Synchronous speed is defined as the rotational speed of which component in an induction motor?

• A. The rotor
• B. The load
• C. The stator's magnetic field
• D. The rotor magnets

Answer: (C)

Synchronous speed is the speed of the rotating magnetic field produced by the stator. In an induction motor, the stator windings are fed with AC to create a magnetic field that rotates at a fixed speed, given by Ns = 120 f / P (where f is the supply frequency and P is the number of poles). The rotor then speeds up to follow this field but never quite reaches it due to slip, so the actual rotor speed is slightly less than Ns. Therefore, the rotational quantity defined as synchronous speed is the stator’s magnetic field, not the rotor, the load, or rotor magnets.