Question

A 10 kW, 400 V, 3-phase, 4-pole, 50 Hz induction motor develops the rated output at rated voltage with its slip rings shorted. The maximum torque equal to twice the full load torque occurs at the slip of 10% with zero external resistance in the rotor circuit. The slip at the full load torque will be?

(a) 2.7%

(b) 5%

(c) 3.7%

(d) 10%

The question was asked in an interview for job.

My enquiry is from Operating (or Performance) Characteristics of Induction Motors in section Induction Machines of Electrical Machines

Answer 1

Correct answer is (a) 2.7%

To explain I would say: Tfl/Tem = 2/((sm/sf)+ (sf/sm))

              1/2 = 2/((0.1/sf)+(sf/0.1))

     s^2-0.4s+0.01=0

     s=0.0268.