A dc machine, operating as a generator, develops 400 V at its armature terminals, corresponding to a field current of 4 A, when the rotor is driven at 1200 r/min and the armature current is zero.

(a) If the machine produces 20 kW of electro- magnetic power, find the corresponding ar mature current and the electromagnetic torque produced.

(b) If the same machine is operated as a motor supplied from a 400-V dc supply, and if the motor is delivering 30 hp to the mechanical load, determine: (i) the current taken from the supply, (ii) the speed of the motor, and (iii) the electromagnetic torque, if the field current is maintained at 4 A. Assume that the armature circuit resistance and the mechanical losses are negligible.

(c) If this machine is operated from a 440-V dc supply, determine the speed at which this motor will run and the current taken from the supply, assuming no mechanical load, no friction andwindage losses, and that the field current is maintained at 4 A.