In a synchronous motor, rotor speed is what relative to the magnetic field?

• A. Rotor lags behind the magnetic field
• B. Rotor turns at the same speed as the magnetic field
• C. Rotor speed is independent of the magnetic field
• D. Rotor runs twice the magnetic field speed

Answer: (B)

In a synchronous motor, the rotor is designed to lock onto and rotate with the stator’s rotating magnetic field. The stator field spins at the synchronous speed set by the supply frequency and the number of poles, and the rotor’s DC-excited magnetic field aligns with that rotating field. Once in step, the rotor follows the field exactly, so there is no slip. Therefore, the rotor speed matches the magnetic field speed.