Optimum battery reconnect for a field controlled electric vehicle

Abstract

An electric vehicle having a field controlled direct current drive motor powered by batteries which are connectable in a high voltage and a low voltage configuration to change the speed range of the drive motor. Switching of the batteries between the high and low voltage connection is accomplished to provide a relatively smooth transition in both directions and to provide for optimum performance and efficiency. Logic is provided to determine the minimum speed at which high voltage operation is feasible and for initiating switching to the high voltage connection at the determined minimum speed. The minimum speed for operation at the high battery voltage connection is determined as a function of battery open circuit voltage, battery resistance, armature resistance, armature current and field flux. When for the known parameters operation at the high voltage connection is possible, the batteries are switched to the high voltage level. A throttle is provided for controlling the speed of the electric vehicle. At certain lower throttle settings, when the batteries are switched from the low voltage connection to the high voltage connection, the field current is varied to match the new torque at the high voltage connection with the torque previously obtained at the low voltage connection. At certain higher throttle settings, it is not practical to match torque before and after switching when high performance is desired. At these higher throttle settings, torques are not matched and batteries are switched to the high voltage connection at the minimum speed allowing for adequate vehicle control. The torque in the new high voltage connection is then greater than the torque in the prior low voltage connection giving high performance and rapid acceleration.