Adjustable-Voltage Shunt-Wound Motor Drive
Due to the requirement of DC power, the application of shunt-wound DC motors has been limited. Extensive use of the shunt-wound motors has been made possible by a combination drive that includes a means of converting AC to DC. A self-contained unit may achieve the conversion of AC to DC. This consists of a separately excited DC generator driven by a constant-speed AC motor, connected to the regular AC supply. An electronic rectifier with suitable controls, connected to the regular AC supply, can also achieve this. The conversion of AC to DC with an electronic rectifier has the advantage of causing no vibrations.
In an adjustable-speed, shunt-wound motor drive, speed control is affected by varying the voltage applied to the armature while supplying a constant voltage to the field.
In addition to providing for the adjustment of the voltage supplied by the converter, to the armature of the shunt-wound motor, the amount of current passing through the motor field may also be controlled. In fact, a single control may be provided to vary the motor speed, from a minimum base speed, by varying the current flowing through the field. With such control, the motor operates at a constant torque up to the base speed and at constant horsepower above the base speed.
Adjustable-speed shunt-wound motor drives are also called DC-adjustable voltage drives. These drives are used for milling machines, boring mills, lathes, and other industrial applications where wide, step-less speed control, uniform speed under all operating conditions, a constant torque acceleration and adaptability to automatic operational control are required.