Red Seal Electrician Practice
MotorsQuestion 1238 of 1506

What is the main difference between a synchronous motor and a brushless DC motor?

  1. The synchronous motor has brushes, the brushless DC motor has none.
  2. The synchronous motor runs on DC, the brushless DC runs on line AC.
  3. The synchronous motor has a sine-wave back EMF, the brushless DC a trapezoidal one.
  4. The synchronous motor has a squirrel cage, the brushless DC a wound rotor.
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Correct answer: The synchronous motor has a sine-wave back EMF, the brushless DC a trapezoidal one.

Both have a rotating stator field and a magnetic rotor. The back EMF shape is the difference.

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

Practice motors

Questions last reviewed: 18 October 2026