Product Manual

There are two basic drive types related to the type of motor controlled –
DC and AC. A DC direct current drive controls the speed of a DC motor
by varying the armature voltage (and sometimes also the field voltage).
An alternating current drive controls the speed of an AC motor by
varying the frequency and voltage supplied to the motor.
DC Drives
Direct current drives are easy to apply and technologically straightforward.
They work by rectifying AC voltage from the power line to DC voltage,
then feeding adjustable voltage to a DC motor. With permanent
magnet DC motors, only the armature voltage is controlled. The more
voltage supplied, the faster the armature turns. With wound-field
motors, voltage must be supplied to both the armature and the field.
In industry, the following three types of DC drives are most common:
DC SCR Drives: These are named for the silicon controlled rectifiers
(also called thyristors) used to convert AC to controlled voltage
DC. Inexpensive and easy to use, these drives come in a variety of
enclosures, and in unidirectional or reversing styles.
Regenerative SCR Drives: Also called four quadrant drives, these allow
the DC motor to provide both motoring and braking torque. Power
coming back from the motor during braking is regenerated back to the
power line and not lost.
Pulse Width Modulated DC Drives: Abbreviated PWM and also called,
generically, transistorized DC drives, these provide smoother speed
control with higher efficiency and less motor heating. Unlike SCR drives,
-85-
A general-purpose DC
SCR drives family. From
left, NEMA 4/12 “totally
enclosed” version, chassis-
mount, NEMA 1 “open”
enclosure.