Electric vehicle traction motor control

Abstract From the Canadian patent

An electric vehicle traction motor control circuit includes a main thyristor in series with the motor armature. A second thyristor is connected in series with a commutating capacitor across the main thyristor which can be turned off by firing the second thyristor to divert the motor current momentarily into the commutating capacitor. A third thyristor is connected in series with an inductor across the commutating capacitor and serves on firing to reverse the voltage residing on the commutating capacitor following commutation. A circuit is connected to detect the cessation of current flow into the commutating capacitor and acts to fire the third thyristor hen such cessation is detected.