Patent ID: 8237384

Claim:
A method of switching a traction motor circuit between motoring and braking a vehicle, the vehicle comprising a pair of traction motors for moving the vehicle and generating electrical energy when the vehicle is decelerating, wherein each said traction motor circuit comprises a field coil and an armature coil, the field and armature coils being connected in series, and wherein each said traction motor circuit comprises a switchable contact reverser operable to switch electrical current in opposite directions of flow through the one of the armature and field coils while maintaining a same flow direction in the other of the armature and field coils; a power source to provide electrical energy to each said traction motor circuit; and a direct current (DC) bus interconnecting each said traction motor circuit and said power source where each traction motor circuit alternates with a free-wheeling current path and free-wheeling time of each traction motor circuit is controlled by selecting a duty cycle for a pair of IGBTs, the method comprising: a) connecting said pair of traction motors in series connection for low-speed motoring and low-speed braking; b) switching each said contact reverser to switch between low-speed motoring and low-speed braking; c) controlling free-wheeling time by selecting the duty cycle for said pair of IGBTs and preventing free-wheeling current buildup by voltage on said DC bus; and d) turning two IGBTs on at once for a buildup portion of the duty cycle and off for a free-wheeling portion of the duty cycle.