Page 116 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

numbers 2, 5, 8, 11, 14, etc.) develop magnetic fields and currents that rotate in a direction opposite to the positive
frequency set. Zero sequence harmonics (harmonic numbers 3, 9, 15, 21, etc.) do not develop usable torque, but
produce additional losses in the machine.
Cogging or magnetic locking
The rotor of a squirrel-cage motor sometimes refuses to start at all, particularly when the voltage is low. This
happens when the number of stator teeth S1 is equal to the number of rotor teeth S2 and is due to the magnetic
locking between the stator and rotor teeth. That is why this phenomenon is sometimes referred to as teeth-
locking.It is found that the reluctance of the magnetic path is minimum when the stator and rotor teeth face each
other rather than when the teeth of one element are opposite to the slots on the other. It is in such positions of
minimum reluctance, that the rotor tends to remain fixed and thus cause serious trouble during starting. Cogging
of squirrel cage motors can be easily overcome by making the number of rotor slots prime to the number of stator
slots and skewing of the rotor conductors.
Method of speed control of 3 phase induction motor

Objectives: At the end of this lesson you shall be able to:
• list the speed control methods from stator and rotor side
• explain the speed control methods of 3 phase induction motor.

In 3 phase induction motor, speed can be controlled from both stator and rotor side

1 Speed control methods from stator side
• By changing the applied voltage
• By changing the applied frequency
• By changing the number of stator poles

2 Speed control from rotor side
• Rotor rheostat control
• Cascade operation
• By injecting EMF in rotor circuit
1 Speed control from stator side

a By changing the applied voltage: Torque equation of induction motor is
k sE 2 R
T 1 2 2
2
R Xs 2 2
2
3 sE 2 R
2 2
2 N s R Xs 2 2
2
2
Rotor resistance R2 is constant and if slip s is small then sX2 is so small that it can be neglected. Therefore, Ta
sE where E is rotor induced emf and E a V
2
2
2
2
And hence T a V , thus if supplied voltage is decreased, torque decreases and hence the speed decreases.
2
This method is the easiest and cheapest, still rarely used because-
1 A large change in supply voltage is required for relatively small change in speed.
2 Large change in supply voltage will result in large change in flux density, hence disturbing the magnetic
conditions of the motor.

b By changing the applied frequency: Synchronous speed (Ns)of the rotating magnetic field of induction
motor is given by,
120 f
N rpm
s
P

103

CITS : Power - Electrician & Wireman - Lesson 70-75 CITS : Power - Electrician & Wireman - Lesson 70-75