Abstract: A chopper and a series circuit of a dynamic braking resistor and a thyristor are connected in parallel with a d.c. motor which is operated as a generator during braking operation. A series circuit of a diode and an inductance element is connected in series with the d.c. motor. A condenser connected between the ground and the junction of the inductance element and the diode forms a filter circuit together with the inductance element. Current generated by the d.c. motor flows into a d.c. source through the diode and the inductance element during regenerative braking operation. An over-voltage detector fires the braking thyristor when the voltage across the condenser becomes greater than a predetermined limit value, as a result the dynamic braking operation is performed.A chopper controller varies the interval between an ON pulse to turn the chopper into ON state and an OFF pulse to turn the chopper into OFF state for adjusting ON time and OFF time of the chopper in response to the current through the d.c.