Abstract: A method and apparatus for detecting and correcting the occurrence of flashover in a direct current electric motor. The polarity and magnitude of voltage on the field winding of the motor is continuously monitored. Detection of a sudden reversal of the polarity of the field voltage accompanied by a rapid increase in magnitude results in the generation of a flashover signal. The flashover signal is coupled to power controlling elements regulating power supplied to the motor and is effective to force removal of power from the motor. In a preferred embodiment the apparatus is disclosed in a two-motor system with two field windings serially connected such that the field voltage monitoring is accomplished on a differential basis.

Abstract: Apparatus for increasing the available regenerative braking energy and for enabling low current control without frequency variation in a time-ratio controlled main thyristor power circuit for a direct current traction motor. A variable impedance element is serially inserted in a motor armature free-wheeling current path between a free-wheeling diode and the motor armature, the variable impedance element preferably comprising a power resistor and parallel connected thyristor. During electrical regenerative braking the thyristor is used to modulate the power resistor to thereby present a continuously variable impedance in the armature current path. During motoring at very low speeds the thyristor can be operated to modulate the power resistor and thus reduce the minimum controllable level of motor current normally limited to commutation current level without the necessity of reducing the main thyristor operating frequency.

Abstract: A d-c motor drive system, useful for propelling and retarding electrically propelled traction vehicles, wherein the armature winding has a first terminal coupled through an armature chopper to a first polarity conductor of a d-c source, and a second terminal coupled through a breaker switch to a second polarity conductor of the d-c source. First and second controlled rectifiers are connected across the first and second conductors in a self commutating circuit such that they conduct sequentially and alternatively. Means are provided for varying their respective conduction time intervals to control the voltage at their junction. Field winding means are coupled from the junction of the controlled rectifiers to the junction of the armature second terminal and the breaker. During the motoring mode with the breaker closed field current flows in a first direction.