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Unit -5
AC Machines
2 marks
What is the principle of induction motor?
Answer: Principle of induction motor:
An induction motor is such an electromechanical device which converts electrical
energy into a mechanical energy. Principle of induction motor is based on mutual induction.
When a three phase supply is given to the stator conductors a rotating magnetic field is
developed in the stator which is of constant value. When the rotor is placed in this magnetic
field it experiences force by which the rotor starts rotating in the direction of magnetic field.
2. Define Slip and Slip speed in an induction motor?
Answer:
Slip in Induction motor is defined as the ratio of relative speed between magnetic
field and rotor to the synchronous speed and is given as follows
Where,
(rpm)
(rpm)
And
%slip=
Slip speed:
Slip speed is probably defined as the relative speed between rotating magnetic field and rotor speed.
3. What is synchronous speed and write its expression.
Answer:
The speed of magnetic field produced in a three magnetic field is termed as
synchronous speed. The synchronous speed (Ns) of the stator field is directly related to the supply
frequency and the number of (magnetic) poles in the stator and is expressed in revolutions per
minute [rpm]:
Where, f= supply frequency
P=number of stator poles
4. What is the working principle of alternator?
Answer:
The working principle of alternator depends upon Faraday's law of
electromagnetic induction which says the current is induced in the conductor inside a magnetic
field when there is a relative motion between that conductor and the magnetic field.
5. Define Pitch factor and Distribution factor?
Answer:
Pitch factor:
Pole pitch in an Alternator is defined as the distance from centre of one pole to the centre
of the next pole.
Coil span in an Alternator is defined as the distance between two conductors which are
connected end to end.
When the coil span is less than the pole pitch, the coil is said to be short pitch coil and the
factor is termed as Pitch factor.
Distribution factor:
If the windings in the Alternator are confined only to particular slots leaving all the
remaining slots empty, the winding is termed as concentrated winding. If the windings are
distributed uniformly in all the given slots the winding is said to be distributed winding and the
factor is termed as Distribution factor.
3 Marks
What are the losses occurring in an induction motor?
Answer:
The power losses in an Induction motor are as follows
Core or Iron losses (Pi):- These losses occur in the stator and rotor parts of the induction
motor. These are further classified as Stator core loss and Rotor core loss. These losses are
of hysteresis and eddy current losses and occur in the stator and rotor core due to the
alternating flux
When three phase supply is given to the stator winding of an Induction motor, three
phase currents flow in the windings. As a result of this current a constant rotating magnetic
field is produced in the air gap. Due to the reversal of magnetization of the core material,
the loss occurred is termed as Hysteresis loss.
Hysteresis losses = kh f Bm(1.6)
During the operation of the Induction motor small currents flow in the core material
which is termed as eddy currents. The loss in the induction motor because of these currents
is termed as Eddy current loss and it is given
Eddy current losses = ke f2Bm 2t2
ii) Copper losses:The loss in an Induction motor because of the ohmic resistance of both the stator and rotor
windings are termed as Copper losses.
Total Cu losses, Pc = I12R1 + I22R2
Where I12R1 = Stator copper loss
I22R2 = Rotor copper
loss
I1 = Stator current
I2 = Rotor current
R1 = stator resistance
R2 = rotor resistance
These losses can be minimized by designing the winding with low resistance.
iii) Mechanical losses:
An induction motor involves moving parts in conversion electrical energy into mechanical
energy so it has bearings and brushes attached to the shaft. Therefore friction and windage losses
take place which are termed as mechanical losses. These losses can be minimized by using grease
and lubricants in the moving parts.
2. Explain the torque slip characteristics in an induction motor?
Answer:
The torque slip characteristics of an Induction motor is a graph showing the variation of
torque with respect to slip.
At low slips the value of SX2 << R2 so we get the torque as T= 0.159 * sE22/R2. From
this it is clear that torque is directly proportional to slip and graph is a straight line
passing through centre.
At high slips the value SX2 >> R2 so we get the torque as T= 0.159 * E22R2/sX22. From
this it is clear that torque is inversely proportional to slip and the graph is
rectangular hyperbola.
3. Mention any three differences between a transformer and induction motor?
Answer:
TRANSFORMER
INDUCTION MOTOR
The secondary winding .is stationary.
The secondary winding (rotor) is
rotating in nature.
short Rotor winding is always short circuited.
Secondary winding is not
circuited.
No-load current is about 1% of full load. No-load current is approximately 30 to
50% current.
Frequency of primary and secondary Frequency of stator current (f) and rotor
currents are same.
current (sf) are not the same.
4. Compare salient pole and cylindrical pole rotors?
Answer:
Salient pole rotor
Cylindrical rotor
This is also termed as projected poleType rotor where all the poles are projected
Out from the surface of the rotor.
These rotors have large diameters and smaller
axial lengths.
The field winding is provided on the pole shoe.
This is also termed as non-salient pole type rotor
or non-projected pole type rotor.
These rotors have small diameters and large axial
lengths.
The unslotted portions of the cylinder acts as
poles.
As mechanical strength of salient pole rotors is Cylindrical rotor is mechanically strong and
less. It is preferred for low speed applications.
preferred for high speed applications.
5. What is coil and coil span in an alternator?
Answer:
The Alternator consists of windings in the form of single layer and multi-layer in nature.
These windings are wound in such a way that maximum Emf is induced in the conductors.
When one end of the conductor is connected to the other end of the conductor, it is termed as
one turn. These turns in particular number is termed as coil.
Coil span in an Alternator is defined as the distance between one side of the coil to the
other side of the coil.
10 marks
Explain the construction of a 3 phase Alternator?
Answer:
ALTERNATOR:
An alternator is an a.c generator that converts mechanical energy to electrical energy in
the form of alternating current by using Faradays law of electromagnetic induction. It says that
when there change in flux in conductor an Emf is induced at the ends of the conductor.
CONSTRUCTION:
An alternator consists of
1. Stator
2. Rotor
STATOR:
Stator is a stationary part in an Alternator. It consists of core (i.e. silicon steel laminations) and
slots to hold the armature winding and it i s laminated to reduce the eddy current losses.
The choice of material of the core is steel to reduce the hysteresis losses. The entire core is
fabricated in a frame made of cast iron.
The three phase windings are placed in these slots and serves as the armature windings of the
Alternator. The armature windings are always connected in star and the neutral is connected
to ground.
These armature conductors are cut by the magnetic field produced by the rotor poles and an Emf
is induced at the terminals of the windings of the three phase Alternator.
This induced Emf is given by E= Kc Kd 4.44 ϕfTph, where Tph is number of turns per phase.
ROTOR:
The rotor is rotating part of the Alternator. It is like a flywheel having alternate north and south
poles attached to its outer rim. It carries a field winding which is supplied with dc current
through two slip rings by a separate dc source. This dc source is generally a small dc generator
mounted on the shaft of the Alternator.
There are two types of rotors:
1. Salient pole rotor
2. Cylindrical pole rotor.
1. SAILENT POLE ROTOR:
A salient pole is a magnetic pole that is projected out of the rotor surface. So, it is termed as
projected pole type rotor.
Salient type rotor has non-uniform air-gap and two or four poles. These poles are made up of
thick lamination. . This type of rotor consists of large number of projected poles (called
salient poles), bolted on a magnetic wheel.
The poles and pole shoe are also laminated to minimize the eddy current losses.
Alternators featuring this type of rotor are large in diameters and short in axial length. As a
mechanical strength of salient pole rotor is less, it is preferred for low speed applications.
2. CYLINDRICAL POLE ROTOR:
This is also termed as non-salient type rotor. It consists of smooth solid steel cylinder having
number of slots to accommodate the field winding. The unslotted portion of cylinder acts as
poles.
The central polar areas surrounded by the field winding placed in slots.
The field coils are so arranged around these polar areas that flux density is maximum on the
polar central line and gradually falls away on either side.
These Rotors have small diameter and large axial length.
The main advantage of cylindrical rotor is, it is mechanically strong and preferred for high speed
applications.
It is used for steam driven turbo Alternator which run at very high speeds of 3600 rpm.
Derive the EMF equation of an Alternator?
Answer: An alternator is an a.c generator that converts mechanical energy to electrical energy
in the form of alternating current by using Faradays law of electromagnetic induction. It says that
when there change in flux in conductor an Emf is induced at the ends of the conductor.
The Induced EMF in the case of 3 phase alternator is given by Faraday’s Law of Electromagnetic
Induction.
i.e,
E = Induced Emf
Φ
= Flux per pole, in Webbers
P = Number of poles
Ns = Synchronous speed in r.p.m.
f = Frequency of induced e.m.f. in Hz
Z = Total number of conductors
dΦ =change in flux or total flux produced by poles
dΦ = Φ*P
and
dt = change in time
Consider a single conductor placed in a slot. The average value of e.m.f. induced in a conductor
Eavg = dΦ/dt
Total flux in one revolution = Φ * P
Time taken for one revolution = 60/Ns seconds.
Eavg per conductor = ΦP / (60/Ns)
= Φ (PNs/60)
We know that, Ns= (120f/P)
Eavg = (ΦP / 60) * (120f/P)
Eavg = 2Φf.
This is induced EMF in a single turn of a coil
We know that in an Alternator, Number of conductors (Zph) = 2* Number of turns (Tph)
EMF per turn = 2 x (e.m.f. per conductor= 2 x (2 f Φ)
Eavg = 4 f Φ volts
Average Induced EMF in each phase is given by Eavg = Tph
Average Eph = Tph
x (Average e.m.f. per turn)
x4fΦ
The induced Emf is an alternating quantity. The form factor is 1.11 for a sinusoidal waveform.
Form factor = (R.M.S.) values/Average values = 1.11
Erms = 1.11 x Average value
EMF = 1.11 * 4 f Φ Tph
EMF = 4.44 Φ f Tph
volts
volts
EMF =Kc Kd 4.44 f Φ Tph
Kc = Pitch factor
3.
volts
and
Kd = distribution factor
Explain working principle of 3-phase Induction motor?
Answer:
An electrical motor is such an electromechanical device which converts electrical
energy into a mechanical energy. In case of three phase AC operation, most widely used
motor is 3-phase induction motor as this type of motor does not require any starting
mechanism. For better understanding the principle of three phase induction motor, the basic
constructional features of this motor must be known to us. This Motor consists of two major
parts:
Stator: Stator of three phase induction motor is made up of numbers of slots to construct
a 3 phase winding circuit which is connected to 3 phase AC source. The three phase
winding are arranged in such a manner in the slots that they produce a rotating magnetic
field after 3-phase AC supply is given to them.
Rotor: Rotor of three phase induction motor consists of cylindrical laminated core with
parallel slots that can carry conductors. Conductors are heavy copper or aluminum bars
which fits in each slots & they are short circuited by the end rings. The slots are not exactly
made parallel to the axis of the shaft but are slotted a little skewed. This arrangement
reduces magnetic humming noise & can avoid stalling of motor.
Working of Three Phase Induction Motor:
The stator of the motor consists of overlapping winding offset by an electrical angle
of 120°. When the primary winding or the stator is connected to a 3 phase AC source, it
establishes a rotating magnetic field which rotates at the synchronous speed.
Secrets behind the rotation:
According to Faraday’s law an Emf induced in any circuit is due to the rate of change of
magnetic flux linkage through the circuit. As the rotor winding in an induction motor are
either closed through an external resistance or directly shorted by end ring, and cut the
stator rotating magnetic field, an Emf is induced in the rotor copper bar and due to this Emf
a current flows through the rotor conductors. Here the relative speed between the rotating
flux and static rotor conductor is the cause of current generation. Hence as per Lenz's law
the rotor will rotate in the same direction to reduce the cause i.e. the relative velocity.
Thus from the working principle of three phase induction motor it may observed
that the rotor speed should not reach the synchronous speed produced by the stator. If the
speeds equals, there would be no such relative speed, so no Emf induced in the rotor, & no
current would be flowing, and therefore no torque would be generated. Consequently the
rotor cannot reach the synchronous speed. The difference between the stator (synchronous
speed) and rotor speeds is called the slip. The rotation of the magnetic field in an induction
motor has the advantage that no electrical connections need to be made to the rotor.
4. Explain the concept of rotating magnetic field in an induction motor?
Answer:
Rotating magnetic field can be defined as field or flux having constant amplitude but whose
axis rotates in a plane at a certain speed .e.g. Permanent magnet rotating in a space produces a
rotating magnetic field .similarly if an arrangement made to rotate the poles, with constant
excitation supplied, the resulting magnetic field is rotating magnetic field .so a field produces in
the air gap of a rotating field type alternator is of rotating type.
PRODUCTION OF ROTATING MAGNETIC FIELD
Consider a three phase winding displaced in a space by 120°, supplied by a three phase
A.C. supply. The three phase currents are also displaced from each other by 120° and the flux
produced by each phase currents is also sinusoidal in nature and all three flux are separated from
each other by 120°if the phase sequence of the windings is1-2-3, then the mathematical equation
for the instantaneous values of the fluxes ∅1, ∅2, ∅3 can be given as,
…………………………………………………………………. (1)
………………………………………… (2)
………………………………………… (3)
As windings are identical and supply is balanced the amplitude of each flux is same i.e. ∅m. the
wave form of three fluxes are shown in the figure.(4) while the assumed positive direction of these
fluxes in space are shown in the figure .(5) assumed positive direction means whenever the
instantaneous value of the flux is positive,in vector diagram it must be represented along its
assumed positive direction.aand if flux has negative instantaneous value then must be represented
in opposite direction to the assumed positive direction,in the vector diagram. [1]
Let ∅1, ∅2, and ∅3 be the instantaneous values of the fluxes. The resultant flux ∅T, at any
instant is given by phasor combination of ∅1, ∅2, and ∅3 at that instant .let us find out ∅T at four
different instant 1, 2, 3, and 4 respectively at θ= 0°, 60°, 120°, and 180°.
Case 1:
θ=0°, substituting in equation (1), (2), (3) we get,
=0
And
……………………………..vector sum
Show positive values in assumed positive directions and negative in opposite directions to assumed
positive directions. Hence vector diagram looks like as shown
BD is perpendicular drawn from B on ‘∅T’
Since
OD=DA=∅T/2
Since in
So
OBD,
BOD=30°
=OD/OB = (∅T/2)
0.866∅m
∅T = 2*0.866∅m *
∅T = 1.5∅m
So magnitude of resultant flux is 1.5 times the maximum value of an individual flux.
Case 2:
θ=60°
Substituting in equations 1), (2), (3) we get,
=
So
is positive and
is negative so vector diagram looks like as shown
It can be seen from the figure that ∅T = 1.5∅m
So magnitude of the resultant is same as before but it is rotated in space by 60° in space in
clockwise direction, from its previous position.
Case 3:
θ=120°
Substituting in equations 1), (2), (3) we get,
=
0
So
is positive and
is zero and
is negative. so vector diagram looks like as shown
∅T = 1.5∅m
So magnitude of the resultant is 1.5∅m, same as before but it is rotated in space by 60° in space in
clockwise direction, from its previous position.
Case 4:
θ=180°
Substituting in equations 1), (2), (3) we get,
=0
So
is zero and
is positive and
is negative. So vector diagram looks like as shown
From figure it can be prove easily that,
∅T = 1.5∅m
So magnitude of the resultant flux is once again 1.5∅m but it is further rotated by 60° in clockwise
direction from its position for θ=120°.
The resultant of the three alternating fluxes, separated from each other by 120°, has constant amplitude
of 1.5∅m where ∅m is maximum amplitude of an individual flux due to any phase.
The resultant always keeps on rotating with a certain speed in space.
This is nothing but satisfying the definition of a rotating magnetic field. Hence we can conclude
that the 3- phase stationary winding when connected to a three phase A.C. supply produces a
rotating magnetic field.
5. What are the losses in induction motor and explain them briefly?
Answer:
There are two types of losses occur in three phase induction motor. These losses are
Constant or fixed losses
Variable losses.
Constant or Fixed Losses:
Constant losses are those losses which are considered to remain constant over normal
working range of induction motor. The fixed losses can be easily obtained by performing no-load
test on the three phase induction motor. These losses are further classified as Iron or core losses,
Mechanical losses and Brush friction losses.
Iron or Core Losses:
Iron or core losses are further divided into hysteresis and eddy current losses. Eddy current
losses are minimized by using lamination on core. Since by laminating the core, area decreases
and hence resistance increases, which results in decrease in eddy currents. Hysteresis losses are
minimized by using high grade silicon steel. The core losses depend upon frequency of the supply
voltage. The frequency of stator is always supply frequency, f and the frequency of rotor is slip
times the supply frequency, (sf) which is always less than the stator frequency. For stator frequency
of 50 Hz, rotor frequency is about 1.5 Hz because under normal running condition slip is of the
order of 3 %. Hence the rotor core loss is very small as compared to stator core loss and is usually
neglected in running conditions.
Mechanical and Brush Friction Losses:
Mechanical losses occur at the bearing and brush friction loss occurs in wound rotor
induction motor. These losses are zero at start and with increase in speed these losses increases. In
three phase induction motor the speed usually remains constant. Hence these losses almost remain
constant.
b. Variable Losses:
These losses occur due to the current flowing in stator and rotor windings. As the load changes,
the current flowing in rotor and stator winding also changes and hence these losses also changes.
Therefore these losses are called variable losses. The copper losses are obtained by performing
blocked rotor test on three phase induction motor.
6. Derive the expression for torque in an Induction motor?
Answer:
The torque produced by three phase induction motor depends upon the following three factors:
Firstly the magnitude of rotor current, secondly the flux which interact with the rotor of three phase
induction motor and is responsible for producing Emf in the rotor part of induction motor, lastly
the power factor of rotor of the three phase induction motor.
Combining all these factors together we get the equation of torque as
Where, T is the torque produced by induction motor,
φ is flux responsible for producing induced Emf,
I2 is rotor current,
Cosθ2 is the power factor of rotor circuit.
The flux φ produced by the stator is proportional to stator Emf E1. i.e φ ∝ E1
We know that transformation ratio K is defined as the ratio of secondary voltage (rotor voltage) to
that of primary voltage (stator voltage).
Rotor current I2 is defined as the ratio of rotor induced Emf under running condition , sE2 to
total impedance, Z2 of rotor side,
And the total impedance Z2 on rotor side is given by ,
Putting this value in above equation we get,
s= slip of Induction motor
We know that power factor is defined as ratio of resistance to that of impedance. The power factor
of the rotor circuit is
Putting the value of flux φ, rotor current I2, power factor cosθ2 in the equation of torque we get,
Combining similar term we get,
Removing proportionality constant we get,
Where ns is synchronous speed in r. p. s, ns = Ns / 60. So, finally the equation of torque becomes,
7. Explain the slip torque characteristics of a 3-phase Induction motor?
Answer:
The torque slip curve for an induction motor gives us the information about the variation
of torque with the slip. The slip is defined as the ratio of difference of synchronous speed and
actual rotor speed to the synchronous speed of the machine. The variation of slip can be obtained
with the variation of speed that is when speed varies the slip will also vary and the torque
corresponding to that speed will also vary. The curve can be described in three modes of operationdescribed in three modes of operation
Motoring Mode: In this mode of operation, supply is given to the stator sides and the motor always
rotates below the synchronous speed. The induction motor torque varies from zero to full load
torque as the slip varies. The slip varies from zero to one. It is zero at no load and one at standstill.
From the curve it is seen that the torque is directly proportional to the slip. That is, more is the slip,
more will be the torque produced and vice-versa.
Generating Mode: In this mode of operation Induction motor runs above synchronous speed
and it should be driven by a prime mover. The stator winding is connected to a three phase supply
in which it supplies electrical energy. Actually, in this case, the torque and slip both are negative
so the motor receives mechanical energy and delivers electrical energy. Induction motor is not
much used as generator because it requires reactive power for its operation.
Braking Mode: In the Braking mode, the two leads or the polarity of the supply voltage is changed
so that the motor starts to rotate in the reverse direction and as a result the motor stops. This method
of braking is known as plugging. The kinetic energy stored in the revolving load is dissipated as
heat. Also, motor is still receiving power from the stator which is also dissipated as heat. So as a
result of which motor develops enormous heat energy. For this stator is disconnected from the
supply before motor enters the braking mode.