Motor Control Center:
A Motor Control Center is a modular cabinet system for powering and controlling motors in a factory. Several MCCs in a factory may be powered from a main switchgear which in turn gets its power from a transformer attached to the incoming line from the power company. MCCs are very common sights in factories with heavy machinery, although the purpose they serve is changing.
A typical MCC cabinet is a grey NEMA 12 enclosure (although higher NEMA ratings are available for harsher environments) with a number of small doors arranged in rows and columns along the front and flat, mostly featureless back and sides. Behind each door is a unit called a bucket which connects the electrical devices inside to the electrical bus bars behind it. An MCC with only one column of buckets is called a single-section MCC.
Very often in the US there will be three bus bars to carry three-phase, 480 Volt, 60Hz electricity. The buckets are intended to contain a circuit breaker, a motor starter, and a control power transformer, although their actual contents will vary widely according to the factory's needs. The circuit breaker will have a handle that goes through the door to shut off the power without opening it.
Several years ago, MCCs housed almost all the control for motors in an industrial setting. The trend lately, however, has been to use the MCC only to provide points from which to distribute power to electrical panels. In this case, the MCC bucket will have only a circuit breaker and a handle to shut the circuit breaker off; the electrical panel would be a separate enclosure which houses all the motor control for a specific machine. The power and popularity of programmable logic controllers for control and the size of high-horsepower variable frequency drives have contributed to this trend, as they do not fit in a standard MCC very well. As long as those are going into a separate enclosure, why not put everything there?
Cables to supply power to the MCC are usually fed from the top left. The first bucket allows connection of the supply cables to the bus bars running along the back wall of the MCC. The exposed cables behind this door may have current transformers around them to display the load on the MCC to current meters mounted in the door's face. One CT is usually sufficient, unless the possibility for severely unbalanced loads exists in which case each cable may have a CT and a meter.
The next bucket down from the infeed is often a circuit breaker panel for lighting and electrical outlets. The next bucket down from there typically houses the transformer providing power to the circuit breaker panel. This is separate from the control power transformers, which supply electricity to the buttons, lights, and sensors for the motors each is attached to.
If the MCC is powering a large number of high-horsepower motors, it may have power factor correction capacitors in one of the buckets to balance the large amount of inductive load.
Control power (the wires that turn the motors on and off, as opposed to the ones that actually power the motors) can be fed from any point along the top, usually through conduit penetrating the top. The exception is in food grade environments, where penetrations are preferred in the sides for ease of cleaning the top. There is a space called a wireway along the top (and sometimes along the bottom as well) of the MCC, above the doors, for the control power wiring to be fed along the columns of buckets. Between each column of doors is a long, narrow door called a vertical wireway to run the wiring down to the appropriate bucket. A terminal strip along the side of the bucket allows the field wiring to be connected to the bucket, separating the wiring inside the bucket from the wiring outside the bucket for easy installation and removal.
Buckets are available in several sizes, typically about a foot and a half wide and a foot tall depending on manufacturer and model. This is usually sufficient for a circuit breaker and a motor starter to be mounted side-by-side with space left over for a control power transformer and a relay or two. Since they are modular, the buckets are available in various heights to enclose larger parts, such as variable frequency drives or soft starters.
The bucket is intended to be easily removable so the parts can be put into it in a more convenient location away from the MCC. It slides on rails and snaps onto the bus bars in the back of the MCC, and has a terminal strip mounted on the side to attach the field wiring from the vertical wireways. This allows all the bucket wiring to be done prior to final installation as well.
The door of the bucket will have a handle through it if there is a circuit breaker inside. This is to comply with OSHA regulations to provide a means of disconnecting the power from the equipment. The handle should have a latch of some kind built in to provide a way to lock the breaker in the off position. If the bucket contains a motor starter, it may also have a number of other devices in the door such as a reset button to clear faults, a light to indicate that the motor is running, a meter to measure the load on the motor, and a "Man/Off/Auto" switch to select the control method.
Although the purpose of MCCs is changing from a centralized motor control location to simply a power distribution center, they are unlikely to disappear from factories entirely. The larger units are very expensive and have a long useable lifespan, meaning there won't be any reason to scrap them any time soon. As a power distribution center, they are very useful for providing a centralized location to shut off the power to a large number of devices for maintenance. There is also the matter of inertia: Since existing MCCs have been converted for power distribution, new power distribution needs will likely be met by buying more MCCs. Their flexible, modular design and built in lock-out handles perform the job very well.