DC Drives Field Voltage Control

To control the speed of a DC motor below its base speed, the voltage applied to the armature of the motor is varied while the field voltage is held at its nominal value. To control the speed above its base speed, the armature is supplied with its rated voltage & the field is weakened. For this reason, an additional variable-voltage field regulator is needed for DC drives with field voltage control. Field weakening is the act of reducing the current applied to a DC motor shunt field. This action weakens the strength of the magnetic field & thereby increases the motor speed. The weakened field reduces the counter emf generated in the armature; therefore the armature current & the speed increase. Field loss detection must be pro vided for all DC drives to protect against excessive motor speed due to loss of motor field current.

DC drives with motor field control provide coordinated automatic armature & field voltage control for extended speed range & constant-horsepower applications. The motor is armature-voltage-controlled for constant-torque, variable-horsepower operation to base speed, where it s transferred to field control for constant-horsepower, variable-torque operation to motor maximum speed.