Red Seal Electrician Practice
MotorsQuestion 1215 of 1506

Why is a synchronous motor not self-starting on a fixed 50 or 60 Hz supply?

  1. The field does not rotate at all until the rotor has reached its full speed.
  2. The supply frequency is too low to magnetize the stator iron.
  3. The rotor field has to be reversed at each new start.
  4. The field already turns at line speed, so a stopped rotor cannot lock to it.
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Correct answer: The field already turns at line speed, so a stopped rotor cannot lock to it.

A stopped rotor cannot follow a field that already turns at line speed, so it needs help to get up to speed.

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

Practice motors

Questions last reviewed: 18 October 2026