A 10-pole induction motor is supplied by a 6-pole alternator, which is driven at 1400 rpm. If the motor runs with a slip of 5%, what is its speed?

• A. 1575 rpm
• B. 1425 rpm
• C. 798 rpm
• D. 882 rpm

Answer: (C)

Slip of the induction motor,
S = (Ns - Nr)/Ns
Ns = (120*f)/P
Poles (P) = 6

1400 = (120*f)/6
f = 70 Hz
For P = 10
Ns = (120*70)/10
= 840 rpm
Nr = Ns(1 - S)
= 840*(1 - 0.05)
= 798 rpm