Regenerative braking protection for an electrically-propelled traction

Abstract

In an electrical propulsion system on board a moving traction vehicle (such as a rapid transit rail car) having current collectors in contact with an energized third rail, controllable switch means is used to connect the system to the current collectors. From time to time the current collectors traverse gaps between adjacent sections of the third rail, in which event regenerative braking operation of the system will be prevented unless the next third rail section is energized by a wayside source of voltage. The switch means comprises the parallel combination of a diode that conducts current when the system is operating in a motoring mode and a reversely-poled thyristor that conducts regenerative braking current once a firing signal is applied thereto. To detect the energized status of the third rail, an electrical filter network is excited by the third rail voltage and derives an output value representative of the amplitude of an a-c ripple component of such voltage having a fundamental frequency that characterizes the wayside voltage source, and after each third rail gap is traversed a firing signal is applied to the thyristor as soon as this value exceeds a predetermined minimum level.