Question

Explain why an induction cannot develop torque when running at synchronous speed? Define the slip speed of an induction motor and deduce how the frequency of the rotor currents and magnitude of the rotor emf are related to slip?

Answer 1

If induction motor run at synchronous speed, then the relative speed of of the rotor flux and stator flux will be zero, hence the torque angle will be zero and subsequently the motor fails to develop torque.

slip speed is defined as the difference between the synchronous speed and actual speed of the motor.

slip speed = Ns - Nr

The ratio of slip speed to synchronous speed is defined as slip of the motor.

s = (Ns-Nr)/Ns

The frequency of the rotor current will be reduced directly corresponding to slip.

fs ∝ Ns

fr ∝Ns - Nr

=> fr/fs = Ns-Nr/Ns

=>   Fr = sFs

Similarly the magnitude of the rotor emf will be reduced directly corresponding to slip.

Es ∝ Ns

Er ∝Ns - Nr

=> Er/Es = Ns-Nr/Ns

=>  Er = s Es