Three Phase Induction Motors
The injected e.m.f. in the rotor of induction motor must have

Zero frequency
The same phase as the rotor e.m.f.
High value for the satisfactory speed control
The same frequency as the slip frequency

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Three Phase Induction Motors
Slip of an induction motor is negative when

Rotor speed is less than the synchronous speed of the field and are in the same direction
Magnetic field and rotor rotate in opposite direction
Rotor speed is more than the synchronous speed of the field and are in the same direction
None of these

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