3 Phase Induction Motor-

Expression for Torque-

T=

(V^2)* (R2)/(w syn)((R eq)^2 +(Xeq)^2)s

Where V- Equivalent Thevenin resistance

R2- Rotor resistance referred to stator

w syn- Synchronous angular speed

R eq- Equivalent Thevenin Resistance plus Rotor resistance at given slip s

X eq- Equivalent Thevenin Reactance plus Rotor Reactance

3 phase Synchronous Motor/Generator

Torque T= V*Ef*sin(∆)/ Xs

Where

V- Applied Voltage

Ef- Induced Voltage

∆- Load/Torque Angle

Xs- Synchronous reactance

Thus when field excitation(Ef) is zero, then T for synchronous machine is zero but for induction machine T isn’t equal to zero!

Thus induction machines are self starting wheres Synchronous machines aren’t!

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Interview Questions

MCQ or objective

Yes, it is self starting. A magnetic field is induced in the Rotor which tries to catch up with that of the stator and makes the Rotor rotate.The Rotor is of Squirrel cage construction and produces magnetic Poles. Yes ! 3 phase induction motors are self starting and we need to incorporate a suitable starter device .

You are welcome to read more Important Interview Questions and MCQs.