Answer Posted / subhasish(rimo) sengupta

f=1/T. when,T = time period for one full cycle.
in T seconds, 1 revolution is completed.
So, in 1 second,1/T revolutions or f full waves is formed.

If there are P no. of poles,then P/2 no. of full waves will be formed in one full revolution of the field.
Now, if n=rotation of the field in 1 second,
then f=(P/2)*n
or,n=2f/p.
So, N=(60*2*f)/P , (when N= rotation per min.)
or, N=120f/P.

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