Abstract: A controlled switch 4 provides current pulses derived from a battery 3 to a motor 1, having a series field 2 under command of a control unit 8 which provides command pulses for repetitive closing of the controlled switch 4. Current through the motor is sensed by a sensing resistor 6 and connected to a safety circuit 9 which, upon sensing of current flow in the absence of a control pulse from the unit 8, disconnects current to the motor by opening a controlled switch 5. To distinguish between (a) current flowing from the battery 3 l and (b) circulating current flowing during pulse gaps in a free-wheeling diode 7 connected across the field and armature of the motor, voltage across the free-wheeling 7 is sensed, thereby effectively sensing the conduction state of the free-wheeling diode and, if the free-wheeling diode is conductive, disabling the safety circuit to prevent spurious operation thereof during flow of inductive, circulating current through the bypass or free-wheeling diode under normal operation.