Red Seal Electrician Practice
Motor controlsQuestion 1275 of 1506

Why must the voltage to an induction motor be decreased when a drive lowers the frequency?

  1. The reactance rises, so the current falls too far to start.
  2. The slip rises above 100%, so the rotor would lock.
  3. The reactance falls, so the current rises and the core could saturate.
  4. The rotor stops acting as a squirrel cage at low frequency.
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Correct answer: The reactance falls, so the current rises and the core could saturate.

Lower frequency means lower reactance, so the voltage must follow it down.

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

Practice motor controls

Questions last reviewed: 18 October 2026