Question

What is slip

What are the factors on which the torque of an induction motor depends?

Slip

In practice, the rotor never succeeds in 'catching up' with the stator field. If it really did so, then there would be no relative speed between the two, hence no rotor e.m.f., no rotor current and so no torque to maintain rotation. That is why the rotor runs at a speed which is always less than the speed of the stator field. The difference in speeds depends upon the load on the motor.

The difference between the synchronous speed Ns and the actual speed N of the rotor is known as slip. Though it may be expressed in so many revolutions/second, yet it is usual to express it as a percentage of the synchronous speed. Actually, the term 'slip' is descriptive of the way in which the rotor 'slips back' from synchronism.

% slip N=S-N/Ns

X 100

Sometimes, Ns—N is called the slip speed. Obviously, rotor (or motor) speed is N=Ns(l—s).

Torque of induction motor

Starting torque of an induction motor depends on the supplied voltage rotor resistance and rotor reactance

And can be given as follow

Ts α (Vs)2 (R2) / (R2)2 + (X2)2

V = voltage supplied.

R2 And X2 = rotor reisitasnce and reactance respectively

If supply voltage is altered it changes the standing torque considerably

While running torque is given as

Tr α (Vs)2 (R2) / (R2)2 + (SX2)2 where S is slip