Red Seal Electrician Practice
Motor controlsQuestion 1273 of 1506

Why can the autotransformer in a reduced-voltage starter be much smaller than a continuous-duty unit?

  1. It carries less than half the motor current while running.
  2. It runs at a lower frequency than the motor it feeds.
  3. It only supplies the control circuit of the starter.
  4. It is only in service for a short start interval.
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Correct answer: It is only in service for a short start interval.

The autotransformer works only during the short start and is then switched out. So it can be sized smaller than a unit that runs all the time.

Source: Lessons in Electric Circuits II: AC (Kuphaldt), 13.7: Tesla Polyphase Induction Motors

Practice motor controls

Questions last reviewed: 18 October 2026