Motor control centers are usually used for low voltage three-phase alternating current
motors from 208 V to 600 V. Medium-voltage motor control centers are made for large
motors running at 2300 V to around 15000 V, using vacuum contactors for switching
and with separate compartments for power switching and control.
Motor control centers have been used since 1950 by the automobile manufacturing
industry which used large numbers of electric motors. Today they are used in many
industrial and commercial applications. Where very dusty or corrosive processes
are used, the motor control center may be installed in a separate air-conditioned
room, but often an MCC will be on the factory floor adjacent to the machinery controlled.
A motor control center consists of one or more vertical metal cabinet sections with
power bus and provision for plug-in mounting of individual motor controllers. Very
large controllers may be bolted in place but smaller controllers can be unplugged
from the cabinet for testing or maintenance. Each motor controller contains a contactor
or a solid-state motor controller, overload relays to protect the motor, fuses or
a circuit breaker to provide short-circuit protection, and a disconnecting switch
to isolate the motor circuit. Three-phase power enters each controller through separable
connectors. The motor is wired to terminals in the controller. Motor control centers
provide wire ways for field control and power cables.
Each motor controller in an MCC can be specified with a range of options such as
separate control transformers, pilot lamps, control switches, extra control terminal
blocks, various types of thermal or solid-state overload protection relays, or various
classes of power fuses or types of circuit breakers. A motor control center can
either be supplied ready for the customer to connect all field wiring, or can be
an engineered assembly with internal control and interlocking wiring to a central
control terminal panel board or programmable controller.