PURPOSE: To bring the short-circuit switch of a charging resistor to a non-contact state by connecting the charging resistor in series with a capacitor connected together with a reactor between a DC motor controlled by means of a chopper and a power supply and connecting a semiconductor element in parallel with the charging resistor.

CONSTITUTION: A current collector 1 is connected to a main motor armature winding 8 through a filter consisting of a unit switch 2, a high-speed breaker 3, the reactor 6 and the capacitor 7. The charging resistor 5 is connected in series with the capacitor 7, and a thyristor 4A and a diode 4B are connected in parallel with the resistor 5. Consequently, the switch 2 is turned ON, and the capacitor 7 is charged through the resistor 5. The thyristor 4A is ignited and the resistor 5 is short-circuited at the point of time when charging is completed. The operation of the chopper 12 is started at the same time as short circuit. Accordingly, the short-circuit switch of the resistor can be brought to the non- contact state and a maintenance-free state. The device can be miniaturized in size and lightened in weight.