage a cross R have a phase difference φ.
In actual practice it is common to use 3 similar RC sections as shown in fig. Each RC
section is designed to produces a shift of 600. In this way a total phase shift 1800.
Rc phase shift oscillator
A phase oscillator employs an R-C n/w and is commonly called R-C phase shift
oscillator.
132
Circuit analysis
The phase shift oscillator is shown. The collector is connected to be base through phase
shift network. It may be seen that the phase shift network consists of 3 similar sections.
R1C1 R2C2 and R3C3. The values are so selected that each section produces a phase
shift of 600. Thus the total phase shift of 1800 is produced by the 3 sections. Generally
R1, R2 and R3 and made equal. C1, C2 and C3 are made equal.
OPERATIONS
When the circuit is switched on, current through R2 starts increasing became of biasing.
This charging current induces voltage across R2 through C3. The voltage across R2 leads
the voltage across R3 by 600. Sine there R.c section are phase shift of 600x3 = 180. A
further phase shift of 1800is produced due to the transistor properties. So a total shift of
3600 is produced. There fore, the fraction of the o/p fed to the input is in phase with it.
133
FREQUENCY OF OSCILLATIONS
When R1 = R2 =- R3 and C1 = C2 = C3 = C, the operating frequency is given by F = 1/
2πCR √6.
CLIPPING CIRCUITS
A circuit used to change the shape of an input wave by clipping or rewing a portion of it
called a clipping circuit.
The important diode clipper are
(1) Positive clipper (2) biased clipper (3) Combination clipper
POSITIVE CLIPPER
A circuit that removes positive half cycle of the signal input voltage) is called a positive
clipper.
During the positive half cycle of input voltage, the diode D is forward biased and
conducts heavily. Ideally it act as a closed switch and hence the voltage across the diode
or load is zero and hence positive half cycle is clipped off. In other words positive half
cycle does not appear at the output.
134
During the negative half cycle, diode is reverse biased and behave as an open switch.
Then the current flow through RL and R which are connected is series. In this condition
circuit behaves as a voltage divider, white the out put voltage is taken across RL.
NEGATIVE CLIPPER
To remove the negative half cycle of the input signal the only thing to be done is to
reverse the polarity of the diode connected across load.
In the above explanation, the diode is considered to be an ideal one. If second
approximation of diode is considered the barriar potential (0.7v) for is and 0.3v for Ge) of
diode will be taken into consideration. Then the output wave shape for +ve and –ve
clipper will be as show in fig.
t
135
BIASED CLIPPER
A clipper used to remove a small portion of positive or negative half cycle of the signal
voltage is called a biased clipper.
WORKING
During the positive half cycle, the diode will be forward biased only when the input
voltage is greater than the battery voltage. When the input voltage is greater than +ve
(battery voltage) the diode behaves as a short and the output equal +ve. The output will
stay at +v so long as the input voltage is greater than +v. Before the supply voltage rives
of +ve volts, the diode is revenue biased and current flows through R and RL which are is
series. Thus voltage appear across RL or at the output.
During the negative half cycle diode is reversee biased therefore, current flow through R
and RL connected is series. As per voltage divider action almost whole of the voltage
(RL>>R) appears across RL (at output) as show here in fig.
136
COMBINATION CLIPPER
A clipper used to remove small portion of positive as well as a small portion of negative
half cycle of the signal voltage is called a combination clipper.
During positive half cycle when the input signal voltage is more than +V2, diode D1
conducts heavily while diode D2 remains severe biased. There fore, positive half cycle
beyond _v, volt is clipped off. On the other hand, during negative half cycle when the
input signal voltage is more than –V2 volts. Poise D2 conducts heavily while diode D1,
remain revenue biased. Therefore negative half cycle beyond -V2 volts is clipped off.
Hence it is seen that no diode conducts between +v1 and –V2 volts. Therefore during this
period most of the input voltage appear across the load or output. This clipper is mostly
employed to generate square wave.
137
CLAMPING CIRCUITS
A circuit that shift either positive or negative peak of the signal at a desired dc level is
know as damping circuit or clamper.
A clamping circuit adds a dc component to the signal. Here input signal is a since wave
having a peak to peak value Vm.
POSITIVE CLAMPER
A circuit that shifts the signal in the +ve side in such a way that negative peak of the
signal falls on the zero level is called a positive clampers.
A positive clamper is show in fig. It consists of a diode D and a capacitor C. The output
is taken across load resistor RL.
Working.
C
D
RL
During negative half cycle of the input signal, the diode conducts heavily and acts like a
closed switch. The capacitor C charged to vm at the negative peak of the signal with the
polarity as marked. Slightly beyond the negative peak the diode stops conduction through
138
it and behaves as am open switch. The charged capacitor (Vm) just behaves as a battery
which adds the signal voltage during its positive half cycle. During +ve half cycle of the
signal
the diode is reverse biased and acts as an open switch. The resultant output
voltage coming across the load resistor RL will e : output voltage = Vm + Vm = 2Vm (at
positive peak)
Negative clamper
A circuit that shifts the signal in the negative side in such a way that the positive peak of
the signal falls on the zero level, is called negative clamper.
NEGATIVE CLAMPER
A circuit that shifts the signal in the negative side in such a way that the positive peak of
the signal falls on the zero level, is called negative clamper.
139
A negative clamper is show. It contains a diode D and a
capacitor C. The
only
difference in the –Ve clamper circuit is that the polarity of the diode is reversed. Because
of this reason the circuit acts -2Vm as a negative clamper.
During the positive half cycle of the signal, the capacitor is charged to Vm with polarity
show. Note that voltage across the capacitor is opponing the input voltage ie.
Output voltage = V-vm
DIFFERENTIATING CIRCUIT
A circuit that give an output voltage directly proportional to the derivative of its input is
known as a differentiating circuit. o/p α d/dt (input)
C
R
Output (Vo)
140
A differentiating circuit is shown. It is just an RC series ckt where output is taken across
the resistor. The output across R will be derivatives of the input. In order to achieve good
differentiation the following two conditions should be satisfied.
i) The time constant RC of the circuit should be smaller than the time period of the
input wave.
ii) The value of Xc should be 10 more times larger than R at the operating frequency.
When an Rc circuit fulfils the above conditions then the voltage that appears across R
will be the devivative of the i/p.
OUTPUT WAVE FORMS
When the input fed to a differentiating circuit is dc (or contant voltae) its output will be
zero. It si became the dilative of a content is zero.
The physical behavior o he circuit under this conditions may be explained as under.
When i/p is constant dc, contrarily current flows through the circuit and then reduces to
zero as soon as the
capacitor is fully charged. The output is show in fig.
141
WHEN I/P IS A SQUARE WAVE
V1
t
V0
t
B
V1A
C
F
t
V0
t
When the input fed t a differentiating circuit is a square wave, output will be consisting of
sharp narrow pulse as shown.
During OA part of the input wave, its amplitude changes abruptly hence its output wave
(differentiating this part) will be a sharp narrow phase. Whereas during AB part of the
input wave, its magnitude is constant there fore, its output will be zero. Again a spike is
obtained when the wave changes abruptly in the negative direction (C0) but spike is
opposite.
142
This constant RC of the circuit is very small w.r.t time period of input wave and Sc >> R,
the capacitor will become fully charged during the early part of each half cycle of he
input wave. During the remainder part of the half cycle, the output of the circuit will be
zero because the capacitor voltage entreaties the input voltage and there can be no current
flowing through R. Thus we shall get sharp pube at the output during the start of each
half cycle of the input name while for the reminder part of the half cycle of input wave,
the o/p will be zero.
ii)When input is a sawtooth wave : The o/p will be a rectangular wave:
During the period OA of the input name, its amplitude charges at a constant rate and
therefore, the differentiated wave has a constant value for each constant rate of change.
During the period AB of the input wave, the change is less abrupt so that output will be a
very narrow pube of rectangular form.
iii) When input is a some wave output is a cosine wave
143
EXPONENTIAL SWEEP CIRCUIT
Figure represents a basic saw tooth voltage generator switches periodically changes its
position from point 1 to point 2. The switching action causes a capacitor C to be
alternately charged and discharged. The resulting wave form of o/p voltage is shown.
Time T1 is called sweep time and is represented by Ts and time T2 is called the retrace or
retain time and is represented by Tr. The return or retrace time is required is display
system to return the beam to its original starting point before the next cycle begins.
When the switch is in position 1 the capacitor tends to charge to the supply voltage V
called the aiming potential. Before the capacitor can charge completely the switch s is
brought to position 2 and the capacitor discharged until the switch is returned to position
1 and thus the cycle is repeated. Sweep voltage amplitude Vs = V2 – V1 sweep linearly is
a measure of how close to the ideal the exponential wave form comes.
INTEGRATING CIRCUIT
A circuit in which output voltage is directly proportional to the integral of the input is
know as an integrating circuit.
144
Vi
C
Out put = V0
An integrating circuit is a simple RC series circuit with output taken across the capacitor
C as show in fig. The following conditions should be fulfilled for faithful integration.
1. The value of R and C are selected is such a way that the time constant (RC) of the
circuit should be very large than the time period of the input wave.
2. The value of R should be 10 times or more than 10 times larger than Xc(ie,
R≥10Xc ) at the operating frequency. When an Rc circuit fulfith the above
conditions then the voltage that appears across C will be the integral of the input.
Wave shaping by integrating circuit
The wave shape of output wave obtained from an integrating circuit depends upon the
time constant (RC) of the circuit and wave shape of the input wave.
When input is a square wave
When input fed to an integrating circuit is a square wave, the output will be a triangular
wave as shown in fig. As integration means runnation, there fore output from an
integrating circuit will be the sum of the input waves at any instant. This sun is zero at 0
145
and goes on increasing till it becomes maximum at B, then ruination goes on decreasing
and becomes zero at F.
A
B
O
C
D
F
T
E
V1
t
V0
o
T
t
146
ASTABLE MULTIVIBRATOR
The stable or free running multivibrator generates square wave with out any external
triggering pulse. It has no stable states. It switches back and forth from one state to the
other remaining in each state for a time depending upon the discharging of capacitive
circuit.
The circuit shows a basic symmetrical transistor astable multivibrator in which
components in one half of a circuit are identical to their counter part in the other half. The
square wave output can be taken from collector point of Q1 and Q2.
When the supply voltage +vcc is applied one transistor will conduct more than the other
due to small difference in their operating characteristics. Initially let us assume that Q1 is
conducting and Q2 is cut off. Then Vc1 the output of Q1 is equal VCE (sat) ie,
approximately zero volt and Vc2 = +vcc.
147
At this instant C1, charges exponentially with a time constant R1, C1 towards the supply
voltage through R1 and corresponding VB2 also increases exponentially to wards Vcc.
When VB2 crosses the cut in voltage Q2 starts conducting and VC2 falls to VCE (sat)
also VB1 falls due to capacitive coupling b/w collector of Q2 and base of Q1 there by
deriving Q1 into off state. Now the rise in voltage VC1 coupled through C1 to the base
of Q2 causing small over shoot in voltage VB2. Thus Q1 is off and θ2 is ON. At this
instant VB is –ve, VC1 = Vcc, Vc2 = VCE (sat), VB = VBE (sat)
When Q1 is off and Q2 is ON, the voltage VB1 increases exponentially with a time
constant R2 C2 towards Vcc. Therefore Q1 is driven into saturation and Q2 is cut off.
Now voltage level is VB1 = VBE (sat), Vc1 = VCE (sat), VB2 is negative and Vc2 =
Vcc. It is clear that when Q2 is ON, the falling voltage Vc2 permits the discharging of the
capacitor C2 which drives Q1 into cut off. The rising voltage Vc1 feed backs to the base
of Q2 tending to turn it on. This process is called regenerative if R1=R2=R and
C1=C2=C we have a symmetrical multivibratior with outputs at the two capacitor having
the same wave forms but out of phase with each other.
T = 1.386 Rc and F = 1/T = 1/1.386 Rc Where T = T1 + T2
(on time of Q1, T1 = 0.693 R1C1 for Q2 T2 = 0.693R2C2)
148
149
CATHODE RAYTUBE (CRT)
Cathode ray tube is a specially constructed vaccum tube in which an electron beam
controlled by electric and magnetic field generates a visual display of input electrical
signals on the fluorescent screen. The three important parts are : electron gum, a
deflection system and a flouresecen screen.
CONSTRUCTION
The electron gum consists of several electrodes mounted at one end of the tube and
electrical connection are given top them through base pins. The deflection system
consists of two parts of parallel metal plates mounted at the neck of the tube. They are
oriented at mutually perpendicular direction to watch other and to the axis of CRT. The
connection to the plates are also given through the base pins.
The screen consists of a tin coating of phase porous deposited on the inner face of the
wide and of the glass envelope. The inner surface of the inner flare of the envelope is
coated with a conductive graphite coating called aquadag.
150
It is connected internally to the accelerating anode. A power supply provides the required
potentials to the various elements of CRT.
WORKING
In the CRT, the electron gum generates an electron beam, focusses it and accelerates it
towards a fluorescent screen located at the farther and of the tube. The screen emits a
small round glow at he point where energetic electron strike. the electron beam may be
moved to any sport on the screen with the help of the deflection system.
(a) ELECTRON GUN (K)
The cathode is heated indirectly and the emitted electrons pass through the control grid
(g) which is held at negative potential. The intensity of the glow produced at the screen is
determined by the number of electron striking the screen. Thus by varying he –ve dc
voltage on the grid, the intensity of the luminous spot on the screen controlled. The grid
voltage is made more –ve the electron beam will be cut off.
The electrodes A1 and A3 are internally connected and held at a higher positive potential
of few KV. A2 is maintained at smaller positive potential. A1 accelerates the incoming
electron and reduces the interaction between control grid G and focusing anode A2. A
node A2 and grid G does the prefocussing of the electron beam. A2 and A3 further
focuses the electron beam to a fine point on the fluorescent screen.
151
The focus of the beam is adjusted by varying the positive potential on A2. The anode A3
imparts further acceleration to the electron as they emerge out of the electron gun.
B) DEFLECTION SYSTEM
The electron emerging from the electron gun are allowed to pass through two types of
deflection plates before they reach the flourescent screen. The two types of deflection
system are electro static type and the electromagnetic type.
In electro static deflection type two pairs of metal plates are employed for deflecting the
electron beam. The two plates in each pair are aligned strictly parallel to each other. The
two pairs of plates are mounted at right angles to each other and also at right angle to the
path of the electrons.
One pair of plates is arranged horizontally near to anode A3 such that electron passes b/w
them. When a potential difference is applied top the plate, a uniform electric field is
produced in the vertical direction. The field acts normal to the beam and causes a vertical
152
deflection of the beam. This set of deflection plates are called vertical deflection plates
(VDP) and are also called y plates.
The other pair of plates are oriented vertically and produce uniform horizontal electric
field when a potential difference is applied between them. They came a horizontal
deflection of the beam and thus are called horizontal deflection plates (HDP) and are also
called X – plates.)
When voltage are not applied to x plates and y plates, the electron beam travels along the
CRT axis and strikes at the geometrical centre of the viewing screen.
153
When dc voltage are applied to both the x and y plates, the electron beam will be acted
upon by two forces due to vertical and horizontal electric fields and gets deflected along
the direction of their resultant.
154
C) FLOURESCENT SCREEN
The interior surface of the circular front face of the CRT is coated with a thin transluscent
layer of phorphour. This coating glows at the point where it is struck by the electron
beam. The coating is made thin to allow the light to pass through the screen material and
glass so that it can viewed from outside the crt.
D) AQUADAG WATING
Electron reaching the screen charge it negatively and repel the electron arriving after
wards. This will reduce the no. of electron reaching the screen leading to a decrease in the
brightness of the glow. Thus the electron are to be conducted away
The cathode assumes a +ve charge as the electron are emitted from it in large numbers.
This also reduce the intensity of the beam. Thus cathode is to be replenished
with
electrons. The aquadag coating is a conducting coating used to complete the electrical
circuit from screen to cathode. The electron striking he fluorescent screen also came
secondary emission of electrons. There electron are attracted by the aquadag coating
which is electrically connected to the anode A3. The electrons are returned to cathode
through the ground.
155
CATHODE RAY OSCILLOSCOPE – CRO
Vertical
Input signal amplifier
to CRT
HT SUPPLY
LT supply
CRT
To all circuit
Trigger
circuit
Sweep
generato
rrrr
Horizontal
amplifier
An oscilloscope can display and also measure many electrical quantities like ac/dc
voltage, time phase relationship, frequency and a wide range of waveform characteristics
like time, fall time and over shoot etc.
As seen from the block diagram of an oscilloscope, it consists of following major sub
systems.
1) Cathode ray tube (CRT) it displays the quantity being measured.
2) Vertical amplifier – it amplifier the signal wave form to be viewed.
3) Horizontal amplifier – it is fed with a saw tooth voltage which is then applied to
the x – plates
4) Sweep generator – it produces raw tooth voltage wave form used for horizontal
deflection of the electron beam.
5) Trigger circuit, it produces trigger pulses to start horizontal sweep.
6) High and low voltage power supply
156
NORMAL OPERATION OF A CRO
The signal to be viewed or displayed on the screen is applied across the y plates of a
CRT. But to see its wave form or pattern it is essential to spread it out horizontally from
left to right. It is achieved by applying a saw tooth. Voltage wave (produced by a time
base generator ) to x plates. Under these conditions the electron beam would move
uniformly from left to right there by graphing vertical variation of the input signal verses
time. Due to repetitive tracing of the viewed wave form, we get a continuous display
because of persistence of vision. However for getting a stable stationary display on the
screen. It is essential to synchronize the horizontal sweep of the beam with the input
signals across the y plates. the signal will be properly synchronized only when it
frequency equals the sweep generator frequency. In generals for proper synchronization
of time base with the signal the conditions is Tsw – nTs, where Ts is the time period of
the signal and n is an integer.
If n = 1 then Tsw = Ts ie time periods of the sweep voltage and input signal voltage are
equal then one cycle of the signal would be displayed as show here.
One the other hand if Tsw = 2Ts,t hen two cycle of the signal voltage would be displayed
as shown
157
Obviously, 3 full cycles of the input voltage would be spread out on the screen when Tsw
= 3Ts.
MEASUREMENT OF VOLTAGE AND CURRENT
CRO can be used for measurement of voltage of any electrical signal as the electrostatic
beam is directly proportional to the deflection plate voltage.
For measurement of direct voltage, firstly the spot is centered on the screen with out
applying any voltage signal to the deflection plates. Then direct voltage to be measured is
applied b/w a pair or deflection plates and deflection of the spot is observed on the
screen. the magnitude of deflection multiplied by the deflection factor gives the value of
direct voltage applied. Usually the screen is calibrated for fixed operating conditions so
by reading the scale voltage can be measured directly by CRO.
In case of measurement of alternating voltage of sinusoidal wave form, it is applied b/w a
pair of deflection plates and the length of the straight line is measured. Knowing the
deflection sensitivity the peak to peak value of applied ac voltage can be determined.
158
The run value of Ac voltage applied will be equal to this peak value divided by 2√2 for
sinusoidal wave form.
For measurement of current the current under measurement is passed through a known on
inductive resistance and the voltage drop across it is measured by CRO, as mentioned. the
current can be determined by simply by dividing the voltage drop measured by the value
of non inductive resistance when eh current to be measured is very small in magnitude
the voltage drop across the non inductive resistance is manfully amplified by calibrated
amplifier.
FREQUENCY MEASUREMENT
The unknown frequency can be accurately determined with the help of a CRO the steps
of the procedure are
1. A known input is applied to the horizontal input and unknown frequency to the
vertical input
2. The various controls are adjusted
3. A pattern with loops is obtained
4. The number of loops cut by the horizontal line gives the frequency on the vertical
plate (fv) and no. of loops cut the vertical line gives the frequency on the
horizontal plates (fH)
Fv
=
No. of loop cut by horizontal line
Fh
=
No. of loops cut by vertical line.
159
For eg. Suppose during the freq. measurement test, a pattern show here is obtained. Let is
further assume that frequency applied to horizontal plate is 2500 Hz. If we draw
horizontal and vertical lines, we find that one loop is cut by the horizontal line and two
loops by the vertical line.
Fv
=
No. of loop cut by horizontal line
Fh
=
No. of loops cut by vertical line
Fv = ½
2000
Fv. 2000 x ½ = 1000 Hz
Unknown frequency is 1000 Hz.
MODULE – 5
Power semiconductor devices : power diodes – SCRS principle of operating of SCRI –
two transistor analogy of SCR – characteristics – SCR rating (basic principle only) High
frequency heating – induction and dielectric heating – resistance heating resistance
welding – block schematic of resistance welding scheme.
WORKING
The load RL is connected in series with the anode. The working of an SCR is explained
1 action of anode voltage
160
G No Voltage
A
J1
J3
K
P n p n
V
Consider an SCR, where anode is made +ve w.r.t cathode and the gate voltage is kept at
zero. With this voltage the juns J1 and Te are forward biased, where as jns J2 is reverse
biased. Hence the flow of current in the circuit is blocked ie, the restore is cut off.
However if anode to cathode voltage V is increased a stage is reached when breakdown if
Jns J2 occurs and the thyristor suddenly switches to highly conducting state. In this stage,
the load current is limited by supply voltage V and load RL.
The maximum anode voltage (anode in +ve w.r.t cathode) at which SCR switches from a
non conducting (off state) to a conducting (ON) state with out gate voltage is known as
forward break over voltage.
If anode is made – ve w.r.t cathode and the gate voltage is kept at zero junction J1 are
reverse biased, where as junction I2 is forward biased. Under such conditions, SCRs do
not conduct current, it remain in cut off position. However if this reverse voltage is
increased, a stage reaches when breakdown of junction J1, and J3 occurs and thyristor is
turned to a highly conducting state. This maximum voltage a they sitar dm tolerate with
out conduction is know as severs break down voltage.
161
MODULE – V
POWER ELECTRONCIS
The branch of electronics which deals with the control of power handled by a system is
called power electronics. This branch deals with those semi conductor devices which can
act as controlled switches such as silicon controlled rectifier (SCR) trac, Diac, UJT etc. )
SILICON CONTROLLED RECTIFIER
A silicon controlled rectifier (SCR) is a three terminal semicondcutor device which can
be used as a controlled switch to perform various functions such as rectification inversion
and regulation of power flow.
SCR can be designed to handle current upto several thousand ampere at voltage 1kv and
more. That is why it is considered to be one of the most important device of power
electronics. The other commercial name of SCR is thyristor. It is a unidirectional power
switch which is mostly used to obtain controlled dc output from an ac input.
Construction
Gate (G)
ANODE
(A)
Cathode
P
n
p
n
(K)
(G)
(K)
(a)
162
An SCR is a pnpn semi conductor device consisting of three pn junctions J1, J2 and J3. It
is as if an ordinary diode and a transistor are connected in one limit. Three terminals are
taken one from the outer p-type material called anode (A) second from the outer n-type
material called cathode (k) and third one from the other p – type materials placed in b/w
and called gate (g)
2.ACTION OF GATE VOLTAGE
A
+ G
K
p n p n
+
-
Consider the SCR circuit where anode is +ve w.r.t cathode suppose anode voltage is less
than forward break over voltage so that thyristor does not conduct. If a +ve voltage is
applied to gate, the gate current Ig flows in the gate circuit. The flow of gate current helps
in making current flow across Jns J2 ie, It will gets break down. This switches the
thyristor to highly conducting state at the anode voltage which is quite less than the
forward break down voltage. The gate voltage has the ability to trigger the scr at a low
value anode voltage, but it loses it all control on the SCR current after triggering. there
before in order to off the SCR circuit the anode voltage has to be reduced to zero.
THUS
1) An SCR has two states : N and off states
2) SCR will conduct if forward voltage is more than forward break down voltage
3) An SCR can be turned ON at much cover forward voltage y the application of
proper gate current
163
IMPORTANT TERMS
BREAK OVER VOLTAGE
The minimum forward voltage, gate being open, at which SCR starts conducting heavily
is called break over voltage.
PEAK REVERSE VOLTAGE
The maximum reverse voltage (cathode +ve) which can be applied to an SCR with out
conducting is called peak reverse voltage.
Holding current
The maximum anode current gate being open at which SCR is turned off from on
condition is called holding current.
FORWARD CURRENT RATING
The maximum anode current that an SCR can handle with out destruction is called
forward current rating of an SCR.
SCR RATINGS
There are a large number of ratings for scr. For selection of SCR’s one should see the
manufactures specifications. According to supply of load requirements one can select can
SCR. Out of many ratings the most important are voltage, current gate and temperature
ratings.
164
1.VOLTAGE RATINGS
Ac mains to which SCR is connected may have transients some of them may occur
regularly and may occur irregularly. SCR should be capable to with stand these transits.
Figure shows a typical ac come with transits. the different voltage are:
VRWM = it is the crest working reverse voltage neglecting transients
VRRM = it is the repetitive peak reverse voltage. It is the peak of the regular transients
VRSM = it is the non repetitive peak reverse voltage. It is the peak value of the surge
voltage that lasts up to 10ms, but does not repeat irregularly
VDWM – it is the crest working OFF state voltage. This corresponds to the peak voltage
applied is forward direction.
VDRM – it is the repetitive peak of state voltage.
VDSM – it is the peak non – repetitive off state voltage
VT – it is the continuous On state voltage b/w cathode and anode.
165
Dv
dt
- This gives the maximum rate of rise of the anode voltage which will not trigger
the SCR.
2.current ratings – The important current ratings are as follows
ITAV – it is the average ON state current
IRMS – it is the R.M.S value of the On state current
ITRM – it is the respective peak ON state current, this can be drawn by th SCR, when I TAV
and IRMS are with in limits.
Di/dt – it gives the maximum rate of rise of ON state current, di/dt should not exceed
beyond the specified value of SCR. Higher value di/dt may case hot points in the juns and
the device may damage because of the temperature rise.
3.Gate ratings – The main gate ratings are given below
VFGM – it is the peak forward gate voltage
VRGM – It is the peak reverse gate voltage
IFGM – it is the peak forward gate current
I RGM - It is the peak reverse gate current
PGM – it is the peak gate power
166
PGAV – it is the average gate power
IGT – it is the minimum instantaneous triggering current
IRGs – It is the reverse gate current
VGT – It is gate rigger voltage
TQ – It is the then off time.
T0 = It is the turn on time.
TEMPERATING SATINGS
These are five factors which determine the dissipation within the SCR,. These factors are
i) Forward conduction
ii) Forward leakage power loss
iii) Reverse leakage power less
iv) Gate power loss
v) Turn – ON loss
SCR should be used with proper heat sinks so that their temperature does not exceed
beyond specified value.
V-I CHARACTERISTICS OF AM SCR
A graph b/w anode – cathode voltage (v) and anode current (I) at constant gate current is
called V-I characteristics of an SCR.
167
The voltage applied b/w anode and cathode (V) of SCR is plotted on X axis. This sit he
voltage supplied the power source.
Forward characteristics.
The curve b/w V and I when forward voltage applied is known as forward characteristics
of an SCR.
When gate current is zero.
As power supply voltage is increased from zero there is a small leakage current which
flows through anode. This is due to leakage of minority carries through junction is. This
leakage current also flours through load. Its value is extremely small ( μ A) and remains
fairly constant as the voltage is increased. On other words, the SCR is said to be in OFF
state. This small leakage current does not affect the load at all and for all practical
purposes the load is considered to be off.
168
Now when the power supply voltage, which is equal to voltage drop across anode an
cathode of SCR reaches the forward break over voltage, SCR at once begin to conduct
and the forward V across thyristor suddenly drops as shown by dotted curve AB. Full
conduction is reached rapidly. Under such conditions, most of power supply voltage
appears across the load, as drop across thyristor is hardly V as compared to several
hundred volts across load. The SCR now acts as a truly closed switch allowing power to
flow through the load circuit. It will continue to flow as long as holding current for
thyristor is maintained. The forward break over voltage is the critical factor here is
turning thyristor in the ON state.
ii)WHEN GATE CURRENT FLOWS
The above consideration apply for zero gate current ie when gate circuit is open when a
small gate voltage is applied the gate current starts flowing. the flow of gate current helps
in making current flow across junction I2 ie, is breaking it down. Due to high positive
potential on the anode side of this junction electron in n-type material are virtually pulled
into p-type lly holes in p type are pulled across J2 into n-type material by the negative
potential on right side of J2. In other words it is clear that break over voltage will
decrease with the increase, in gate current Ig. Reverse characteristics- when anode is
connected to negative ad cathode to +ve of power supply, the curve b/w V and I is called
reven characteristics. In this case, junction J1, and J3 reverse biased while J3\2 is forward
biased. IT is clear that by increasing the reverse voltage beyond a certain value (punt D)
will result in the break down of the junction J1, J2 and thyristor can no longer block the
flow of current through it. However this voltage is very high as compared to the forward
voltage.
169
TWO TRANSISTOR MODEL OF A THYRISTOR
The principle of thyristor operation can be explained with the help of its two transistor
model. Fig (a) shows schematic diagram of a thyristor. From thin fig, two transistor
model is obtained bisecting the two middle layer, along the dotted line, in two separate
levels as in (b). In this fig junctions J1, - J2 and J2 – J3 can be considered to constitute
pnp and npn transistor separately. The circuit representation of the two – transistor
model of a thyristor is shown lin (c)
A
A
A Ia
Ia
p
G
J1
J1
p n
∞11B1
p
n
n
J2
n
p
p J3
J2 p
n
J2
p J3
IB1 = Ic2
Q1
Ic2
p
Iy
Q2 n
IB2
∞21B2
G
n
n
K
n
IK
K
In the off state of a transistor, collector current Ic is related to emitter current IE as Ic = ∞
IE + ICBO.
Where α is the common base current gain and ICBO is the common base leakage current
of collector – base jns of transistor.
170
For transistor Q1 in current IE = anode current Ia and Ic – collector current Ic1. There
fore for Q1.
IC1 = α1 Ia + ICBO1
α1 = common base current gain of Q1.
ICBO1 = common base leakage current of Q1.
for transistor Q2 the collector current Ic2 is given by
IC2 = αIK + ICBO2
α2 = common – base current gain of θ2.
ICBO 2 = common – base leakage current of Q2.
IK = emitter current of θ2.
The sum of two collector currents given by above eqns is equal to the external circuit
current IA entering at anode termials A.
Ia = Ic1 + c
Ia = ∞1 Ia + IcBo1 + ∞2 IK + IcBo2
When gate current is applied, then Ik = Ia + Ig. Substituting thin value of Ik is above eqn.
Ia = ∞1 Ia + ICBO1 + ∞2 (Ia + Ig) + ICbo2.
171
Ia = ∞2Ig + ICBO1 + ICBO2
1 – (∞1 + ∞2)
For a silicon transistor, current gain ∞ is very low3 at low emitter current with an
increase in emitter current, ∞ builds up rapidly. With gate current Ig = 0 and with
thyristor forward biased (∞1 + ∞2) is very low as per eqn and forward leakage current
some what more than (ICBO1+ICBO2) flows. If by some means, the emitter current of
two component transistor can be increased so that (∞1 + ∞2) approaches unity, then as
per eqn Ia would tend to become infinity thereby turning on the device. Actually external
load limits the anode current to a safe value after the thyristor beings conduction. The
methods of tuning on a thyristor are the methods of making ∞1 + ∞2 to approach unity.
SWITCHING CHARACTERISTICS OF THYRISTORS
The switching or dynamic or transient characteristics of SCR or thyristor is discussed
below. During turn on and turn of processes, a thyristor is subjected to different voltage
across it and different currents through it. The time variation of the voltage across a
thyristor and the current through it during turn on and turn off processes give the dynamic
or switching characteristics of a thyristor.
SWITCHING CHARACTERSTICS DURING TURNS ON
A forward biased thyristor is usually turned on by applying a positive gate voltage
between gate and cathode so there is a transition time from for ward off state to forward
on state. This transition time called thyristor turn on time is defined as the time during
which it changes from forward blocking state to final on state. This turn on time is
divided into three intervals (1) delay timely (2) rise times (3) spread time (tp)
172
(1) DELAY TIME (tp)
Td is measured from the instant at which gate current reaches 0.9 Ig to the instant at
which anode current reaches 0.1 where Ig is the final value of the gate current and Ia is
the final value of the anode current. The delay time may also be defined as the time
during which anode voltage falls from Va to 0.9 Va where Va is the initial value of anode
voltage. It si also defined as time during which anode current rises from for ward leakage
current to 0.1 Ia where Ia is the final value of anode current.
The delay time can be decreased by applying high current and more forward voltage
between anode and cathode. The delay time is a fraction of microsecond.
173
RISE TIME (tr)
The rise time tr is the time taken by the anode current to rise from 0.1 to 0.9 Ia or it is
time required for forward voltage to fall from 0.9 Va to 0.1 Va.
SPREAD TIME (tp)
It is the time taken by the anode current to rise from 0.9 to Ia.
174
(or) it is the time taken for the forward blocking voltage to fall from 0.1 Va to the on state
voltage drop. (1 to 1.5 v) Total turn on time of am SCR in equal to the sum of delay time,
rise time and spread time.
During turn on SCR may be considered to be a charge controlled device. A certain
amount of charge must be injected into the gate region for the thyristor conduction to
begin. This charge is directly proportional to the value of gate current. There for higher
the magnitude of gate current the lesser time it taken ti inject their charge. The turn on
time can there fore be reduced be using higher value of gate current. When gate current is
several times higher than the minimum gate current required a thyristor is said to be hard
fired or over driven. Hard firing or over driving of a thyristor reduces its turn on time
and enhances its di/dt capability. )
DYNAMIC CHARACTERISTICS DURING TURN OFF PROCESS
SCR horn off or commutation means that it has changed from ON to OFF state and is
capable of blocking the forward voltage.
Once the thyristor starts conducting gate has no control on the device.SCR can be turned
off by reducing the anode current below a level called holding current for a sufficiently
long time. During that time all the excess charge carriers in the four layer are recombined
and a depletion layer is developed across the junction J2 and thyristor is able to with
stand a forward voltage.
Turn off time is the time during which the charge present in the silicon structure decays
to a level near its equilibrium off state. This process is not instantaneous it takes a
definite amount of time.
175
The SCR can be trussed off by reducing the anode current below holding current. If for
ward voltage is applied to the SCR at the moment its anode current falls to zero, the
device will not be able to block this for ward voltage as the charge carrier in the four
layer are shill favorable for conduction. The device will there fore go into conduction
immediately even through gate signal is not applied.
In order to avoid such as occurrence, it si essential that the thyristor is reverse biased for a
finite period after the anode current has reached zero. This is forced communication.
176
TURN OFF CHARACTERISTICS OF SCR
Turns off time tq of a SCR is defined as the time between the instant anode current
become zero and the instant SCR regains forward blocking capability. Turns off time in
divided into 2 in travels.
177
Tq = trr + tgr. Where trr is the reverse recovery time and tgr is the gate recovery time. At
instant t1, anode current become zero, after t1, anode current builds up in the reverse
direction with the same di/dt slope as before t1. The reason for reversal of anode current
after t1, is due to the presence of carrier slotted in the four layer.
Reverse , recovery during this time the reverse current removes the excess carrier from
the and junctions J1, and J3 between the instant t1 and t3.
Revere recovery current flows due to the sweeping out of holes from top layer and
electrons from bottom n – layer.
At instant t2 when about 60% of the stored charges are removed from the outer 2 layer.
Carrier density across J1, and J3 begin to decrease and with this, reverse recovery current
also starts decaying. The reverse current de cay is fast is the beginning but gradual there
after. The fast decay of recovery current cases a reverse voltage across the device due to
circuit inductance. This is the reverse voltage surge which appear across the thyristor
terminals and may therefore damage it (To avoid this RC element is used for protection
across the SCR)
At instant t3 when reverse recovery current has fallen to nearly zero value end junction J1
and J3 recovers and SCR is able to block the reverse voltage.
178
GATE RECOVERY TIME (Tgr) : During this the rapped charge carriers in J2 is
eliminated by recombination. This is possible if a reverse voltage is maintained across
SCR. There fore junction J2 is forward biased and some time is required to recombine
these charge carriers. This time is called gate recovery time (tgr.) Tgr is measured b/w t3
and t3. At instant t4 a depletion layer is developed across J2 and a forward voltage can be
applied b/w anode & cathode.
At instant t4 a depletion layer is developed across J2 and a forward voltage can be
applied b/w anode and cathode.
Thyristor turn off time tw is in the range of 30 to 150 μsec.
The thus off time directly depend on the magnitude of forward current di/dt at the time of
communication and junction temperature increase in these factors increase turns off time.
Turns off time decrease with an increase is the magnitude of reverse voltage particularly
in the range to ) to 50V. This is because a high reverse voltage across the thyristor
sweeps away all excess majority charge carriers away from the outer o region and n
region and also recombination of charge carrier across the junction J2 made fast.
Tq – applicable to an individual SCR.
Tc – applicable to power circuit containing more SCRC
In case of more SCRS, then off time provided to the SCR by practical circuit is called
circuit their off time tc. It is defined as the time b/w the instant anode current become
zero and the instant revere voltage due to practical circuit reaches zero. This tc must be
greater than tq for reliable turn off.
179
Thyristor with slow turn of time (50-100 m sc) are called converter grade SCRs and those
with fast turn off time (3 – 50 y sec) are called inverter grade SCRs.
POWER DIODES
Power diode is a two layer, two terminal p-n semiconductor device. TI ahs one pn –
junction formed by alloying diffusing or epitaxial growth. The two terminals of diode are
called anode and cathode.
DIODE V-I CHARACTERISTICS
When anode is positive with respect to cathode, diode is said to be forward biased. With
increase of the source voltage Vs from zero value, initially diode current is zero. From Vs
= O to cur in voltage, the forward diode current is very small. Cut in voltage is also
known as threshold voltage or turn on voltage. Beyond cut in voltage, the diode current
rises rapidly and the diode is said to conduct. For silicon diode, the cut in voltage is
around 0.7b. When diode conducts, there is forward voltage drop of the order of 0.8 to
IV.
Anode
cathode
p
I
n
I
A
C
V
Reverse
leakage
Current
RRM
i
Vs
Forward voltage drop
Vs
wt in voltage
Vs
Reverse
break
down
When cathode is positive w.r.t anode, the diode is said to be reverse biased. In the
reverse biased condition of the diode a small reverse current called leakage current, of the
180
order of micoramperes or miliamperes flows. The leakage current increases slowly with
the reuses voltage until break down or avalanche voltage is reached. A breakdown
voltage diode is turned on in the reversed direction. If current in the reversed direction in
not limited by a series resistance, the current will become quite high to destroy the diode.
The reverse avalanche break down of a diode is avoided by operating the diode below
specified peak repetitive reverse voltage VRRM. Diode manufactures also indicate the
value of peak inverse voltage of a diode. this is the largest reverse voltage the diode may
be subjected to during its working.
HIGH FREQUENCY EDDY CURRENT HEATING
For heating an article by eddy – current it is placed inside a high frequency ac current
carrying coil .The alternating magnetic field produced by coil sets up eddy currents in the
article which gets heated up. Such a coil is known as heater coil or work coil and the
material to be heated in known as charge or load. It is the eddy current loss which
produces the heat. The eddy current loss We α B2 F2. This can be controlled by
controlling flux density B and supply frequency f. This loss is greatest on the surface of
the materials but decreases as we go deep inside.
ADVANTAGES OF EDDY CURRENT HEATING
1) Negligible wastage of heat because heat is produced in the body to be heated
2) It can take place in vacuum or other special environments.
3) Heat can be made to penetrate any depth of he body by selecting proper supply
frequency
APPLICATIONS
1)surface hardening 2) anealing 3) soldering
181
INDUCTION HEATING
This heating process makes use of the currents induced by the electro magnetic action in
the charge to be heated. This is based on the principle of transformer working. The 1 0
wdg which is supplied from an ac source is magnetically coupled to the charge which
acts as a short circuited secondary of single turn. When an ac is applied to 10, it induces
voltage in the 20 ie, charge. The 20 current heats up the charge in the same way as any
electric current does while passing through a resistance.
If V is the voltage induced in the charge and R is the charge resistance, then heat
produced = V2/R. The value of current induced in the charge depends on 1) magnitude of
current 2) turn ratio of the transfer 3) coefficient of magnetic coupling.
Low frequency induction furnaces employed for the meeting of metals are of two types 1)
core type furnace 2) coreless type furnace
a) The core type induction furnace
In this furnace, melting is rapid and clean and temperature can be controlled
easily. Inherent stirring action of the charge by electromagnetic forces ensures
greater uniformity of the end product.
b) Vertical core type induction furnace
This furnace is widely used or melting and refining of brakes and other non
ferrous metals. It is suitable for continuous operation. It has a pf of 0.8 – 0.85 with
normal frequency its efficiency is about 75% and its standard size varies from 60300kw, all signal phase.
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DI-ELECTRIC HEATING
It is also called high frequency capacitive heating and is used for heating insulator like
wood, plastics and ceramics etc. Which cannot be heated easily and uniformly by other
methods.
The supply frequency required is b/w 10-50 Mhz and applied voltage is up to 20kv. The
overall η of this process is 50 %
When a practical capacitor is connected across an ac supply it draws a current which
leads the voltage by an angle φ, which is a little less than 900 or falls short of 900 by an
angle. It means that there is a certain component of the current which is in phase with the
voltage and hence produces some loss called dielectic loss. At normal frequencies this is
very low but at freq. of 50 MHz or so, this loss becomes so large that it is sufficient to
heat dielectric in which it takes place. the insulating material to be heated is placed
between two conducting plates in order to forms a parallel plate capacitor.
Slab to be heated
ADVANTAGES OF DIELECTRIC HEATING
1. Uniform heating
2. Heating becomes fast with increasing frequency
3. Only method for heating bad conductor of heat
4. In flammable articles like plastics an wooden products etc. can be heated safely
5. Heating can be stopped immediately when desired.
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Application : gluing of multilayer plywood boards 2) For preheating of plastic
compounds 3) for baking of biscuits and cakes 4) for dehydration of food which is then
sealed in airtight container. 5) for quick drying of glue used for book binding purposes.
CORELESS INDUCTION FURNACE
The three main parts of the faunae are 1) the primary coil 2) a ceramic crucible
containing charge which forms the secondary and the frame which includes supports and
tilting mechanism. The furnace contains no heavy iron core with the result that there is no
continuous path for the magnetic flux. The crucible and the coil are relatively light in
construction can be conveniently tilted for pouring.
Crucible
Primary wdgs
charge
Such furnaces are used for steel production and for melting of non ferrous metals like
brass, bronze, copper an aluminum along with various alloys of these elements. Special
application include vacuum meting in a controlled atmosphere and melting for precision
casting where high frequency heating is used. It has wide use is electronic industry and
also in soldering brazing hardening and annealing and sterlizing surgical instruments etc.
ADVANTAGES ARE : 1) fast is operation 2) produce most inform quality of product.
3) an be operated intermittently 4) operation is free from smoke, dirt 5) used for all
industrial application requiring heating and meting 6) Have low operating costs 7)
charging and pouring is simple.
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RESISTANCE HEATING
It is based on the I2 R effect. When current is passed though a resistance elements I2R
loss takes place which produces heat. There are 2 methods of resistance heating
a) Direct resistance heating 2) Indirect resistance heating
b) Direct resistance heating
Electric supply
Electrode high resistance powder
Furnace
charge
The materials to be heated is treated as a resistance and current is passed through it. The
charge may be in form of power, small solid pieces or liquid. The 2 electrodes are
inserted in the charge and commented either to ac dc supply. For 1φ or dc, 2 electrodes
and for 3φ, 3 electrodes are required. When the charge is in the form of small pieces, a
power of high resistivity material is sprinkled over the surface of the charge to avoid
direct short circuit. Heat is produced when current passes through it. This method of
heating has high efficiency because the heat is produced in the charge itself.
B)IN DIRECT RESISTANCE HEATING
In this method of heating electric current is passed through a resistance element which is
placed in an electric oven. Heat produced is proportional to I2R losses in the heating
element. The heat produced is delivered to the charge either by radiation or connection or
by a combination of two.
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Electric
Supply
cylinder
Heating
Element
Charge
Jacket
Some time, resistance is placed in cylinder which is surrounded by the charge placed in
the jacket. This arrangement provides uniform temperature. Moreover, automatic
temperature control can also be provided.
ELECTRIC WELDING
It is the process of joining two pieces of metal of same type by it to a plastic stage by
heating and then fused together. The different types of electric welding are as follows.
Electric welding
Resistance welding
Bult
flash spot
Butt
Projection
ARC welding
Steam Energy
storage
metal carbon
are
are
Atomic Helium
hydrogen
or
Argon
arc
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Resistance welding
A heavy current is passed through the joint to be welded. The resistance of the joint
generates heat and causes the metal to melt and fusion at the point of contact. Different
types of resistance welding are as follows.
BUTT WELDING
Rods, pipes and wires are wedded this way. One part is held in a fixed clamp and the
other is moved in a moving clamp. Both parts are pressed together and a heavy current is
passed from the welding transformer.
Butt welding
FLASH BUTT WELDING
In this case the two different pieces to be welded are kept with a small gap between.
When voltage is applied arcing occurs and due to this the unevenness is removed and
when the metal attains sufficient temperature they are pressed together. The two pieces
are joined together with a fin around which is easily removed by filing.
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SPOT WELDING
It is applied to welding of sheets. Welding is carried out at certain points and used for
plate. Plate to be welded are placed over lapping each other, between two electrodes.
Welding current flows through the tip of the electrode producing spot weld. The electrode
tips are made of copper or copper alloy and are water cooled. Welding current depends
on the size of the plates are varies from 1000 to 10,000 A. current flows only for a
fraction of a second to a few seconds.
Electrode
PROEJCTION WELDING
It is a modified form of spot welding. One of the plate is having special projection and
welding occurs at these points. Plate electrodes are used.
STEAM WELDING
Wheel or roller type electrodes are used. The plates to be welded are placed together in
between the two roller which presses and a continues weld in obtained.
ENERGY STORAGE WELDING PROCESS
Metals like aluminium, magnesium etc. require a very high current for short interval
which will cause the following problems.
1.This will cause a fluctuation in the supply system.
2. If the welding interval is shortened, the energy supplied is reduced and then the quality
of weld will be poor.
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In energy storage system energy storage system energy is stored from the line and
released suddenly at the time of welding aluminium, magnesium and such other metal
have the following properties.
1. Since specific resistance is less, very high current is resulted.
2. Thermal conductivity is high and hence the temperature produced at the contact
point conduct away.
3. On melting, they become very thin liquid and runs off.
4. At low temperature they stick to electrodes having copper.
The solution is that welding should be done at high pressure in a very short time and
using very high current.
There are two types
A. Capacitor discharge welding
Capacitor bank of 2000 to 3000 mf is used for energy storage and is changed of
3000 V from a grid controlled rectifier.
Welding transformer
3d Ac supply
3d guid controlled
Rectifier
dc
+
-
The no load voltage of the rectifier is 6000V. An automatic system cuts off the capacitor
from rectifier when it is charged to 3000V through an ignitron switch the capacitor
discharged and thus high transient current will be produced in the secondary to weld the
material. In the arc no residual flux should be present.
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MAGNETIC ENERGY STORAGE WELDING
Contractor coil
3φ Ac supply
Wedding transformer
3φ grid controlled
Rectifier
dc
In this type energy is stored in the magnetic field of the transformer and is equal to ½ Li2.
Dc voltage is controlled so that the current in the 10 increases gradually and avoids
preheating. Preheating causes deformation when sufficient energy has been stored the
contactor opened and flux collapses and high current induced in the secondary. The KVA
demand on the line is more in magnetic energy storage welding than in capacitor
discharge system. But high voltage rectifier and expensive capacitor bank are eliminated
and hence low initial cost.
RESISTANCE WELDING EQUIPMENT
It consists of welding transformer, electrodes and control equipments for controlling
pressure, temperature and time. The pressure control may be pedal type. It manually
operated very experienced operator is required. Controls of three types 1) constant time
2) current activated 3) energy actuated modern practice is to apply high current for short
duration, may be 1/100 sec to 1/10 sec. Cosntant time can be achieved by a mechanical
switch operated by a cam. Welds upto 300 per minute is possible. If more is required
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electronic control is necessary using grid controlled ignitrons
When the tubes conduct, it will be available at the welding transformer. When it is not
conducting voltage drop will be high and hence no welding current. By suitable control
ckt, starting and stopping of welding can be controlled.
ELECTRONIC RESISTANCE WELDING UNIT
Heat controller.
Timer
Power
circuit
SCR
Transformer
breaker
Contractor
Relay
Weld
Transformer
Electronic controls in resistance welding make use of SCR contractors which allow the
welding current to flow through the work pieces not only in precisely controlled
magnitude but also for a definite instant this result in perfect welding of sheets.
Sophisticated equipments like bodies of air planes, automobiles etc. need perfect welding
which can’t be controlled manually.
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