A motor control center (MCC) is an assembly to control some or all

electric motor An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate for ...

s in a central location. It consists of multiple enclosed sections having a common power bus and with each section containing a combination starter, which in turn consists of motor starter,

fuse Fuse or FUSE may refer to: Devices * Fuse (electrical), a device used in electrical systems to protect against excessive current ** Fuse (automotive), a class of fuses for vehicles * Fuse (hydraulic), a device used in hydraulic systems to protect ...

s or

circuit breaker A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the ris ...

, and power disconnect. A motor control center can also include push buttons, indicator lights,

variable-frequency drive A variable-frequency drive (VFD) is a type of motor drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and, depending on topology, to control associated voltage or current va ...

s, programmable logic controllers, and metering equipment. It may be combined with the electrical service entrance for the building. MCC's are typically found in large commercial or industrial buildings where there are many electric motors that need to be controlled from a central location, such as a

mechanical room A mechanical room, boiler room or plant room is a room or space in a building dedicated to the mechanical equipment and its associated electrical equipment, as opposed to rooms intended for human occupancy or storage. Unless a building is serve ...

electrical room An electrical room is a room or space in a building dedicated to electrical equipment. Its size is usually proportional to the size of the building; large buildings may have a main electrical room and subsidiary electrical rooms. Electrical equip ...

Voltage ranges

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.Robert W. Smeaton (ed) Switchgear and Control Handbook 3rd Ed., Mc Graw Hill, New York 1997 , chapter 26

Usage

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.

Components

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.

Specifications

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.

Fire protection

Motor control centers usually sit on floors, which are often required to have a

fire-resistance rating A fire-resistance rating typically means the duration for which a passive fire protection system can withstand a standard fire resistance test. This can be quantified simply as a measure of time, or it may entail other criteria, involving eviden ...

Firestop A firestop or fire-stopping is a form of passive fire protection that is used to seal around openings and between joints in a fire-resistance-rated wall or floor assembly. Firestops are designed to maintain the fire-resistance rating of a wall ...

s may be required for cables that penetrate fire-rated floors and walls.

References