Red Seal Electrician Practice
MotorsQuestion 1254 of 1506

Why can an induction rotor never run exactly at synchronous speed and still produce torque?

  1. At that speed the stator field would stop rotating in space.
  2. At that speed no flux cuts the rotor, so no torque is made.
  3. At that speed the rotor winding opens up by itself and stays open.
  4. At that speed the line frequency doubles at the stator terminals.
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Correct answer: At that speed no flux cuts the rotor, so no torque is made.

Torque depends on the flux cutting the rotor, and that needs slip.

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

Practice motors

Questions last reviewed: 18 October 2026