Propulsion system

Summary form only given. The propulsion system of low-voltage dc (-600 V) transit cars is complicated by the continual breaking of heavy dc currents in the typical multistep motor controller. Semiconductor controllers have two difficulties: they are vulnerable to high voltage impulses, and they require the ministrations of electronic technicians who may be hard to attract to the railroad industry. The author suggests an arrangement using two old and well-proven devices to control the flow of power to the traction motors. One is the venerable synchronous converter running inverted, that is, converting dc to polyphase ac, for example, 12 phase ac. The second device is the polyphase variable autotransformer, receiving its power from the converter and driving separately excited dc traction motors through diodes. Coasting is effected by simply backing off the variable autotransformer.