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This is the speed at which a motor will run at a known voltage and frequency, called synchronous speed. The formula for obtaining synchronous speed is S = (120 X F) / P S = speed in rpm F = frequency in hertz P = number of poles ( Example: S = 120 x 60 = 7200 / 2 = 3600) Numbers of poles 60 hz speed 50 hz speed 2 3600 3000 4 1800 1500 6 1200 1000 8 900 750.