Discussion
Super-synchronous operation of a doubly fed machine requires slip power to
Answer: C
Correct answer: flow into the rotor from the supply
The rotor electrical power is \( P_r \approx -sP_{ag} \), so above synchronous speed the slip is negative and \( P_r \) takes the same sign as the air gap power. For a doubly fed machine motoring above synchronous speed that means power must be injected into the rotor from the supply, while below synchronous speed the slip power flows the other way and is recovered to the supply rather than burnt in a resistance. A doubly fed generator reverses both directions, since its air gap power flows outward.
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