A 100 hp 60 Hz six pole 460 V three phase wound rotor induction motor develops a full-load torque at 1152 r/min with the rotor short-circuited. An external noninductive resistance of 1 Ohm is placed in series with each phase of the rotor, and the motor is observed to develop its rated torque at a speed of 1116 r/min. Calculate the resistance per phase of the original motor.
Synchronous speed n1: n1 = (120f1)/pole numbers
f1 is the system frequency f1 = 60Hz, with the total pole numbers at 6 poles.
n1 = (120*60)/6 = 1200 r/min
Slip S: S = (n1-n)/n1 where n is the motor speed,
For n = 1152 r/min S1 = (1200-1152)/1200 = 0.04
For n = 1116 ...
The answer calculates the exact resistance per phase of the motor, given certain parameters of torque speed and resistance. This is solved using formulas for speed and properties of electromagnetic torques.