1.

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»

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%.


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