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MCC Panels

Motor Control Center (MCC) panels control electric motors from a central location in large industrial and commercial applications. Generally, these panels consist of one or more enclosed sections with a communal power bus. Each of these sections houses power contactors, protective relays, and isolators alongside other control and indication devices. In this article, we will review the purposes of MCC panels and their design process and examine a typical diagram.
MCC Panels

Motor Control Center (MCC) panels control electric motors from a central location in large industrial and commercial applications. Generally, these panels consist of one or more enclosed sections with a communal power bus. Each of these sections houses power contactors, protective relays, and isolators alongside other control and indication devices. In this article, we will review the purposes of MCC panels and their design process and examine a typical diagram.

Purpose of MCC Panels

Electric motors perform vital functions in industrial applications as they drive several pieces of equipment, such as fans, pumps, and conveyor belts. As the number of equipment on the factory floor increases, it becomes difficult to manage control and performance. Hence, the use of MCC panels to provide the following functions:

  • MCC panels ensure the slow and gradual start of motors to avoid any damage. Depending on the requirements, types of starters include DOL, Star-Delta, Soft, and Auto Transformer starters.
  • Delivers operational control by maintaining operational speeds to protect machines, materials, motors, and operators. It also provides direction reversal and operational speed adjustments to match process requirements.
  • Emergency shutdown according to requirements.
  • Motor protection from overcurrent, overload, low voltage, phase failure, phase reversal protection, single phase prevention, and ground fault protection.

The presence of MCC panels within any facility offers several benefits to the operators. Some of the benefits include:

  • Reduction in equipment downtime as MCC panels can quickly identify faulty parts.
  • There is a central administration of controls, and operations are rapid.
  • Another advantage of MCC panels is expandability. The addition of new sections and units is seamless.
  • Using MCC panels saves significant factory floor space, and their installation is simple.
  • Features such as emergency shutdown, motor protection relays, phase reversal relays, and single-phase preventers improve safety.

Design Process for MCC Panels

The design process for MCC panels begins with determining the total power demand from the motors. On the basis of this demand, MCC panels are classified into low voltage centers, which operate from 230 V to less than 1,000 V. Medium voltage centers operate within a 1,000 V to 15,000 V range. Whatever the ratings may be, the design of the panels must conform to current revisions from regulatory bodies. Some of them include IEC 60529, IS 8623, UL 845 & 489, NEMA ICS 1 & 2, and NEC 450. Requirements from these industry codes cut across several aspects of the design of MCC panels, as the sections below highlight.

General Requirements

  • Avoid placing MCC panels in hazardous locations. Protection must be available to prevent moisture intrusion. Also, areas shall be well-ventilated with a minimum temperature of 32℉ and a maximum temperature of 104℉.
  • MCC assembling and installation shall occur on a smooth, level surface to ensure proper alignment of sections. In addition, the surface under the MCC shall be of non-combustible material.
  • The location of MCC panels shall allow adequate room for the removal and installation of units. Spacing should be a minimum of 3 feet in front and 0.5 inches at the back of front-of-board constructions. In damp locations, the spacing between the back of the MCC and the wall can be up to 6 inches. In contrast, an additional 3 feet should be in the rear of back-to-back constructions.

Enclosure Design

  • Enclosure design should follow NEMA 1 general purpose, 12 industrial duty, or 3R rainproof non-walk-in, depending on project requirements.
  • MCC shall consist of one or more standard vertical sections put together using bolts to form a rigid, self-supporting, free-standing assembly. Also, there shall be room for adding sections to either end, with steel barriers providing isolation between adjacent sections.
  • MCC construction shall be dead back and dead front with provision for all access from the front. Also, construction shall make it possible to install the assemblies back-to-back, free-standing, or against the wall.
  • The overall height of the enclosure shall not exceed 90 inches, excluding the base. The total width of one section shall be 20 inches, while base channels up to 1.5 inches in height shall be removable. 
  • The enclosure for MCC panels shall have easily connectable assemblies of up to four vertical sections. In addition, each shipping section shall have lifting angles on the tops.

Wireways

  • Each section shall have a minimum of 6-inch and 12-inch horizontal wireways at the bottom and top, respectively. Moreover, these wireways shall run the full length of the MCC to enable power and control cable connections between units of different sections.
  • There shall be provision of a full-depth vertical wireway in each section that accepts modular plug-in units. It is necessary for this vertical wireway to connect with both the bottom and top horizontal wireways, as well as have isolation from unit interiors.
  • Access to wireways shall not require opening control unit doors; rather, they shall have separate hinged doors. 
  • The assembly of MCC panels shall have the proper grounding of individual units. Moreover, the maximum resistance between individual starter enclosures and the MCC ground bus shall not exceed 0.005.

Diagram

In a typical factory setup, power arrives first at the switchgear before it is stepped down to the required levels by transformers. Next, it goes to the switchboard for onward distributions to the various panels, such as the MCC, as the layout below shows.

Courtesy: Paktech Point

The control of individual sections and units within the MCC enclosure is achieved by the PLC or DCS control panel. More sophisticated systems offer remote control access with smart MCCs. The adoption of smart MCCs is increasing as they offer quicker communication, thereby improving efficiency and minimizing downtime.

Courtesy: ForumAutomation