Electrical Machines I

A 400 V, 4-pole, 50 Hz, 3 phase slip ring induction motor has a star connected stator winding and a star connected rotor winding. At standstill the voltage between the two slip rings is 190V. The stator impedance is 0.5 + j2.5 Ω. The rotor resistance and reactance at standstill are 0.06 Ω and 0.3 Ω respectively. The stator to rotor turn ratio is 1: 1. The motor consumes 600 Watts at no load. Calculate: i) Speed at which the motor develops the maximum torque. ii) Efficiency of the motor when it is running at 1,380 rpm.