Control system and method for operating a DC motor

A system applicable for use in an electric vehicle, for example, for controlling a separately-excited DC motor, permits field current control over all operating speed ranges of the motor. Below base speed a current limiting resistor is placed in series with the armature winding and the torque is directly proportional to field current. Above base speed, the resistor is shorted, and the speed is inversely proportional to field current. A feedback network is responsive to acceleration and deceleration input signals for controlling the magnitude of field current. A logic network provides automatic control, including switching battery interconnections, for changing the operating speed range of the motor by changing the level of voltage applied across the armature winding. Relatively low current switching of the batteries is provided by first summing with the error signal in the feedback loop a ramp signal, for forcing the magnitude of the armature current towards zero, second opening a switch to disconnect the batteries from the armature winding, third switching the batteries into a new circuit configuration, fourth a differential amplifier for subtracting from the new level of battery voltage the present armature voltage, produces an output signal having a level analogous to the difference therebetween for summing with the error signal for controlling the magnitude of field current to equate the battery and armature voltages, and lastly closing a switch to reconnect the battery configuration across the armature winding.