A 50 Hz, 3-phase induction motor has a full load speed of 1440 r.p.m. The number of poles of the motor are

A 50 Hz, 3-phase induction motor has a full load speed of 1440 r.p.m. The number of poles of the motor are

(A) 4. 
(B) 6.
(C) 12. 
(D) 8.

Explanatory answer:

We know, N = (1-S)Ns

1440 = (1-S) Ns

Therefore, Ns = 1440/(1-S)

Ns = 120F/P = 120*50/P = 6000/P

always Ns will be closer to N. i.e. 1440

Let P=2, Ns=3000 rpm (not close to N)
Let P=4, Ns=1500 rpm (close to N)

Therefore, P=4 for N=1440