A 440-V, 60-Hz, six-pole, wye-connected, squirrel-cage induction motor with a full-load speed of 1170 r/min has the following parameters per phase referred to the stator: R₁ = 0.2 , R'₂ = 0.1 , X_l1 = 0.75 , X_l2 = 0.7 , and X_m = 20. Consider the motor to be fed by a six-step inverter, which in turn is fed by a six-pulse, fully controlled rectifier.

(a) Let the rectifier be fed by an ac source of 440 V and 60 Hz. Find the rectifier firing angle that will obtain rated fundamental voltage across the motor.

(b) Calculate the inverter frequency at 570 r/min and rated torque when themotor is operated at a constant flux.

(c) Now let the drive be operated at a constant V/f ratio. Compute the inverter frequency at 570 r/min and half the rated torque. Neglect the derating due to harmonics and use the approximate equivalent circuit of Figure, with jXm shifted to the supply terminals.