PURPOSE: To suppress the rush current by inserting a motor-generator circuit between a filter reactor and a thyristor chopper and connecting it in parallel with a filter capacitor.

CONSTITUTION: Power on a line 2 is supplied to a main motor M through a pantograph 3, a switch 4, a filter reactor LF and a thyristor chopper 10. A motor-generator 1 is inserted between the filter reactor LF and the thyristor chopper 10 through a contactor 5 and is connected to a power supply in parallel with a filter capacitor CF. A rush current according to a boosting of voltage or a detachment of the pantograph from the line is now suppressed by inductance of the filter reactor, therefore an evil influence can be prevented from being exerted on the motor-generator.