Download UNIT – VI UNIT-6: SPECIAL MACHINES Explain the constructional

UNIT – VI
UNIT-6: SPECIAL MACHINES
1. Explain the constructional features and principle operation of a single phase induction motor.
(OR)
Why the Single Phase Induction Motor is not SELF STARTING?
(OR)
Show that a single phase winding when excited by a single phase supply produce two equal and opposite
revolving fields.
ANS:
CONSTRUCTIONAL FEATURES OF 1- PHASE INDUCTION MOTOR
Constructionally, a 1- Ф induction motor is more or less similar to that of a 3-Ф induction
motor except that its stator is provided with a single phase winding and a starting mechanism
like centrifugal switch or relay etc.
Rotor: The rotor construction is identical to that of a 3- Ф squirrel cage induction motor. It
consists of a laminated core with skewed slots on periphery. The conductors and end rings are
formed by forcing the molten material (copper or aluminum) into the slots. In fact the rotor of
any 1- Ф induction motor is inter-changeable with that of a poly phase induction motor.
Stator: Its core construction is similar to the stator core of
3-phase induction motor. The stator slots are distributed
uniformly and usually 1- Ф double layer winding is
employed. A simple single phase winding would produce no
rotating magnetic field and no starting torque. It is,
therefore, necessary to modify or split the stator winding
into two parts, such as running winding or main winding
and starting wind or auxiliary winding, each displaced in
space by 900 on the stator to make the motor self starting.
The below figure shows the construction of a single phase
induction motor.
OPERATING PRINCIPLE OF 1-Ф INDUCTION MOTOR
Constructionally this motor is similar to poly-phase induction motor except that
(i) its stator is provided with a single phase winding
(ii) a centrifugal switch is used in some types of motors, in order to cut out a winding only
used for starting purpose.
A single-phase induction motor consists of a single phase winding mounted on the stator and a
squirrel cage winding on the rotor as shown in the above fig.The rotor bars are short circuited
by end rings. When the stator is energized with the single phase supply, it produces an
alternating flux and this flux cuts the rotor conductors. During the positive half cycle, the flux
induces a voltage in the rotor and the resulting current produces a torque. The rotor tends to
rotate in one direction. During the negative half cycle, the torque produced tends to rotate the
rotor in opposite direction. Thus the rotor is subjected to these rapid flux reversals and due to
its inertia it cannot move. This type of torque is known as pulsating torque. That is why the
single phase induction motor is not self starting
• This peculiar behavior is explained in two ways:
– Double field revolving theory.
– Cross field theory.
DOUBLE FIELD REVOLVING THEORY:
• This theory makes use of the idea that an
alternating
uni-axial
quantity
can
be
represented by two oppositely rotating vectors
of half magnitude.
• Accordingly, an alternating sinusoidal flux
can be represented by two revolving fluxes,
each equal to half the value of the alternating
flux and each rotating synchronously in
opposite direction
• Let the alternating flux have a maximum
value Φm
• Its component fluxes A & B will each be
equal to Φm/2 revolving in clockwise and
anticlockwise directions respectively.
• After some time, when A & B would have
rotated through an angle +θ and –θ, the resultant flux would be ΦmCOSθ.
• After a quarter cycle of rotation fluxes A and B will be oppositely
directed, so resultant flux would be zero. • After half a cycle, fluxes
A and B will have a resultant of -2 * Φm/2 = - Φm • After three
quarters of a cycle, again the resultant is zero and so on. • If we plot
the resultant flux against θ between limits θ = 00 to θ = 3600, then
the following curve is obtained.
• SLIP : It may be noted that if the slip of the rotor is s w.r.t. the
forward rotating flux ( i.e. one which rotates in the same direction of
the rotor), then its slip w.r.t backward rotating flux is (2-s).
• Slip w.r.t. forward rotating flux s = (Ns-N) / Ns
i.e N / Ns = (1-s)
• Slip w.r.t. backward rotating flux S = (Ns – (-N)) / Ns
= 1 + (N / Ns) = 1 + (1 – s ) = ( 2 – s )
• Each of the two component fluxes,
while revolving round the stator, cuts
the rotor, induces an emf and this
produces its own torque.
• Obviously the two torques (Forward
& backward) are oppositely directed,
so that the net or resultant torque is
equal to their difference.
• Figure shows both torques and
resultant torques for slips between 0
and 2.
• At standstill s=1 and (2-s)=1. •
Hence Tf and Tb are numerically
equal but, being oppositely directed,
produce no resultant torque.
• That explains why there is no starting torque in a single phase induction motor.
• However if the rotor is somehow started in clockwise direction, the clockwise torque starts
increasing at the same time, the anti-clockwise torque starts decreasing, net torque in
clockwise increases which accelerates the motor to full speed.
CROSS FIELD THEORY
Standstill condition of Motor
• The stator winding is excited by a single phase supply.
• This supply produces an alternating flux Φs which acts along the
axis of stator winding.
• Due to this flux, emf gets induced in rotor conductors due to
transformer action.
• As rotor is a closed one, this emf circulates current through rotor
conductors.
• The direction of rotor current is so as to oppose the cause
producing it as shown in figure.
• When Φs acts in upward direction, the conductors on left
experience force from left to right, while conductors on right
experience force from right to left.
• Hence overall force, torque is zero. Rotor is stationary.
Condition for rotation
• We know that there must exist two fluxes separated by some angle in space so as to produce
rotating magnetic field.
• According to Cross Field theory, the stator flux can be resolved
into two components which are mutually perpendicular, one
acting along the axis of stator winding and another acting
perpendicular to it. How to rotate?
• Assume an initial push is given. (AntiClockwise)
• Due to rotation, rotor physically cuts the stator flux and
dynamically induced emf gets induced in the rotor.
• The direction of this emf (E2N) is in phase with stator flux Φs.
• This circulates a current through rotor which is I2N. This
current produces its own flux Φr.
• The axis of Φr is at 900 to the axis of stator flux. Hence this
rotor flux is called CROSS FIELD.
• Due to high reactance of rotor, the rotor currentI2N and flux Φr lags the emf by 900
• Thus Φr is in quadrature with Φs in space and lags Φs by 900 in time phase. Hence the above
two such fluxes produce RMF.
2. (a) Explain how to make 1-Ф IM Self Start.
(b) Explain the STARTING Methods of single phase Induction motor with vector diagram.
ANS:
(a) MAKING 1-PHASE INDUCTION MOTOR SELF STARTING
We know that a single phase induction motor is not self
starting. To overcome this draw back and make the motor
self starting it is temporarily converted into a two phase
motor during starting period. For this purpose the stator of
a single phase motor is provided with an extra winding
known as starting or auxiliary winding in addition to the
main or running winding. The windings are spaced 900
electrically apart and are connected in parallel across the
single phase supply as shown in fig.
The impedance of the windings differ and currents in the main and auxiliary windings are
phase shifted from each other. As a result of this, a rotating stator field is produced and the
rotor rotates. When the motor speed is about 75% of synchronous speed, the auxiliary winding
disconnected from the circuit. This is done by connecting a centrifugal switch in the auxiliary
winding, which is used for starting purpose only. That is why it is called starting winding.
Under running condition, a single phase induction motor can develop torque only with main
winding. That is why it is called running winding.
(b) STARTING METHODS OR TYPES OF 1- PHASE INDUCTION MOTORS
We have seen that some means should be used to start the single phase induction motor.
Mechanical methods are impractical and, therefore, the motor is started temporarily converting
it into two-phase motor. Single phase induction motors are usually classified according to the
auxiliary means used to start the motor. They are classified as follows.
1. Split phase motor (or) Resistance start motor
2. Capacitor start motor
3. Capacitor run motor
4. Capacitor start capacitor run or two-value capacitor motor
5. Shaded pole motor
SPLIT PHASE OR RESISTANCE START-MOTOR
This fig shows a split phase or resistance start induction motor. It has a squirrel cage rotor and
its stator has two windings-a main or running winding and a auxiliary or starting winding. The
two windings are displaced 90o in space and are connected in parallel across the 1-Ф supply as
shown in the fig.
The centrifugal switch will be in its closed position when the machine is at stand still. Once the
motor picks up 70% to 80% of its rated speed, it opens its contacts. The main winding will have
high reactance and low resistance where as the auxiliary winding will have high resistance and
low reactance. At stand still both the windings are connected in parallel across the supply.
When the supply is given, the currents flow through the main winding (IM) and auxiliary
winding (IA) and there will be a phase displacement Ф (< 900) between these two currents [as
shown in fig.] and hence the resultant magnetic field is rotating, which produces a Torque in
the rotor. If the auxiliary winding is permanently connected in the circuit, it causes power
losses because of its highly resistive nature. Hence a centrifugal switch (SC) is used to
disconnect the auxiliary winding from the circuit when the motor picks up speed. The torquespeed characteristic of this motor is shown in fig, which also shows the speed ‘N0’ at which the
centrifugal switch operates.
In order to provide high resistance and low inductance the starting winding is wound with
fewer turns of the fine wire and placed on the top of the stator slots. The main winding, on the
other hand is wound with thicker wire and placed at the bottom of the stator slots. The idea
behind it is that the current through the starting winding is nearly in phase with the line
voltage than is that of through the main winding. This gives the effect of splitting the single
phase applied to the motor terminals.
For motors rated about 100W or more, a centrifugally operated switch is used to disconnect the
starting winding. For smaller motors a relay is often used. The relay is connected in series with
the main winding. At the time of starting, a heavy current flows in the relay coil causing its
contacts to close. This brings the starting winding into the circuit. As the motor reaches its
predetermined speed of the order of 70% to 80% of synchronous speed, the current through
the relay coil decreases. Consequently the relay opens and disconnects the auxiliary winding
from the main supply and the motor then runs only on the main winding.
Characteristics:
Resistance split phase motor has the following characteristics
 The starting torque is 100% to 250% of the rated value.
 The breakdown torque is upto 300%.
 The efficiency of the motor is 55% to 65%.
 The power factor of the motor is 0.5 to 0.65.
 The power rating of this motor is in the range of ½ HP to 1 HP.
Applications:
Split-phase motors are cheap and they are most suitable for easily started loads where
frequency of starting is limited. The common applications are washing machines, air
conditioning fans, food mixers, grinders, centrifugal pumps, lathes, small drills etc.
CAPACITOR START MOTOR
In split-phase motor, high auxiliary winding circuit resistance creates phase angle between the
currents of the main and auxiliary windings. This phase difference between I A and IM can also
be produced by connecting a capacitor in series with auxiliary winding. Fig. shows the
schematic diagram of a capacitor start motor. It has a cage rotor and its stator has two
windings namely, the main winding and the auxiliary winding or starting winding. The two
windings are displaced 900 in space. A capacitor C is connected in series with auxiliary
winding. A centrifugal switch SC is also connected, as shown in fig. In this case both the
windings may be of similar
nature.
When the motor is switched on to the supply mains, current IM drawn by the main winding
lags the supply voltage V by a large angle, where as, IA drawn by the auxiliary winding leads V
by a certain angle. The two currents IM and IA are out of phase with each other by nearly 800
to 900 as compared to nearly 30o for a split-phase motor Their resultant current I is small and
is almost in phase with ‘V’ as shown in fig.1.8(a), and causes the motor to start. Once the
motor has picked up speed, the centrifugal switch or relay operates and opens the auxiliary
winding circuit. The motor is so named because it uses the capacitor only for the purpose of
starting. The starting torque of a capacitor start motor is of the range of 3 to 4 times to its full
load torque where as for a split phase motor it is about 1.5 times only. Typical torque-speed
curves of a capacitor start motor is as shown in fig.
Characteristics:
Capacitor start induction motor has the following characteristics
 The starting torque is 250% to 400% of the rated value.
 The breakdown torque is up to 350%.
 The efficiency of the motor is 55% to 65%.
 The power factor of the motor is 0.5 to 0.65.
 The power rating of this motor is in the range of 1/8 HP to 1 HP.
Applications:
Capacitor start motors are used for loads of higher inertia where frequent starts are required.
These motors are most suitable for pumps and compressors, and therefore they are widely
used in refrigeration and in air-conditioner compressors. They are also used for conveyors and
some machine tools.
CAPACITOR RUN OR PERMANENT SPLIT CAPACITOR MOTOR
This motor is similar to the capacitor start motor except that the starting winding and the
capacitor are connected in the circuit at all times. It has one running winding and one starting
winding in series with a capacitor as shown in fig.7.9. Since capacitor remains in the circuit
permanently, this motor is known as permanent split capacitor run motor and behaves
practically like an unbalanced 2-phase motor. Consequently, it produces a uniform torque. The
motor is therefore less noisy during operations. Since the capacitor is always in the circuit, this
type of motor has no starting switch.
In this motors, the capacitor used is designed for continuous
duty and is of oil filled type. Since the same capacitor is
used for starting and running, it is obvious that neither
optimum starting nor optimum running performance can be
obtained because value of capacitance used must be a
compromise between the best value for starting and that for
running. One unique feature of this type of motor is that it
can be easily reversed by an external switch provided its
starting and running windings are identical. One serves as
the running winding and the other as the starting winding
for one revolution of rotation. For reverse rotation, the one
that previously served as a running winding becomes the starting winding and while the former
starting winding serves as the running winding.
Advantages:
A single value capacitor motor possesses the following advantages
 No centrifugal switch is required.
 It has high efficiency.
 It has higher power factor because of permanently connected capacitor.
 It has a higher pull-out torque.
Characteristics:
A single value capacitor motor has the following characteristics
 The starting torque is 50% to 100% of the rated value.
 The breakdown torque is up to 250%.
 The efficiency of the motor is 60% to 70%.
 The power factor of the motor is 0.75 to 0.9.
 The power rating of this motor is in the range of 1/8 HP to 1 HP.
Applications:
The low value of the capacitor results in small staring torque which is about 50 to 100% of the
rated torque. Consequently these motors are used where the required starting torque is low
such as air moving equipments i.e fans, blowers, oil burners and voltage regulators. They are
also used to drive office machinery.
CAPACITOR START CAPACITOR RUN OR TWO VALUE CAPACITOR MOTOR
Fig. shows the schematic diagram of a capacitor start
capacitor run or two value capacitor motor. It has a
squirrel cage rotor and its stator has two windings
namely the main winding and the auxiliary winding.
The two windings are displaced 900 in space. The motor
uses two capacitors CS and CR. One capacitor CS is used
for starting purpose and another capacitor CR is used
for running purpose. The two capacitors are connected
in parallel at starting. In this motor, we can get high
starting torque because of two capacitors.
In order to obtain a high starting torque, a large current
is required. For this purpose, the capacitive reactance X in
the auxiliary (starting) winding should be low. Since
XS=1/(2π fCS) , the value of CS should be large. The
capacitor CS is short time rated and electrolytic. During
normal operation, the rated line current is smaller than
the starting current. Hence the capacitive reactance
should be large. Since XR=1/(2π fCR) , the value of CR
should be small. It is long time rated for continuous
running and is of oil filled paper construction. As the
motor approaches synchronous speed, the capacitor CS is
disconnected by centrifugal switch SC. The capacitor CR is
permanently connected in the circuit. Two-value capacitor
motors are quiet and smooth running. They have a higher efficiency than the motors that run
on the main windings alone. The torque-speed characteristics of a two-value capacitor motor is
shown in fig.
Advantages:
A capacitor start capacitor run induction motor possesses the following advantages
 High starting torque
 High efficiency
 High power factor
Characteristics:
A capacitor start capacitor run induction motor has the following characteristics
 The starting torque is 200% to 300% of the rated value.
 The breakdown torque is up to 250%.
 The efficiency of the motor is 60% to 70%.
 The power factor of the motor is 0.75 to 0.9.
 The power rating of this motor is in the range of 1/8 HP to 1 HP.
Applications:
Two value capacitor motors are used for loads of higher inertia requiring frequent starts where
the maximum pull out torque and efficiency required are higher. They are used in pumping
equipment, refrigerators, air compressors etc.
SHADED POLE MOTORS
A shaded-pole motor is a simple type of self self-starting single phase induction motor. It
consists of a stator and a cage type rotor. The
stator is made up of salient poles. Short circuited
coils or shading coils are placed on slot cut
portion of each pole. This part of the pole is
known as shaded part and the other is known as
un-shaded part. Shading coils can be of thick
single turn in the form of a ring or have a number
of short circuit turns. The cross- sectional view of
a shaded pole type motor is as shown in fig.
When an alternating current flows in the field
winding or exciting winding surrounding the
whole pole, an alternating flux is produced in the
field core. A portion of this flux links with the
shading coil, which behaves as a short circuited secondary of a transformer and hence a
voltage is induced in the shading coil, and this voltage circulates a current in it. The induced
current produces a flux called the induced flux which opposes the main core flux. The shading
coil, thus, causes the flux in the shaded portion A to lag behind the un-shaded portion B of the
pole. At the same time, the main flux and the shaded pole flux are displaced in space less than
90o. Since there is time and space displacement between the two fluxes, a rotating magnetic
field is produced. Under the action of rotating flux a staring torque is developed on the rotor.
The direction of the rotating field (flux) is from the un-shaded to the shaded portion of the pole
i.e the direction of the rotation is in clockwise. The movement of the flux around the stator may
be more clearly illustrated in the following discussion.
When the flux is increasing rapidly along OA during the time period t1 as shown in fig.(a), then
it induces large e.m.f and consequently large current in the short circuited shaded band or
shaded part or shaded pole so as to oppose the cause of its production according to Lenz’s law.
Thus the shading band current produces upward increasing flux in the shaded part to oppose
the original flux Φ through it. Due to this opposition, negligible flux passes through shaded
part and most of the flux Ф is concentrated at the un-shaded part. When the flux is increasing
along AB during the time period t2 as shown in fig.(b), the flux Ф has negligible variation. So it
cannot induce any e.m.f and current in the shaded band. As a result there is no opposition to
the flux Ф. Hence it is uniformly distributed over the unshaded and shaded parts of the pole.
This gives the shift of axis of flux Ф towards the centre of the entire pole.
When the flux is decreasing along BC during the time period t3 as shown in fig.(c), it again
induces large e.m.f and current in the shaded band. This current produces flux which aids the
main stator flux. This gives rise to the concentration of flux in the shaded part .i.e. the axis of
the flux shifts towards the center of the shaded part as shown in fig.(c). The above analysis
indicates that the flux Ф rotates through pole from un-shaded part to shaded part during the
half cycle of the stator current. During the negative half cycle, it further rotates through the
next pole in the same direction. In short a rotating magnetic field is produced by shaded pole
construction, which rotates at synchronous speed in the direction from un-shaded part to
shaded part.
A typical torque-speed curve for shaded pole motor is as shown below.
Merits and demerits:
The merits of shaded pole induction motors are
 Rugged construction.
 Cheaper in cost.
 Smaller in size.
 Requires little maintenance
The demerits of shaded pole induction motors are
 Very low starting torque.
 Low efficiency.
 Low power factor.
Characteristics:
The characteristics of shaded pole induction motors are
 The starting torque is 40% to 60% of the rated value.
 The breakdown torque is up to 150%.
 The efficiency of the motor is 25% to 40%.
 The power factor of the motor is 0.25 to 0.4.
 The power rating of the motor ranges up to 40W.
Applications:
These motors are widely used in low torque applications though their efficiency is very low.
Because of its low starting torque the shaded pole motor is generally used for small fans, toys,
hair driers, ventilators, relays and electrical clocks.
3. Write a short notes on AC SERVO MOTOR.
ANS:
BASIC PRINCIPLE OF AC SERVO MOTOR:
A special case of the two-phase motor is the AC servomotor. This is a high-slip, high torque
motor, designed specifically for control systems, and it has a relatively linear torque-speed
curve. The maximum torque occurs when the speed is zero. When the motor is running, the
speed is inversely proportional to the load torque; put another way: the lighter the load, the
faster the motor runs. This is very similar to the way a DC motor behaves.
The two windings are called the main winding
and the control winding. The main winding is
connected to an AC source, usually 120 Vac.
The control winding is driven by an electronic
circuit. Electronic circuit has two functions- (1)
Causes the phase to be either leading or lagging
the main winding (thereby controlling the motor
direction) and (2) sets the magnitude of the
control-winding voltage, which determines the
speed.
Typically, the maximum control winding voltage
is about 35 Vac. If the control winding has 0 V, the motor will coast to a stop, even though the
main winding is still connected to the line voltage. This is different from a normal induction
motor that will continue to run on a single phase.
APPLICATIONS:

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Servo Motor in Solar Tracking System
Robotic Vehicle
Servo Motor Applications AS Camera Auto Focus
Servo Motor in Conveyors
Servo Motor in Robotics