A 220V, 750 rpm 200A separately excited dc motor has an armature & field resistances of 0·05Ω & 20Ω respectively. Load torque is given by the expression TL=500-0·2N N-m where N is the speed in rpm. Speeds below-rated value is obtained by armature voltage control with a full field and the speeds above rated are obtained by field control at rated voltage. The armature is fed from a 3 phase fully controlled rectifier with ac source voltage (line) of 170V, 50Hz, and the field is fed from a half-controlled single-phase rectifier with a single-phase source voltage of 250V, 50Hz. 

Drive operates under continuous conduction. 

Calculate firing angles for the speed of 

a) 600 rpm 

b) 1200 rpm.