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ELECTRICAL MACHINES:
Chapter 3 : 3 phase induction motor
Group leaders:
GUIDED BY MALIKA PAWAR
130230111044: Sarkale Priya P.
130230111041: Prajapati Roshni R.
130230111052: Tandel srushti Y.
130230111049:Shukla priti
130230111040:Prajapati Rinkal K.
130230111018:Divyani Bhargava
Advantages of induction motor over dc motor Low maintenance requirement since the squirrel cage induction
motor does not use the commutators and brushes.
 Ruggedness , smaller size and weight.
 Low cost.
 They can operate at higher speeds, of the order of 12,000 RPM.
This is again possible as the brushes are absent so no friction.
 It can produce sufficient torque.



low starting torque.
Lagging and low power factor.
Speed control by electrical methods is not easy.
application of induction motor:

Fans , pumps , extruders , conveyors , in food and
chemical industries .
There are two main parts of a 3-phase induction motor.
1. Stator
2. Rotor
1.stator:
•
It is the stationary part of the motor.
•
This part is made of silicon steel stampings.
•
The stampings are slotted. when complete
stator is assembled, the through slots are
formed on the inner side of the stator.
•
The slot may be open type, close type or
semi type. in this slot, 3 phase winding is placed.
•
The three ends of this windings are brought out
into the terminal box where 3 phase
A.C supply can be connected.
•


Squirrel cage
No slip rings or
brushes.
Simple construction.

Starting torque can
not be adjusted.

Less rotor copper loss.

Higher efficiency.





Wound rotor
Slip rings and brushes
are used .
Complicated
construction.
Starting torque can be
adjusted by adjusting
the external
resistance.
High rotor copper loss.
Low efficiency.
Torque produced by induction motor is depend on three factor.
1.
the rotor power factor under the running condition.
2. The rotor current under running condition.
3. The part of rotating magnetic field which induced emf in the rotor winding.
the mathematical expression for torque is given by,
Tα Φ I2r cosΦ2r
where , Φ= flux that induces emf in the rotor.
I2r =rotor current under running condition.
cosΦ2r =rotor power factor under running condition.
We can write the torque equation of an induction motor as follows:
Φ α E1 where E1= stator voltage and E1 α E2 i.e. Φ α E2
I2r =
s E2
R2² + (s X2) ²
AND
cosΦ2r = R2
=
Z2r
R2
R2² + (s X2) ²
P2
AC input
power P1
Pm
Stator copper
and iron loss
Po
Pout
Gross
mechanical
power
electrical input
power to rotor
Rotor copper
loss
Fnction and
windage
losses
(mechanical
losses)
Efficiency of an induction motor:
% ηm = Pout * 100
Pi
There are 3 types of starters.
 Only for cage motor:
1. stator resistance starters.
2. star delta starter..
3. Autotransformer starter.
 Only for slip ring motor:
1. Rotor resistance starter
 For both types of motors:
1. D.O.L starter.



1.
2.
3.
4.
5.

1.
2.
3.
4.
5.
6.
Speed control techniques:
Speed control techniques for squirrel cage induction motor:
stator voltage control.
V/T control .
Variable f,constant V control.
Variable current, constant F control.
Controlling number of poles.
Speed control techniques for wound rotor motor:
stator voltage control.
Variable f,constant V control.
Variable current, constant f control.
Constant resistance control.
Rotor resistance control.
Slip power recovery systems (cascade control) controlling number of
poles.
There are two test:
1. No load test.
2. Blocked rotor test.
1. No load test:
the set up for no load
test is shown below in figure…
The squirrel cage induction motor have small or moderate torques and it is
not possible to use external resistances for increasing the starting torque.
 These are constant speed motor.
 so these motors are preferred for the following applications:
1. Fans and motors.
2. Grinders.
3. Drilling machines.
4. Lathe machines.
5. Blowers and Water pumps .
6. Printing machines.

Application of Slip ring induction motor:
1. Hoist .
3. Lifts.
5. Elevators.
2. Cranes.
4. Compressors.