Red Seal Electrician Practice
MotorsQuestion 1239 of 1506

A brushless DC motor must hold a constant speed. What is needed?

  1. A larger air gap between the rotor and the stator iron
  2. A phase locked loop tied to a reference frequency
  3. A centrifugal switch on the rotor shaft
  4. A wound rotor with slip rings and brushes
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Correct answer: A phase locked loop tied to a reference frequency

Its speed is not fixed by the line, so feedback to a reference is needed.

Source: Lessons in Electric Circuits II: AC (Kuphaldt), 13.6: Brushless DC Motor

Practice motors

Questions last reviewed: 18 October 2026