Speed of Induction motor is given by, N= 120f/p
I am searching the derivation of this formula.if anybody have an idea pl. help.
Nobody can help you derive that formula for an induction motor since it is the formula for a synchronous motor. Try Googling, if that fails look in any textbook on electrical machine theory.
This formula is much too easy to require a "derivation." It's just a matter of counting poles as they go by, one per each half of a sine wave.
Please refer to any good text book on AC Machines.
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