Red Seal Electrician Practice
MotorsQuestion 1304 of 1506

Why does a 2-phase AC servo motor have a rotor with the iron core removed, leaving an aluminum cup?

  1. It raises the starting current.
  2. It lets the motor run on DC.
  3. It increases the inertia for smooth speed.
  4. A low mass rotor accelerates faster.
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Correct answer: A low mass rotor accelerates faster.

Less inertia gives fast response in servo loops.

Source: Lessons in Electric Circuits II: AC (Kuphaldt), 13.10: Other Specialized Motors

Practice motors

Questions last reviewed: 18 October 2026