Find the frequency of the motor in hertz. Most commercial motor operate at between 50 and 60 hertz. In addition, count the number of winding poles inside the motor.
Multiply the motor's frequency by the constant 120. Using the example of a motor that operates at 55 hertz, the result would be 120 x 55 = 6600.
Take the result of the first part of the equation and divide by the number of poles in the motor. If the motor in the above example had four poles, the solution would be 6600 / 4 = 1650 RPM.