An induction motor can run at synchronous speed when____________?
Correct answer: D. e.m.f. is injected in the rotor circuit
- A. it is run on load
- B. it is run in reverse direction
- C. it is run on voltage higher than the rated voltage
- D. e.m.f. is injected in the rotor circuit
Explanation
At synchronous speed, slip and the naturally induced rotor emf are zero, so an ordinary induction motor cannot produce torque there. Injecting a suitable emf into the rotor circuit can maintain rotor current and torque at synchronism.
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