Electrical Drives
DC Controllers
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.
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