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.

Abstract: Variable pulsing of a dc source without change of polarity is used for lossless control of the speed of a compound dc motor. The pulsed dc supply is fed in series through the series field winding, armature and reversing switch system and the shunt field winding has one terminal connected to the pulsed dc source and its other terminal connected respectively through two conductively poled diodes to the opposite terminals of the armature winding. Two anti-kickback devices are also provided, one of which operates during reverse-current braking. The reversing switch system has two independently actuatable single-pole double-throw switches of which the respective arm contacts are connected to the ends of the armature winding, one fixed contact of each of these reversing switches being connected to the series field winding and the other pair, likewise connected together, being connected to one terminal of the dc supply.