Types of Motor Controllers

Ac Primary Resistor

TYPES OF MOTOR CONTROLLERS

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

Manual

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

Magnetic

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.

Across-the-Line

Autotransformer

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

develop.

amount of current during starts, thereby preventing

motor damage and overloading the power system. In

Reactor

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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