PURPOSE: To inhibit inrush currents from an electric-car line by increasing shunt field currents in proportion to the speed of voltage rise of the electric-car line when the voltage of the electric-car line suddenly rises during the control of power travelling.

CONSTITUTION: When power travelling is controlled, a comparison amplification arithmetic circuit 16 controls the field chopper 10 in response to deviation between the output of a shunt field current pattern generating circuit 13 and the output of a shunt field current detector 12. When the voltage of the electric-car line suddenly rises, output proportional to the rate of the voltage is given to the comparison amplification circuit 16 from a differentiation circuit 21, the flow rate of the field chopper 10 is extended, and the shunt field currents are increased. Accordingly, the back electromotive force of a main motor 6 increases, and inrush currents from the electric-car line are inhibited.