Question

For a three phase induction motor, maximum torque is double the full load torque and starting torque is 1.6 times the full load torque. To get a full load slip of 5%, percentage reduction in the rotor resistance should be?

(a) 62.7%

(b) 75%

(c) 60%

(d) 35%

This question was posed to me in unit test.

I want to ask this question from Operating (or Performance) Characteristics of Induction Motors in portion Induction Machines of Electrical Machines

Answer 1

Correct answer is (a) 62.7%

To explain I would say: Slip at which maxim torque occurs is calculated as smT=0.5.

             r2/x2=0.5

    for full load slip of 0.05,  1/2 = 2/((0.05/sf)+(sf/0.05))

    smT2 = 0.186

    reduction in resistance = (0.5×2-0.186×2)/0.5×2 = 63%.