For three-phase induction motors which of the following is the least efficient method of speed control ?
Correct answer: C. Rheostat control
- A. Cascade control
- B. Pole changing
- C. Rheostat control
- D. Combination of cascade and pole changing
Explanation
Rheostat or rotor-resistance control wastes substantial power as heat in the external resistance, so its efficiency is low. Cascade, pole-changing and combined methods use the motor power more effectively.
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