Find out which of the two motors:

Two three-phase induction motors while connected across a 400 V, 50 Hz supply are running at 1440 and 940 RPM respectively. Find out which of the two motors is running at higher slip.

Solution

We know that f = 50 Hz and we may determine the synchronous speed for

P = 2, N _s  = (120 × 50 )/2 = 3000 RPM

P = 4, N _s   = (120 × 50 )/4 = 1500 RPM

P = 6,     N _s   = (120 × 50 )/6 = 1000 RPM

An induction motor runs at a speed somewhat less than the synchronous speed. Numbers of poles of the motor running at 1440 rpm must be 4 and N_s = 1500 rpm.

% Slip of the motor (A),

S_A   = ((N_s - N_r) /N_s) × 100 = (1500 - 1440/1500) × 1000 = 4%

Numbers of poles of the motor running at 940 rpm must be six and

N_s = 1000 rpm.

The slip of the motor (B),

s _B   = (1000 - 940 )/1000= 6%

Therefore the slip of the motor running at 940 rpm is higher than the slip of the motor running at 1440 rpm.