ELECTRICAL MACHINES : DET30043

4.0 THREE PHASE INDUCTION MOTOR

An electrical motor is an electromechanical device which converts electrical energy into
mechanical energy. An induction motor or asynchronous motor is an AC electric motor in
which the electric current in the rotor needed to produce torque is obtained
by electromagnetic induction from the magnetic field of the stator winding. An induction
motor can therefore be made witout electrical connections to the rotor. An induction
motor's rotor can be either wound type or squirrel-cage type. This type of motor does not
require an additional starting device. These types of motors are known as self-starting
induction motors.

4.1 ADVANTAGES
i. Self-starting.
ii. Robust in construction.
iii. Economical (very reliable and having low cost).
iv. Easier to maintenance.
v. High efficiency and good power factor.

4.2 DISADVANTAGES
i. Speed decreases with increase in load, just like a DC shunt motor.
ii. If speed is to be varied, we have sacrifice some of its efficiency.
iii. Speed control of induction motors are difficult. (Variable frequency drives using induction
motors are used in industries for speed control now a days.)

4.3 CONSTRUCTION
Basically there are two types of 3 phase induction motor :
1. Squirrel cage induction motor and
2. Phase Wound induction motor @ (slip-ring induction motor).

(Both types have similar constructed of stator, but they different in construction of rotor.)

4.3.1 STATOR
It is a stationary part of induction motor. The stator of the three phase
induction motor consists of three main parts (see figure 4.1) :
1. Stator frame :
Its main function is to support the stator core and the field winding. It acts
as a covering and provide protection and mechanical strength to all the
inner parts of the machine.

2. Stator core :
The main function of the stator core is to carry alternating flux. In order
to reduce the eddy current losses the stator core is laminated.

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3. Stator winding or field winding :

The slots on the periphery of stator core of the three phase induction motor
carries three phase windings and its supplied by three phase ac supply. The
three phases of the winding are connected either in star or delta depending
upon which type of starting method is used. When this winding is excited by
three phase ac supply it produces rotating magnetic field.

*** The winding wound on the stator of three phase induction motor is
also called field winding.

Figure 4.1

4.3.2 ROTOR
The rotor is a rotating part of induction motor. The rotor is connected to the
mechanical load through the shaft. The rotor of the three phase induction motor
are further classified as-
1. Squirrel cage rotor,
2. Slip ring rotor or wound rotor or phase wound rotor.

4.3.2.1 SQUIRREL CAGE ROTOR

The rotor shape is a cylindrical laminated core with parallel slots for
carrying the rotor conductors, which are not wires but heavy bars of
copper or aluminium or its alloys (see figure 4.2).
The rotor conducting bars are usually not parallel to the shaft, but
are purposely given slight skew.
There are end rings which are welded or even bolted at both ends of the
rotor, these end rings are short-circuited.
Note that fins are cast into the rotor to circulate air and cool the motor
while it's running.

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Figure 4.2

Reasons for Having Skewed Rotor

1. It helps in reduction of magnetic hum, thus keeping the motor quiet,
2. It also helps to avoid “Cogging”, i.e. locking tendency of the rotor.
3. Increase in effective ratio of transformation between stator & rotor,
4. Increased rotor resistance due to comparatively lengthier
rotor conductor bars,
5. and Increased slip for a given torque.

In particular the squirrel-cage ones, which are commonly used to control

pumps, fans, compressors and many other industrial applications.
However, it suffers from the disadvantage of a low starting torque. It is
because the rotor bars are permanently short-circuited and it is not
possible to add any external resistance to the rotor circuit to have a large
starting torque.

4.3.2.2 SLIP RING ROTOR OR PHASE WOUND ROTOR

It consists of a laminated cylindrical core and carries a 3-phase winding,
similar to the one on the stator.
The rotor winding is uniformly distributed in the slots and is usually star-
connected.
The open ends of the rotor winding are brought out and joined to three
insulated slip rings mounted on the rotor shaft with one brush resting on
each slip ring.
The three brushes are connected to a 3-phase star-connected rheostat.
At starting, the external resistances are included in the rotor circuit to
give a large starting torque.

*** These resistances are gradually reduced to zero as the motor runs up to
speed. The external resistances are used during starting period only. When
the motor attains normal speed, the three brushes are short-circuited so

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that the wound rotor runs like a squirrel cage rotor. These resistances
serve to increase the starting torque and ensure smooth starts.

Figure 4.3

What happens when external resistance is added?

In the case of a squirrel cage induction motor, the rotor resistance is very
low so that the current in the rotor is high, which makes its starting
torque poor. But adding external resistance, as in the case of a slip ring
induction motor, makes the rotor resistance high when starting, thus
the rotor current is low and the starting torque is maximum.
Also the slip necessary to generate maximum torque is directly
proportional to the rotor resistance. In slip ring motors, the rotor
resistance is increased by adding external resistance, so the slip is
increased. Since the rotor resistance is high, the slip is more, thus it's
possible to achieve “pull-out” torque even at low speeds.
As the motor reaches its base speed (full rated speed), after the
removal of external resistance and under normal running conditions, it
behaves in the same way as a squirrel cage induction motor.
Thus these motors are best suited for very high inertia loads, which
requires a pull-out torque at almost zero speed and acceleration to
full speed with minimum current drawn in a very short time period.

Advantages of slip ring induction motors :

 High starting torque with low starting current

 Smooth acceleration under heavy loads
 No abnormal heating during starting
 Good running characteristics after external rotor resistances are cut out
 Adjustable speed

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Difference between Slip Ring and Squirrel Cage Induction Motor

NO Slip ring Induction motor Squirrel cage induction motor

Construction is complicated due to

1 Construction is very simple.
presence of slip ring and brushes.

The rotor consists of rotor bars which

The rotor winding is similar to the
2 are permanently shorted with the help
stator winding.
of end rings.

We can easily add rotor resistance by Since the rotor bars are permanently
3 using slip ring and brushes. shorted, its not possible to add external
resistance.
Due to presence of external Staring torque is low and cannot be
4 resistance high starting torque can be improved.
obtained.
5 Slip ring and brushes are present. Slip ring and brushes are absent.
Frequent maintenance is required
6 Less maintenance is required.
due to presence of brushes.
The construction is complicated and The construction is simple and robust
7 the presence of brushes and slip ring and it is cheap as compared to slip ring
makes the motor more costly. induction motor.
This motor is rarely used only 10 % Due to its simple construction and low
8 industry uses slip ring induction cost. The squirrel cage induction motor is
motor. widely used.
Rotor copper losses are high and Less rotor copper losses and hence high
9
hence less efficiency. efficiency.
Speed control by rotor resistance Speed control by rotor resistance
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method is possible. method is not possible.
Slip ring induction motor are used
Squirrel cage induction motor is used in
where high starting torque is
11 lathes, drilling machine, fan, blower
required i.e in hoists, cranes, elevator
printing machines etc.
etc.

4.4 OPERATING PRINCIPLE OF THREE PHASE INDUCTION MOTOR

Working principle of any induction motor is very similar to that of a transformer.
Supply is given to stator windings, and as the supply is AC, alternating flux is produced
around the stator winding. This interaction between the magnetic field and the conductor
induces a current in the bars. This induction action is what gives the motor its names, and
makes it similar to the transformer action, in the fact that a voltage is induced into the rotor
(sometimes called secondary) by current flowing in the stator (sometimes called the
primary). A current flows in the conductors of the rotor, through the short circuiting rings at
the end. This current in turn produces a magnetic field. It is the interaction between the
rotor magnetic field and the squirrel cage bars that induces the torque and causes rotation.

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*** 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 velocity, so no emf
induction in the rotor & no current would be flowing, and therefore no torque would
be generated. Consequently the rotor can not reach at 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.

4.4.1 WHY IS THREE PHASE INDUCTION MOTOR SELF STARTING ?

So what is self starting?
When the machine starts running automatically without any external force to
the machine, then it is called as self starting.

So in three phase, there are three single phase line with 120° phase difference. So
the rotating magnetic field is having the same phase difference which will make
the rotor to move. If we consider three phases R, Y and B, when phase R is
magnetized, the rotor will move towards the phase R winding, in the next moment
phase Y will get magnetized and it will attract the rotor and than phase B. So the
rotor will continue to rotate.

4.4.2 A ROTATING MAGNETIC FIELD

In a three phase induction machine, there are three sets of windings, phase A
winding, phase B and phase C windings. This would result in a balanced three
phase current l1— l3 represent the currents that flow in the three phase windings.
Note that they have a 120◦ time lag between them.

A rotating magnetic field in the stator is the first part of operation. To produce a
torque and thus rotate, the rotors must be carrying some current. In induction
motors, this current comes from the rotor conductors. The revolving magnetic
field produced in the stator cuts across the conductive bars of the rotor and
induces an e.m.f.

The rotor windings in an induction motor are either closed through an external
resistance or directly shorted. Therefore, the e.m.f induced in the rotor causes
current to flow in a direction opposite to that of the revolving magnetic field in
the stator, and leads to a twisting motion or torque in the rotor.

As a consequence, the rotor speed will not reach the synchronous speed of the
r.m.f in the stator. If the speeds match, there would be no e.m.f. induced in the
rotor, no current would be flowing, and therefore no torque would be generated.
The difference between the stator (synchronous speed) and rotor speeds is
called the slip.

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For better understanding of the rotating magnetic field. These are shown as 3-
phase sine wave, input voltage, which has been divided into a few moments, from
t1 to t3.

Figure b1. Figure b2. Figure b3.

Figure 4.4
The position at T1

When T1, supply current in phase l1 are in a state of maximum positive and the
current in phase l2 and l3 in a state of half maximum negative. This will form the
pole of magnetic field are moves like figure b1.

The position at T2

When T2, supply current in phase l1 are in a state of neutral while phase
l2 becomes positive and l3 phase remain negative. This will form the pole
of magnetic field are moves like figure b2.

The position at T3

When T3, supply current in phase l1 are in a state of maximum negative

while phase l2 and l3 in a state of half maximum positive. This will form the
pole of magnetic field are moves like a figure b3.

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4.5 PARAMETERS AND TERMS OF A.C. MOTOR

4.5.1 SYNCHRONOUS SPEED (Ns) –The speed of the magnetic field at the stator.
The rotational speed of the rotor of the induction motor is usually decreased
3% to 4% of synchronous speed.

Formula:
120 f
Synchronous Speed, Ns = unit rpm
P

Ns = synchronous speed
f = supply frequency
p = Number of pole

4.5.2 SLIP is the ratio of the slip speed to the synchronous speed.

Formula:

Ns  Nr Slip speed - the difference between the

%s= x 100
Ns synchronous speed and the rotor speed.

Ns = synchronous speed Formula :

Nr = rotor speed Slip speed = Ns – Nr unit rpm.
% s = Percentage of slip ,

4.5.3 The actual motor speed is the speed of the rotor during carrying a full load it also
known as ‘ROTOR SPEED’. The speed is less then the synchronous speed.

Formula :

Rotor Speed, Nr = Ns ( 1 – s ) unit rpm

4.5.4 Currents frequency induced in the rotor conductors known as 'ROTOR

FREQUENCY'. Rotor frequency is directly proportional to the rotor slip.

Formula :

Rotor Frekuensi, fr = s x fs unit Hz.

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4.5.5 VOLTAGE AND CURRENT INDUCED IN THE ROTOR

Rotor induced emf under standstill condition, Ers.

rotor winding
Ers = (ratio ) x Line Voltage
stator winding

* [E is voltage rs is rotor static]

Rotor induced emf under running condition, Er.

Er = s x Ers

* [E is voltage r is rotor rotate]

Voltage between terminal rotor or between slip ring, (Y connection),

Vt-r .

Vtr = 3 x Er

* [V is voltage tr is terminal rotor]

Rotor reactance under standstill condition or (the actual value).

Xrs = 2 π fL

* [X is reactance and rs is rotor static]

Rotor reactance under running condition.

Xr = s x Xrs

* [X is reactance and r is rotor rotate and rs rotor static]

Rotor current I2 is defined as the ratio of rotor induced emf under running condition, s.Ers
to total impedance, Zr of rotor side,

s E rs E
Ir  @ Ir  r
Zr Zr
and rotor impedance, Zr is given by,

Zr = Rr  (s X rs ) 2
2

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Putting this value in above equation we get rotor current is ,

s E rs E
Ir  @ Ir  r
Rr  ( s X rs )
2 2 2 2
R  X
r r

We know that power factor is defined as ratio of resistance to that of impedance. The
power factor of the rotor circuit is …

R Rr Rr
Cosr  r  @ Cos r =
Zr Rr  ( s X r ) Rr  X r
2 2 2 2

4.5.6 MOTOR POWER FACTOR - Hence when we consider the entire motor circuit
consisting of resistor or inductor and capacitor, there exists some phase
difference between the source voltage and current. The cosine of the phase angle
between voltage and current - or the "cosφ".

Formula :
Pin = electrical power supplied to the stator of three phase induction motor,
VL = line voltage supplied to the stator of three phase induction motor,
IL = line current,
Cos  = power factor of the three phase induction motor.

Pin = 3 x VL x IL x cos 

Pin
Power factor, Cos 
3 x VL x I L

4.5.7 LOSSES IN THE THREE PHASE INDUCTION MOTOR

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-

1. Iron or core losses (into hysteresis and eddy current losses in the stator,
a losses can reduced by using lamination on core).
2. Mechanical losses (Friction and windage - losses are caused by friction in the
bearings of the motor and aerodynamic losses associated with
the ventilation fan and other rotating parts).

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3. Variable Losses :

These losses are also called copper losses. 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.

Figure 4.5 Power stages in induction motor

Known:
Electrical power input to the stator, Pin = √3VL IL cos ϴ

A part of this power input is used to supply stator losses which are stator iron loss and stator
copper loss. The remaining power i.e (input electrical power – stator losses) are supplied to
rotor as rotor input.

Stator copper losses =3 Ist2 Rst ,

Where, Ist = Stator current,

Rst = Stator resistance

Stator output = Rotor Input

Stator output = Stator input – stator losses
Stator losses = Stator copper loss + (Stator iron & friction loss).

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Rotor copper losses = 3 Ir2 Rr

Rotor Copper losses= s x Rotor input
Rotor output = Rotor Input – Rotor copper losses
Rotor output = (1 – s) Rotor input
Rotor output = Mechanical Energy
Mechanical Energy ----> Gross Torque Tg.
Net Power output, Po = Mechanical Energy + Friction and Windage

4.5.8 TORQUE is the ability to rotate the motor. Usually it is expressed in units of
Newton – meters, N-m. For a.c induction motor, the torque occurring between
push and pull a rotating magnetic field with the rotor magnetic field the
resulting by mutual induction. The torque produced by three phase induction
motor depends upon the following three factors:
i. The part of rotating magnetic field which reacts with rotor and is
responsible to produce induced e.m.f. in rotor.
ii. The magnitude of rotor current in running condition.
iii. The power factor of the rotor circuit in running condition.

Formula :

Gross Torque and Shaft Torque :

The torque produced by rotor is gross mechanical torque and due to mechanical
losses entire cannot be available to drive load.

The load torque is net output torque called shaft torque or useful toque.
Therefore, Shaft torque :

where,
Tlost = Torque lost due to mechanical losses
Pout = Motor output
Pm = Mechanical power developed
N = Motor speed

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Starting Torque :
The torque produced by the motor at start is called as starting torque, Tst.
At start, N = 0 and hence slip, s = 1
Therefore, starting torque can be obtained by substituting s = 1 in the equation (2)
i.e., torque equation under running condition.

The torque will be maximum when slip s = R2 / X2 , the maximum value of torque as,

4.5.9 THE EFFICIENCY,  of an electric motor is just output power divided by input power Input
power is your electrical input power, which is (V*I). Output power is your mechanical output
power, which is (speed x torque).
Formula :
Output Power, Pout = hp x 746
Input Power , Pin = 3 x VL x IL x Kos 

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Rotor efficiency of the three phase induction motor ,

Figure 4.6

Figure 4.7

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