Motor control centers: tie drives, starters into integral network: MCCs provide convenient, economical AC drive mounting platforms.(alternating current)

Motor control centers (MCCs) have long been the preferred choice, from among many options, to provide a convenient, economical enclosure for motor control and branch circuit protection in commercial and industrial facilities. There is a school of thought, however, that suggests that MCCs are not the ideal enclosure, since they were not originally designed to house solid-state electronic controls, such as alternating current (ac) variable frequency drives (VFD). As these power electronic devices become more popular, it's important to understand the issues concerning ac drives and MCCs.

Advantages to mounting ac drives in MCCs includes cost. Mounting drives in MCCs can trim installation costs and system complexity. That's because an entire MCC filled with various types of motor controllers can be fed by a single incoming feeder. The single feeder can supply several drives with incoming power, as well as allow future plug-on drives to be accommodated at less expense.

When four or more drives will be installed near each other, it is generally more cost effective to mount the drives in an MCC. Reductions in wiring, conduit, and labor will most likely offset any additional cost of the MCC structure.

Additionally, many plant and building managers are finding that the cost of floor and wall space is at a premium and that they can't afford as much space for electrical equipment. A bank of individual wall-mounted drives typically requires more space than an MCC drive lineup due to installation codes, wire/ conduit bending spa, and minimum workspace requirements. A single wall-mounted drive with conduits requires several feet of width. However, as many as six 3-horsepower (hp) drives can fit in one 20-inch wide MCC section.

MCCs typically meet extended short circuit ratings above those required for individual drives. They are specifically made to withstand the higher fault currents commonly found in industrial environments.

However, several myths persist when it comes to mounting ac drives in MCCs. As with most myths, these tend not to hold up upon closer inspection.

Myth 1. VFDs generate heat, which requires special cooling and air flow in MCC enclosures. While it's true that VFDs generate heat, MCC enclosures are engineered to allow for heat dissipation. Drives produce much more heat than most other motor controllers and venting that additional heat is critical. MCCs are designed to remove heat from the front. The more advanced MCC manufacturers use design tools to optimize the cooling function and create a standard drive solution. For example, an effective way to cool the drive is to remove heat at the source, i.e. the drive's metal heat sink. Such a focused cooling method uses the same components as other enclosures but uses them more efficiently. Also, special provisions, such as air conditioning, can be added to MCCs to cool outdoor installations.