Question

A three phase induction motor is operating at full load at the difference (slip) between synchronous and non-synchronous speeds s. During operation, if the phase sequence of the supply is reversed, what will be the difference (slip) between synchronous and asynchronous speed immediately after reversal?
1. 2 - s
2. 1 + s
3. 1 - s
4. s

Answer 1

Correct Answer - Option 1 : 2 - s

Concept of slip:

• The speed of the rotor structure with respect to stator structure is Nr
• With the rotor running at Nr, the relative speed of the stator rotating magnetic field with respect to the rotor conductors is Ns - Nr in the direction of Ns 
• This relative speed is referred to as slip speed.

Where Ns is the synchronous speed of the machine

\(N_s =\frac{120f}{p}\)

Where,

f is the frequency of the machine

p is the number of poles of the machine

Slip speed = Ns - Nr

\(slip,s = \frac{(N_s - N_r)}{N_s}\)

If the rotor is made to revolve in a direction opposite (backward) to the rotating magnetic field by reversing the supply:

The relative speed between the rotor winding and the rotating magnetic flux becomes Ns + Nr 

Backward slip,

\(s_b=\frac{(N_s + N_r)}{N_s} = \frac{2N_S-(N_s-N_r)}{N_s}=2 - s\)