Does the rotor of a synchronous motor rotate at synchronous speed?

• A. No
• B. Only under no-load
• C. Only at stall
• D. Yes

Answer: (D)

A synchronous motor’s rotor runs at synchronous speed as long as it remains synchronized with the stator’s rotating magnetic field. The rotor is DC excited, and once it locks into that field, it turns at the same speed as the rotating field set by the supply frequency and the machine’s pole count. That speed, the synchronous speed, is determined by ns = 120 f / P, so under normal operation the rotor indeed rotates at synchronous speed regardless of load (until you lose synchronism or stall). So the correct idea is that, when synchronized, the rotor matches the synchronous speed.