What is the slip value for a synchronous motor at its operating speed?

• A. 0
• B. 0.1
• C. 1
• D. 2

Answer: (A)

Slip is how far the rotor lags behind the stator’s rotating magnetic field. In a synchronous motor, the rotor locks to that rotating field and turns at the same speed as the synchronous speed set by the supply frequency and pole count. When the motor is running at its designed operating speed, there’s no relative motion between rotor and field, so the slip is zero. Mathematically, s = (Ns − Nr) / Ns, and at steady-state Nr equals Ns, making s = 0. In contrast, nonzero slip values would indicate the rotor is not yet synchronized with the field, which is not the operating condition for a properly running synchronous motor.