Discussion
Consider the following statements about induction motor speed control.
1. Stator voltage control wastes power in the rotor.
2. V/f control keeps the flux nearly constant.
3. Rotor resistance control suits cage machines.
Which are correct?
Answer: A
Correct answer: 1 and 2 only
Statement 1 is correct: reducing the stator voltage lowers the speed by raising the slip, and the rotor loss \( sP_g \) rises with it. Statement 2 is correct: holding \( V/f \) constant holds \( E/f \), and therefore the flux, nearly constant. Statement 3 is wrong: rotor resistance control needs access to the rotor circuit through slip rings, so it applies only to wound rotor machines, never to a cage machine whose rotor bars are permanently short-circuited.
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