Abstract: A d.c. motor whose armature is supplied from an a.c. power source through an a.c./d.c. thyristor converter and is subject to damaging commutator flash-over, as from loss of a.c. voltage during regeneration is provided with a protective circuit which includes a normally charged capacitor and a diverter resistor which are connectable across the converter and motor armature, respectively, by SCR means in response to a fault condition such as either predetermined high output current from the converter (i.e., indicative of high armature current) or excess a.c. input current to the converter. Connection of the capacitor across the converter and its discharge instantaneously and forcibly but temporarily blocks current flow from the converter thyristors to the motor armature.