Dc motors are energized or started by manually operated switches or magnetically operated contactors. Manually operated switches handle a modest range of motor currents. Magnetically operated contactors usually provide a safer and more convenient means for controlling motor power circuits, particularly in large horsepower ratings.
The inrush currents of line-started dc motors are limited only by the "at-rest" resistance of the circuit and may be ten times the full-load current. Because dc motors have commutators that can be damaged by excessive starting currents, full-voltage starting is not always possible, except for small motors that have relatively high winding resistance. Compound-wound motors up to 7 hp and other dc motors up to 2 hp can usually be line started.
A dc motor can commutate more current at low speeds than at high speeds. With the armature practically standing still, most commutators handle currents as high as six times the full-load rating of the motor without excessive sparking.
A starting or current-limiting resistor must be added for larger motors. Resistance is gradually shorted out as the motor accelerates. The motor counter-emf increases with speed, thus reducing the current. When the motor reaches its base or normal operating speed, all starting resistance is shorted out, and the motor draws its normal running current.
If magnetic starters are used for acceleration, starting resistance can be shorted out in one or more steps, depending on motor size and the smoothness of acceleration desired. There are three methods for obtaining automatic acceleration: time-limit, current-limit, and speed acceleration.
DC Motor Drives: Direct-current motors are used for adjustable-speed drives because their speed is easy to adjust. The speed of a dc motor is a function of the armature and field voltages. Control and adjustment of armature voltages result in a constant-torque drive, while constant horsepower is obtained by field-voltage control.
Dc drives consist of a controlled-voltage power supply, one or more dc motors, and a feedback system which controls dc voltage as a function of speed.
Adjustable-voltage dc drives are equally applicable to single or multiple-motor applications. The power-conversion unit can be sized to supply a single motor or several motors as long as the speed of all motors can be adjusted according to a common signal. Motors do not have to operate at the same speed, however, since adjustable resistors can vary the motor field voltage.
Semiconverters have diode rectifiers in one side of the bridge and thyristors in the other side. A commutating diode is placed across the output. Semiconverter circuits provide unidirectional current and voltage, and cannot be used for reversing or regeneration (where power is fed back into the circuit by the motor). The motor can be reversed by contactors, and automatic dynamic braking (power generated by the motor is fed to a resistor grid) can be furnished for slowdown and stopping.
Converters have thyristors in all legs of the bridge, and can be used for reversing and regeneration applications. These units impose less ripple current on the motor than semiconverters.