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 method and control system for minimizing peak current levels in a direct current traction motor drive system in which the traction motor is powered by periodic current pulses metered from a direct current source by a controllable switch. The control system includes apparatus for comparing actual motor current to commanded motor current and for controlling the duty factor of the controllable switch in a manner tending to minimize any difference between actual and commanded currents. The control system further includes apparatus for changing the repetition rate of the current pulses in response to the magnitude of current in the motor, without effecting the commanded duty factor of the controllable switch, so that the ripple currents which are inversely related to repetition rate are reduced as average motor current is increased to thereby maintain total peak current levels within predetermined limits.

Abstract: Method and apparatus for effecting control of a complementary impulse commutated inverter to enable start-stop operation without power removal and to avoid start-up failures caused by inductive load flux saturation. The invention utilizes a gating logic circuit to always start inverter operation by gating a first predetermined switching element. The gating logic circuit responds to a stop command by terminating operation in a half-cycle in which the first predetermined switching element is not carrying load circuit. In one embodiment upon receipt of a stop command the gating logic circuit terminates inverter operation by applying a last gating signal to the first predetermined switching element, the last gating signal having a time duration less than that required after receipt of a gating signal for current reversal in the inductive load.

Abstract: Apparatus for limiting the peak magnitude of voltage applied to a chopper excited load to the magnitude of the source voltage. In a system including a chopper, a smoothing reactor and a load serially connected to a direct current power source, a clamping circuit is connected in parallel with the chopper and smoothing reactor and provides a current path between the load and the direct current source when load voltage attempts to exceed source voltage as a result of commutation transients in the voltage-turn-off chopper. Where the load is a direct current motor, the clamping circuit in combination with a freewheeling diode provides a suppression circuit for power source transients.