Induction motor:Rotor E.M.F. and Reactance Under Running Conditions

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Rotor E.M.F. and Reactance Under Running Conditions

Let

E2 = standstill rotor induced e.m.f./phase

X2 = standstill rotor reactance/phase, f2 = rotor current frequency at standstill

When rotor is stationary i.e. s = 1, the frequency of rotor e.m.f. is the same as that of the stator supply frequency. The value of e.m.f. induced in the rotor at standstill is maximum because the relative speed between the rotor and the revolving stator flux is maximum. In fact, the motor is equivalent to a 3-phase transformer with a short-circuited rotating secondary.

When rotor starts running, the relative speed between it and the rotating stator flux is decreased. Hence, the rotor induced e.m.f. which is directly proportional to this relative speed, is also decreased (and may disappear altogether if rotor speed were to become equal to the speed of stator flux). Hence, for a slip s, the rotor induced e.m.f. will be s times the induced e.m.f. at standstill.

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(i) rotor current at standstill with the rheostat is in the circuit.

(ii) when the slip-rings are short-circuited and motor is running with a slip of 3%.

(Elect.Engg. I, Nagpur Univ. 1993)

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