Motor Control Centers Defined

By Sal Spada

Category:
Industry Trends

Motor Control Centers (MCCs) are segmented into two distinct classifications: Low Voltage and Medium Voltage.

  • Low Voltage MCCs are used for low voltage three-phase alternating current motors from 230 V to less than 1000 V.
  • Medium-voltage MCCs are used for large motors requiring 1000 V to around 15000 V.

For both classification of MCC, 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 control-ler through separable connectors. The motor is wired to terminals in the controller. MCCs 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 bi-metal and solid-state overload protection relays, or various classes of power fuses or types of circuit breakers. A MCC 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 program-mable controller. An MCC is generally mounted on floors, which are often required to have a fire-resistance rating. Fire stops may be required for cables that penetrate fire-rated floors and walls.

Low Voltage Motor Control Centers

A low voltage motor control center (LV-MCC) is an assembly of one or more enclosed sections having a common power bus and principally containing motor control units. LV-MCCs 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 LV-MCC may be installed in a separate air-conditioned room.

LV-MCCs provide the most suitable method for grouping electrical motor control, automation, and power distribution in a compact and economical package. LV-MCCs consist of totally enclosed, freestanding structures bolted together. These sections support and house control units, a common bus bar for distributing power to the control units, and a network of wire trough and conductor entrance areas for accommodating incoming and outgoing load and control wires. The control units consist of components, such as a combination of motor starters, branch feeder devices, variable frequency drives, programmable logic controllers, soft starters, direct online starters, and meters. Each unit is mounted in an individual, isolated compartment or drawer having its own door. LV-MCCs are an assembly of several motor starters. An LV-MCC 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.

Medium Voltage Motor Control Centers

A Medium Voltage Motor Control Center (MV-MCC) is an assembly of one or more enclosed sections having a common power bus and principally containing motor control units. MV-MCCs are used in many industrial and commercial applications. Where very dusty or corrosive processes are used, the MV-MCC may be installed in a separate air-conditioned room.

MV-MCCs provide the most suitable method for grouping electrical motor control, automation, and power distribution in a compact and economical package. MV-MCCs consist of totally enclosed, freestanding structures bolted together. These sections support and house control units, a common bus bar for distributing power to the control units, and a network of wire trough and conductor entrance areas for accommodating incoming and outgoing load and control wires. The control units consist of components such as a combination of motor starters, branch feeder devices, variable frequency drives, programmable logic controllers, AC drives, soft starters, direct online starters, and meters. Each unit is mounted in an individual, isolated compartment. MV-MCCs are an assembly of several motor starters. A MV-MCC 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.

Motor Control Center

Motor Control Centers by Configuration

Conventional

This configuration consists of one or more enclosed vertical compartmentalized sections bolted together and mounted on the floor. Each vertical section supports and houses one or more combination of control units and various related devices, including push buttons and selector switches. These units are mounted in an individual, isolated compartment having its own door and can be fixed or withdrawable.

Intelligent

An Intelligent MCC uses a smart, electronic protection device in lieu of a conventional thermal overload device, to accommodate for various protection functions, depending on configuration and software. However, each control unit uses devices that have embedded intelligence. Specifically, PLC, Soft Starters, circuit breakers, contactors and AC Drives that can provide information over an internal network which can further support communication networks to higher level systems in the operation. Most common network supported outside of the cabinet includes Profibus, ProfiNet, CCLink, Ethernet\IP, DeviceNet or Modbus). This communica-tion link is often connected directly to the PLC mounted inside the cabinet. The PLC performs protection, communication, control and monitoring functions.

Motor Control Centers by Starter Type

MCC are segmented by starter type, which includes the following segmentation:

Direct On Line

Motor starters referred to as Direct On Line are the simplest form of motor starters. They simply connect the motor directly to the line.

Soft Starter

Motor starters referred to as soft starters ramp the applied voltage up at the start and down at the stop

Variable Speed Drive (AC Drives)

AC Drives are the most advanced forms of motor starters. These systems allow for a full range of speed and acceleration control of the motor. Generally the AC Drive also provides more advanced diagnostics through a network connection.

Motor Control Centers by Circuit Breaking Type

MCC can be segmented by the type of overcurrent protection employed in the panel.

Fused Technique

A fuse is a one-shot device. The heat produced by overcurrent causes the current carrying element to melt open, disconnecting the load from the source voltage.

Circuit Breaker

In addition to providing overcurrent protection, a circuit breaker provides a manual means of energizing and de-energizing a circuit. A circuit breaker allows a circuit to be reactivated quickly after a short circuit or overload is cleared by simply resetting the breaker.

Motor Control Centers by ARC Flash

An arc flash is an electric arc supplied with sufficient electrical energy to cause substantial damage, fire or injury. There are many methods of protecting personnel from arc flash hazards. This can include personnel wearing arc flash personal protective equipment (PPE) or modifying the design and configuration of electrical equipment. The best way to remove the hazards of an arc flash is to de-energize electrical equipment.

The MCC can be segmented into arc flash and non arc proof designs. ARC Flash is specifically a panel design that incorporates technology that will allow personnel to safely open panel doors without removing power from the system.

Motor Control Centers by Standard

Several organizations are involved in establishing standards for the design, construction, and application of motor control centers. The primary stand-ards segmented in this report are NEMA and IEC.

NEMA - National Electrical Manufacturers Association (NEMA) is an or-ganization that develops standards for electrical equipment.

IEC - The International Electrotechnical Commission (IEC) is an organiza-tion based in Geneva, Switzerland with over 50 member nations. IEC writes standards for electrical and electronic equipment practices.

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