MotorsQuestion 1211 of 1506

Why does a small single-phase synchronous clock motor need an auxiliary starting motor or induction cup?

Its rotor is an electromagnet that first needs a separate DC supply to start.
Its stator has too many poles to make any useful field at 60 Hz.
Its coil field only pulses, so the rotor must be sped up first.
Its rotor has to be demagnetized by the supply before each start.

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Correct answer: Its coil field only pulses, so the rotor must be sped up first.

A pulsating field cannot start the rotor. The rotor must be near synchronous speed to lock in.

Source: Lessons in Electric Circuits II: AC (Kuphaldt), 13.2: Synchronous Motors

Questions last reviewed: 18 October 2026

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