Red Seal Electrician Practice
MotorsQuestion 1268 of 1506

At what load is the efficiency of a polyphase induction motor highest?

  1. Near 10% of full load torque
  2. Near 150% of full load torque
  3. At no load
  4. Near 75% of full load torque, not at full load
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Correct answer: Near 75% of full load torque, not at full load

Efficiency at 75% can be a little higher than at 100%, and it falls only slowly down to 25%.

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

Practice motors

Questions last reviewed: 18 October 2026