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Ac Primary Resistor
In an ac primary resistor controller, resistors are
A motor controller protects a motor from damage,
inserted in the primary circuit of an ac motor for both
starts or stops it, increases or decreases its speed, or
starting and speed control. Some of these controllers
reverses its direction of rotation.
only limit the starting currents of large motors; others
control the speed of small motors, as well as limit the
starting current.
A manual (nonautomatic) controller is operated by
Ac Secondary Resistor
hand directly through a mechanical system. The
operator closes and opens the contacts that normally
In an ac secondary resistor controller, resistors are
energize and de-energize the connected load.
inserted in the secondary circuit of a wound-rotor ac
motor for starting or speed control. Although they are
sometimes used to limit starting currents, secondary
resistor controllers usually function to regulate the
In a magnetic controller, the contacts are closed or
speeds of large ac motors.
opened by electromechanical devices operated by local
or remote master switches. Normally, all the functions
Static Variable-Speed
of a semiautomatic magnetic controller are governed by
one or more manual master switches; automatic
A static variable-speed motor controller consists of
controller functions are governed by one or more auto-
solid-state and other devices that regulate motor speeds
matic master switches, after it has been initially
in indefinite increments through a predetermined range.
energized by a manual master switch. Either controller
Speed is controlled by either manual adjustment or
can be operated in the semiautomatic or automatic
actuation of a sensing device that converts a system
mode, depending on the mode of operation selected.
parameter, such as temperature, into an electric signal.
This signal sets the motor speed automatically.
An across-the-line controller throws the connected
load directly across the main supply line. This motor
The autotransformer controller (or compensator) is
controller may be either manual or magnetic, depending
an ac motor controller. It starts the motor at a reduced
on the rated horsepower of the motor. Normally, across-
voltage through an autotransformer, and then it connects
the-line dc controllers are used for starting small (frac-
the motor to line voltage after the motor accelerates.
tional horsepower) motors. However, they also may be
There are two types of compensators: open transition
used to start average-sized, squirrel-cage induction
and closed transition. The open-transition compensator
motors without any damage. This is because these
cuts off power to the motor during the time (transition
motors can withstand the high starting currents due to
period) the motor connection is shifted from the auto-
starting with full-line voltage applied. Most squirrel-
transformer to the supply line. In this short transition
cage motors drive pumps, compressors, fans, lathes, and
period, it is possible for the motor to coast and slip out
other auxiliaries. They can be started "across the line"
without producing excessive line-voltage drop or
of phase with the power supply. After the motor is
connected directly to the supply line, the resulting
mechanical shock to a motor or auxiliary.
transition current may be high enough to cause circuit
breakers to open. The closed-transition compensator
Dc Resistor
keeps the motor connected to the supply line during the
entire transition period. In this method, the motor cannot
In a dc resistor motor controller, a resistor in series
slip out of phase and no high transition current can
with the armature circuit of the dc motor limits the
amount of current during starts, thereby preventing
motor damage and overloading the power system. In
some resistor controllers, the same resistor also helps
regulate the speed of the motor after it is started. Other
A reactor controller inserts a reactor in the primary
dc controllers use a rheostat in the motor shunt field
circuit of an ac motor during starts, and later it short
circuit for speed control.

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