Electrical – How to find the slip speed of a slip ring induction motor without knowing the rotor speed (rated RPM)

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The Rotor of a 4 pole, 50hz slip ring induction motor has a resistance of 5ohm per phase and inductance of 150mH per phase. Calculate the slip speed at which max torque occurs.

I have used Ns = 120f/p to find the synchronous speed, being 1500RPM, but I cannot calculate the Slip speed without knowing the rated RPM of the motor.

Is there a way to calculate this using the given information?

UPDATE:
I believe this is the answer.

ns = 120f/p
= 120×50/4
= 1500 rpm

fr = X/2piL
= 5/(2pi x 0.150)
= 5.3hz (or rpm?)

Slip% = fr/f
= 5.3/50
= 0.106

Slip speed = slip% x ns
= 0.106 x 1500
= 159rpm at 10.6% slip (0.106×100)

Best Answer

Homework, I guess, a theoretical one which assumes ideal stator and no substantial saturation nor stray inductance effects.

The general induction motor theory tells: In that ideal case max. torque occurs at slip=R/X where R is one phase resistance (=load + rotor wire) and X=one phase rotor reactance. Actually also negative slip = -(R/X) is valid for max torque; that has meaning for generators.