Motor control centers are defined as an assembly of one or more enclosed sections having a common power bus, and principally containing motor control units. At their most basic, they can be described as a series of buckets housed in a larger structure, and are used in industrial and manufacturing settings where electric motors are used.
How Motor Control Centers Work
Feeder buckets serve to power and control electric motors. Generally, a single bucket will control a single motor. If there are several electric motors being used, multiple buckets will be required. A vertical housing is used to hold and control all the feeder buckets in one convenient location.
The buckets come in multiple sizes and configurations, and may be fused or circuit breakers. These are also removable which allows the system to be updated if changes are needed. For example, additional motors can be added or removed from the structure and swapped out for updated equipment as needed.
Basics of Control Components
When we think about how a motor is controlled, we can consider manual control and automatic operation. Manual control is fairly simple to understand in the basic application of a human turning on and off a light switch. In automatic operation, however, circuits are able to turn something on and off without human interaction.
Automatic operation uses a wide variety of components in control circuits. These components can vary in complexity from indicator lighting to advanced systems that monitor, protect and control AC motors. In some cases, the interaction of these components is greatly dependent on how they are wired to each other. Often, they are wired directly to a motor control system.
Basic Motor Control in Action
The most basic type of AC motor control involves turning the motor on and off. This is done using a motor starter made up of a contactor and overload relay. The contactor’s contacts are closed to start the motor, and open to stop it. This opening and closing is accomplished electromechanically using start and stop pushbuttons or other pilot devices wired to control the contactor.
There is an overload relay that works to protect the motor by disconnecting power to the motor when it detects an overload. However, because it does not provide short circuit protection for the wiring that supplies power to the motor, it requires a circuit breaker or fuses.
Finding the Best Motor Control Center for Your Application
If you are looking for a quality motor control center, there are certain specifications you should be looking for. These include:
Ampacity: This is the maximum amount of current the main horizontal bus can accommodate without overheating. The minimum ampacity of an MCC main horizontal bus is 600A and 300A for the vertical bus. Maximum values are 2,000A for horizontal and 1,600A for vertical.
The amount of current available varies from installation to installation. The MCC must be rated to withstand whatever fault current it is dealing with. The fault rating withstand of an MCC is determined by the lowest rated device within the system. These typically have standard values like 10,000A, 14,000A, 18,000A and 22,000A.
Bussing Material: The material used for electrical bussing within the MCC is another thing that should be considered. Aluminum is most commonly used and is fairly inexpensive but because it expands and contracts easily, fasteners that are holding it tend to come loose more often resulting in added maintenance. It also isn’t as conductive as other materials so larger amounts are needed.
Copper is more sturdy and conductive. However, some say it is likely to corrode leading to eventual failure in the MCC system. Some attempt to solve this problem by coating the copper with tin or silver but these do not completely protect against the corrosion.
Feeder Cables: Another factor to consider is how the main feeder cables will enter the MCC. The main choices are overhead or underground. Knowing which type of feeder cables you will be using will make installation easier and less costly as you won’t have to deal with unforeseen circumstances that may complicate the process. There are several types of control wiring and it is a good idea to familiarize yourself with the specifications of each to best determine which you should use for your application.
Motor control centers come in handy in modern electrical engineering offering a convenient and centralized system for controlling your machinery. If you are planning on using machinery that incorporates the use of an MCC, it is a good idea to familiarize yourself with the supplies you will be needing for installation and the specifications of the machinery to make the process as simple as possible. Then enjoy the ease of use of your system for years to come.