Consider the 220-V, 1800-r/min dc motor, controlled by a three-phase fully controlled rectifier from a 60-Hz ac source. The armature-circuit resistance and inductance are 1.5  and 30 mH, respectively.

(a) When the motor is operating in continuous conduction, find the ac source voltage required to get rated voltage across the motor terminals.

(b) With the ac source voltage obtained in part (a), compute the motor speed corresponding to α = 60° and T_a = 25 N·m assuming continuous conduction.

(c) Let the motor drive a load whose torque is constant and independent of speed. The minimum value of the load torque is 1.2 N·m.

Calculate the inductance that must be added to the armature circuit to get continuous conduction for all operating points, given that ψ = tan^-1 (ωL_a/R_a) = 1.5 rad, T_an > 0.006, and all points on the T_an-ω_mn plane lie to the right of the boundary between continuous and discontinuous conductions.