Basic Electrical Engineering (Three Phase Induction Motor)

Three Phase Induction Motor

Introduction
Three phase induction motor are simple, rugged, low cost and less maintenance with
suitable characteristics to industrial needs. That is why , three phase induction motors are widely
used. Speed of the motor is the function of supply frequency. In olden days dc motors were
employed wherever large speed variation were required. But due to advancement in power
electronics, Variable Frequency Drives (VFD) and Variable Voltage are Frequency Drives
(VVFD) are nowadays available to vary the speed of three phase induction motors.

Advantages of a three phase induction motor

1. It is simple and rugged in construction
2. Low cost motor
3. Less maintenance is required for this motor
4. Reasonably good power factor and sufficiently high efficiency
5. Self starting motor

Disadvantages
1. Constant speed motor. Difficult to vary the speed as the speed in depends upon supply
frequency. (but nowadays VFD’s are available to vary speed)
2. Starting torque is inferior to that of dc motor.

Three phase induction motor

It consist of a stator and a rotor. The stator is the stationary part and rotor is the rotating
part of the machine. Stator consists of a three phase winding and the rotor consists of short
circuit winding. Stator is fed from a 3 phase supply. Mechanical rotating power is developed in
the rotor due to electromagnetic induction. That is why this motor is named as Induction motor.
It can also be visualized as a rotating secondary transformer in which the electrical energy is
converted into mechanical energy.

Construction
An induction motor consists of two major parts

(1) Stator and

(2) Rotor
Stator

It is made up of a steel frame to enclose a hollow

cylindrical core as shown in the figure 1. Cylindrical core is
made up of thin laminated high silicon steel to reduces
hysteresis and eddy current losses. Predesigned and evenly
spaced slots are provided in the hollow cylindrical core to Figure 1: Stator of 3-phase
hold the three phase winding. The windings (thin insulated
Induction Motor
 Basic Electrical Engineering (Three Phase Induction Motor)

conductors) are interconnected with either delta (∆) connection or star (Y) connection so that
they form definite number of poles. The number of poles decide the speed of the motor. As the
number of poles increases the speed of the motor decreases. A rotating magnetic field will be
produced when the stator is energized by a 3 phase supply. The magnetic field rotates at a speed
120 f
equal to ; where f is the supply frequency and P is the number of poles formed by stator
P
winding. The speed of the rotating magnetic field is known as synchronous speed. The rotating
magnetic filed in the stator induces an emf in the rotor. As the rotor is short circuited, a short
circuit current will flow in the rotor.

Rotor
Rotor has short circuited windings mounted on a shaft with the help of a laminated core.
Windings are placed on the slots provided on the outer periphery of the core. The rotor windings
are classifies into two types:

(1) Squirrel cage rotor

(2) Phase wound or wound rotor

Squirrel cage rotor

Most of the 3 phase induction
motors use this type of rotor due to
its simple construction and robust
operation even in adverse operating
condition. It is made up of a
laminated cylindrical core with slots
in its outer periphery to hold
aluminium or copper bars as shown
in the figure 2. Bars placed in the
slots are short circuited by a metal Figure 2: Squirrel cage rotor of induction motor
ring called End ring. The set of all
bars and end rings together appear like a squirrel cage. A short circuit current flows in the rotor
bars due to induction with the stator. Motors with squirrel cage rotors are called Squirrel cage
induction motor. The main disadvantage of this type is low starting torque. It is not possible to
change the rotor impedances for the purpose of changing its
characteristics.

Phase wound or wound rotor

It is also made up of a laminated cylindrical core but
with 3 phase windings placed in the slots in the outer
periphery. The windings are uniformly distributed in the slots
usually forming a star connection. The three terminals are
taken out through slip rings and brush arrangements as shown
in the figure 3. The schematic diagram is shown in the figure
4. The three terminals taken out can be used to vary the
impedance of the rotor winding. There is a close relationship
Figure 3: Wound rotor of
induction motor
 Basic Electrical Engineering (Three Phase Induction Motor)

between the impedance of the rotor and the starting torque. Therefore the impedance of the rotor
winding can be varied just by connecting a rheostat as shown in the figure 4. The external
resistance is removed after it attains the required speed and runs like squirrel cage motor.

Figure 4: Schematic diagram of wound rotor

Comparison between cage and wound rotor

The advantages of the cage rotor are as follows:

1. Robust construction and cheaper

2. The absence of brushes reduce the risk of sparking
3. Lesser maintenance
4. Higher efficiency and higher power factor

The wound rotor have the following merits:

1. High starting torque and low starting current
2. Additional resistance can be connected in the rotor circuit to control speed

Principle of operation
A 3 phase induction motor has a stator winding which is supplied by 3 phase alternating
balance voltage and has balance 3-phase currents in the winding. The rotor is not excited from
any source and has only magnetic coupling with the stator. Under normal running conditions, the
rotor winding (cage or slip rings) is always short circuited to allow induced currents to flow in
the rotor winding. The flow of 3-phase current in the stator winding produces a rotating magnetic
field of constant amplitude and rotates at synchronous speed. Let us assume that the rotor is at
standstill initially; the rotating stator field induces an emf in the rotor conductor by transformer
action. Since the rotor circuit is a closed set of conductors, a current flows in the rotor circuit.
This rotor current then produces a rotor field. The interaction of stator and rotor field produces a
torque which causes the rotation of the rotor in the direction of the stator field.
As per Lenz’s law, the rotor field will try to oppose the very cause of its production. Thus
it speeds up in the direction of the stator field so that relative speed difference between these two
fields is zero. In this way, the three phase induction motor catches up the speed.

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 Basic Electrical Engineering (Three Phase Induction Motor)

When the rotor is at standstill, the relative motion between the rotor and the stator field is
maximum. Therefore, the emf induced in the rotor and the rotor current are reduced. However,
the rotor cannot attain the speed of the stator field which is equal to the synchronous speed. This
is evidently due to the reason that if the rotor is moving at synchronous speed, there is no relative
motion between the stator field and the rotor. Hence the rotor induced emf and current becomes
zero and the torque becomes zero. This would cause the rotor speed to decrease. As the rotor
speed falls below the synchronous speed, the rotor emf and current continues to increase.
Therefore, the electromagnetic torque continues to increase.

Finally, the rotor speed becomes constant at a value at speed slightly less than of
synchronous speed. The torque developed equals the sum of load torque and the mechanical
losses.

Concept of production of rotating magnetic field

When a 3-phase winding, displaced in space by 1200, are supplied by a 3-phase currents
displaced in time by 1200, a magnetic flux is produced which rotates in space. This causes the
rotor to rotate. The method of analysis is as follows:

Analytical method:
let us consider three identical coils placed 120 0 apart with respect to each other as shown in
figure 5(a).
The coils are supplied with currents having frequency of supply and varying sinusoidally
in time. Each coil will produced an alternating flux along its own axis. Let the instantaneous flux
be given by

φ1 = φm sin ωt − − − −−− −−− −−− 1(a)

φ2 = φm sin ( ωt − 120 0
) −−− −−− −−− − − − 1(b)
φ =φ sin ( ωt − 240 ) − − −
0
−−− − − − − −− 1(c)
3 m

The resultant flux produced by this system may be determined by resolving the components with
respect to the physical axis, as shown in figure 5(b).

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 Basic Electrical Engineering (Three Phase Induction Motor)

Figure 5: Production of rotating magnetic field in 3-phase induction motor

Therefore the resultant horizontal component of the flux is given by

φ = φ −φ cos 60 0 − φ cos 60 0
h 1 2 3

= φ − ( φ + φ ) cos 60 0
1 2 3

1
= φ1 − (φ2 + φ3 ) ×
2
1
= φ sin ωt − ( ) (
⎡φ sin ωt − 1200 + φ sin ωt − 240 0 ⎤ )
2⎣ ⎦
m m m

φm ⎛ 1⎞
= φ sin ω t − × ( 2 sin ωt ) −
m ⎜ ⎟
2 2
⎝ ⎠
3
= φ sin ωt (2)
m
2

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 Basic Electrical Engineering (Three Phase Induction Motor)

Similarly, the vertical component of the flux is given by

3
φ = 0 − φ cos 300 + φ cos 30 0 = [φ −φ ]
v 2 3 3 2
2
3
⎡φ sin (ωt − 240 ) − φ sin (ωt − 120 ) ⎤
0 0
=
2 ⎣ ⎦
m m

3 3
= × φm × 2 cos ωt ×
2 2
3
= φm cos ωt (3)
2

The resultant flux is (figure 5(d)),

3 3
φ = (φ ) 2 + ( φ ) 2 = φ sin 2 ωt + cos 2 ωt = φ (4)
r h v m m
2 2

φv
And tan θ = = cot ωt = tan ( 900 − ωt )
φh

It implies θ = ( 900 − ωt ) (5)

The above equation shows that the resultant flux ( φr ) is free from time factor. It is a
⎛3⎞
constant flux of magnitude equal to ⎜ ⎟ times the maximum flux per phase. However, θ is
⎝2⎠
π
dependent on time and we can calculate θ at different values of ( ωt ) ; when ( ωt ) =0, θ =
2
corresponding to position P in figure 5(c).

π
Similarly, for ωt = , θ = 00 , corresponding to position Q.
2

π
When ωt = π , θ = − , corresponding to position R
2

3π
When ωt = , θ = −π , corresponding to position S
2

It is thus observed that the resultant flux φr rotates in space in the clockwise direction
with angular velocity of ω radians per second.

PN s
Since ω = 2π f and f = , the resultant flux φ rotates with synchronous speed (N ).
r s
120

The following conclusions are drawn from the above discussion:

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 Basic Electrical Engineering (Three Phase Induction Motor)

1. Three phase currents of a balanced supply systems produced a resultant flux of

constant magnitude in air gap of the motor. The magnitude of the flux at every instant
3
is φ m .
2
2. The resultant flux is rotating in nature and its angular velocity is the same as that of
PN s
supply currents. Since ω = 2π f and f = , the resultant flux φr rotates with
120
synchronous speed (Ns).
3. The direction of rotation of resultant flux in the air gap depends upon the phase
sequence. The direction is the same as the phase sequence of the supply.

Speed and Slip:

An induction motor cannot run at synchronous speed. Let us consider for a moment that
is rotor is rotating at synchronous speed. Under this condition, there would be no cutting of flux
by the rotor conductors, and there would be no generated voltage, no current and no torque. The
rotor speed is therefore slightly less than the synchronous speed. An induction motor may also be
called ‘Asynchronous motor’ as it does not run at synchronous speed. The difference between the
synchronous speed and the actual speed of the rotor is called the slip.
Thus, the slip expresses the speed of the rotor relative to the field.
If NS = synchronous speed in r.p.m.
NR = actual rotor speed in r.p.m
The slip = NS - NR in r.p.m (6)
The slip expresses as a fraction of the synchronous speed is called the per-unit slip or
fractional slip. It is denoted by ‘s’.
N −N r
s= s per unit (p.u.) (7)
Ns
N s −N r
Percentage slip s = ×100 (8)
Ns
Frequency of rotor voltage and current
The frequency of voltage and current in the stator must be the same as the supply
PN s
frequency given by f =
120
(9)
The frequency in the rotor winding is variable and depends on the difference between the
synchronous speed and the rotor speed. Hence the rotor frequency depends upon the slip. The
rotor frequency is given by
P ( N s −N r )
(10)
fr =
120
Division of equation (10) by equation (9) gives

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 Basic Electrical Engineering (Three Phase Induction Motor)

f r N s −N r
=
f Ns
N s −N r
But =s
Ns
∴ f r = sf (11)
That is,
Rotor current frequency = per unit slip × supply frequency
When the rotor is stationary (stand – still)
N −N r
N = 0, s= s =1 and f = f
r r
Ns
When the rotor is driven by a mechanical prime over at synchronous speed NS, then s = 0 and f r =
0. Therefore, frequency of rotor current varies from fr = f at stand still (s = 1) to fr = 0 at
synchronous speed (s = 0).

Power Flow Diagram

Starting of induction motor (Squirrel cage)

The following are the commonly used starters for cage motors:

1. Direct On Line Starter

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 Basic Electrical Engineering (Three Phase Induction Motor)

2. Star Delta Starter

3. Autotransformer starter.

Direct On Line Starter:

In the Direct On Line method of starting cage motors, the motor is connected by means
of a starter across the full supply voltage. Figure 6 shows the connection for one type of the
direct on line (D.O.L) starter. It consists of a coil operated contactor C controlled by start and
stop push buttons which may be installed at convenient places remote from the starter. On
pressing the START push button S1 (which is normally held open by a spring) the contactor coil
C is energized from two line conductors L1 and L2. The three main contacts M and the auxiliary
contact A close and the terminals a and b are short circuited. The motor is thus connected to the
supply. When the pressure on S1 is released, it moves back under spring action (open). Even then
coil C remains energized through ab. Thus, the main contacts M remain closed and the motor
continues to get supply. For this reason, contact A is called hold-on contact.

When the STOP push button S2 (which is normally closed) is pressed, the supply through
the contactor coil C is disconnected. Since the coil C is de-energized, the main contacts M and
auxiliary contact A are opened. The supply to motor is disconnected and the motor stops.

Figure 6: Direct On Line Starter

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 Basic Electrical Engineering (Three Phase Induction Motor)

Undervoltage protection

When the voltage falls below a certain value, or in the event of failure of supply during motor
operation, the coil C is de-energized. The motor is then disconnected from the supply.

Overload protection

In case of an overload on the motor, one or all the overload coils (O.L.C) are energized. The
normally closed contact D is opened and the contactor coil C is de-energized to disconnect the supply to
the motor.
Fuses are provided in the circuit for short circuit protection.
Direct on line starter is a simple and cheap method. The starting current may be as large 10 times
the full load currant and the starting torque is equal to full load torque. Such a large starting current
produces excessive voltage drop in the line supplying the motor. Small motors upto 5 kW rating may be
started by D.O.L. starters to avoid supply voltage fluctuations.

Star Delta Starter

This is a very common type of starter and extensively used, compared to the other type of

Figure 7: Star Delta Starter

starter. A stat delta starter is used for a squirrel cage motor designed to run normally on delta
connected stator winding. Figure 7(a) shows the connections of a three phase induction motor
with a star delta starter. When the switch S is in the START position, the stator windings are
connected in STAR (figure 7(b)). When the motor picks up speed, say 80% of its rated value, the

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 Basic Electrical Engineering (Three Phase Induction Motor)

changeover switch S is thrown quickly to the run position which connects the stator windings in
delta (figure 7(c)). By connecting the stator windings, first in star and then in delta, the line
current drawn by the motor at starting is reduced to one third as compared to starting current
with the windings connected in delta. At the time of starting when the stator winding are star
V , where VL is the line voltage. Since the torque
connected, each stator phase gets a voltage L
3
developed by an induction motor is proportional to the square of the applied voltage, star-delta
starting reduces the starting torque to one third that obtainable by direct delta starting.

Autotransformer Starter

An auto-transformer starter is suitable for both star and delta connected motors. In this
method, the starting current is limited by using a three phase auto-transformer to reduce the
initial stator applied voltage. Figure 8 shows the motor with the auto-transformer starter. The
auto-transformer is provided with a numbers of tapping.

Figure 8: Auto-transformer starter.

In practice, the starter is connected to one particular tapping to obtain the most suitable
starting voltage. A double throw switch is used to connect the auto-transformer in the circuit for
starting. When the handle H of the switch is placed in the START position, the primary of the
auto-transformer is connected to the supply line and the motor is connected to the secondary of
the auto-transformer. When the motor picks up the speed, say to about 80% of its rated value, the

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 Basic Electrical Engineering (Three Phase Induction Motor)

handle H is quickly moved to the RUN position. The auto-transformer is disconnected from the
circuit and the motor is directly connected to the line and gets its full rated voltage. The handle is
held in RUN position by the under voltage relay. In case the supply voltage fails or falls below a
certain value, the handle is released and returns to the OFF position. Over load protection is
provided by thermal overload relays.

Starting of induction motor (Slip Ring)

Figure 9 shows the connection of a 3-phase slip ring induction motor with a starter. Full supply
voltage is connected across the stator. Full starting resistances are connected, and thus the supply
current to the stator is reduced. The rotor begins to rotate and the rotor resistances are gradually
cut out as the motor speeds up. When the motor is running at its rated full speed, the starting
resistances are cur out completely and the slip rings are short circuited.

Figure 9: Slip Ring Induction motor starter

its full rated voltage. The handle is held in RUN position by the under voltage relay. In case the supply
voltage fails or falls below a certain value, the handle is released and returns to the OFF position. Over
load protection is provided by thermal overload relays.

Starting of induction motor (Slip Ring)

Figure 9 shows the connection of a 3-phase slip ring induction motor with a starter. Full supply voltage is
connected across the stator. Full starting resistances are connected, and thus the supply current to the
stator is reduced. The rotor begins to rotate and the rotor resistances are gradually cut out as the motor
speeds up. When the motor is running at its rated full speed, the starting resistances are cur out completely
and the slip rings are short circuited.
Figure 9:

Rotor Impedance Starter

Power Electronics Starter

Speed Control of Induction Motor

The speed of an induction motor is given by the relation

s=(Ns-N)x100/Ns
N = Ns (1 – s) or N = 120 / (1 − )
Hence, the speed of an induction motor depends upon three factors i.e., supply frequency f, number of
pole P or slip S. The main methods employed for speed control of induction motors are as follows:
1. Pole changing
2. Stator voltage control
3. Supply frequency control
4. Rotor resistance control
5. Slip energy recovery

By Changing the Number of Stator Poles:

From the above equation of synchronous speed, it can be seen that synchronous speed (and hence,
running speed) can be changed by changing the number of stator poles. This method is generally used for
squirrel cage induction motors, as squirrel cage rotor adapts itself for any number of stator poles. Change
in stator poles is achieved by two or more independent stator windings wound for different number of
poles in same slots. For example; a stator is wound with two 3-phase windings, one for 4 poles and other
for 6 poles. Then for supply frequency of 50 Hz i) Synchronous speed when 4 pole winding is connected,
Ns = 120*50/4 = 1500 RPM ii) Synchronous speed when 6 pole winding is connected, Ns = 120*50/6 =
1000 RPM

Voltage control:
Running torque of induction motor is 2 2 2 2 2+ 2 2 2 Rotor induced emf at standstill; E2
depends on the supply voltage V ∴ E2 α V For low slip region, (sX2) 2 << R2, hence  2 2 2
2 2 2 If supply voltage is reduced below rated value, as per above equation, torque
produced also decreases. But to supply the same load it is necessary to develop same torque hence value
of slip increases so that torque produced remains same. Slip increases means motor reacts by running at
lower speed, to decrease in supply voltage. So motor produces the required load torque at a lower speed.
This method, though the cheapest and the easiest, is rarely used because (i) A large change in voltage is
required for a relatively small change in speed (ii) Due to reduction in voltage, current drawn by the
motor increases. Due to increased current, the motor may get overheated. (iii) This large change in
voltage will result in a large change in the flux density thereby seriously disturbing the magnetic
conditions of the motor.

Frequency Control:
Synchronous speed of rotatin magnetic field of an induction motor is given by, = 120 where,
f = frequency of the supply and P = number of stator poles. Hence, the synchronous speed changes with
change in supply frequency. Actual speed of an induction motor is given as N = Ns (1 - s). However, this
method is not widely used. It may be used where, the induction motor is supplied by a dedicated generator
(so that frequency can be easily varied by changing the speed of prime mover). Also, at lower frequency,
the motor current may become too high due to decreased reactance. And if the frequency is increased
beyond the rated value, the maximum torque developed falls while the speed rises.

By Variation of Rotor Resistance:

Running torque of induction motor is 2 2 2 2 2+ 2 2 2 For low slip region, (sX2) 2 << R2,
and can be neglected & for constant supply voltage E2 is also constant  2 2 2 2 Thus if the
rotor resistance is increased, the torque produced decreases. But when the load on the motor is same,
motor has to supply same torque as load demands. So motor reacts by increasing its slip to compensate
decrease in T due to R2 and maintains the load torque constant. So due to additional rotor resistance R2,
motor slip increases i.e. the speed of the motor decreases.
Advantage
a. By increasing the rotor resistance R2 speeds below normal value can be achieved
b. The starting torque of the motor increases proportional to rotor resistance.

Disadvantage
a. The large speed changes are not possible.
b. The method cannot be used for the squirrel cage induction motors.
c. The speeds above the normal values cannot be obtained.
d. Large power losses occur due to large 12R loss.
e. Due to large power losses, efficiency is low.

Short Answer Type Questions

Q.1. What is the need of a starter to start an induction motor?
Ans. Starter is required to limit the starting current to predetermined value and to provide necessary
protection to the motor.
Q.2. Which starter has the minimum cost?
Ans. Direct on line starter.
Q.3. How much starting current is reduced if the motor is started with stardelta starter?
Ans. it is reduced to 1/3rd.
Q.4. What protections are provided in 3-phase induction motor starters?
Ans. Over-load protection and no-volt protection.
Q.5. Which method is adopted to start a slip-ring induction motor?
Ans. Rotor resistance method is used to start slip-ring induction motors?
 Q.6. What are the factors on which speed of an induction motor depends?
Ans. Speed of an induction motor depends upon supply voltage, supply frequency, slip and number of
poles of the stator.
Q.7. If the speed of an induction motor is decreased by increasing the number of poles, how does it affect
the pf of the motor?
Ans. The pf of the motor will reduce.