The basic idea
A plant with a few motors can give each one its own starter and breaker mounted wherever it is convenient. A plant with dozens or hundreds of motors, such as a processing facility with pumps, fans, conveyors and mixers, quickly runs into problems with that approach. The equipment is scattered, hard to find, hard to maintain and sometimes hard to isolate safely.
A motor control center, often shortened to MCC, solves this by gathering the control and protection equipment for many motors into a single, standardized, floor-standing structure. Power comes in at one place and is distributed to many outgoing motor circuits, each with its own protection and controls. Think of it as a purpose-built cabinet system for motor feeders, as distinct from switchgear that handles the main incoming power and larger feeds.
What is inside
An MCC is built from vertical sections bolted side by side, forming a lineup. Common elements include:
- Main incoming connection. Power arrives from an upstream source, such as a transformer or switchgear, often through a main breaker or switch.
- Horizontal bus. Copper or aluminum bars running through the lineup carry the main power across all sections.
- Vertical bus. In each section, bars take power from the horizontal bus to the individual units.
- Units, often called buckets. Each unit serves one motor or load and contains the equipment for it.
- Control wiring and terminals. Space for wires that connect each unit to the plant control system, pushbuttons and field devices.
- Wireways. Channels for cables, kept separate from the power bus.
The enclosure also provides barriers and doors that limit access to live parts, which is a key safety feature, although opening or servicing any part must still follow proper procedures.
What a typical unit contains
A standard unit for a motor typically includes several devices in a compact arrangement.
- A disconnecting means with short-circuit protection, such as a circuit breaker or a fused switch, which can isolate the unit and protect against faults. The difference between circuit breakers and fuses is covered in the discussion of electrical components.
- A motor starter. A contactor that switches the motor on and off, combined with an overload device that protects the motor against sustained overcurrent.
- Control devices. A control power transformer, pushbuttons, indicator lights, selector switches and sometimes a small relay.
- Optional extras. These can include a soft starter, a variable frequency drive, a reversing starter, communication modules and protection relays.
Different units can be mixed in one lineup, so a single MCC may serve a direct-on-line pump motor, a reversing starter for a gate, a drive for a fan and a soft starter for a large conveyor.
Smart motor control
Modern MCCs often include intelligence. Electronic overload relays and motor protection devices can measure current, track running hours and report fault conditions. When connected through industrial networking to the plant control system, they let operators see motor status and trends from a control room rather than walking to the cabinet. Information from these devices can feed maintenance planning, for example by highlighting a motor that draws gradually rising current.
Why plants use them
The benefits tend to be practical.
- Centralized location. Starters and controls are in one place, usually in an electrical room away from dust, heat and washdown areas. This helps their reliability and keeps maintenance staff out of harsh process areas.
- Standardized construction. Units follow similar layouts, which simplifies training and spares.
- Easier maintenance. Many designs use withdrawable units that can be taken out of the lineup for service or replaced as a unit, which can reduce downtime. Some designs are fixed rather than withdrawable.
- Space efficiency. Combining many circuits in one lineup usually takes less floor area than separate wall-mounted starters.
- Organized power distribution. A shared bus feeds many motors from one source, and each circuit has its own protection so a fault on one motor does not necessarily take down the others.
- Expansion. Many lineups are built with spare units or spare space, so new motors can be added later with less disruption.
Design considerations
Several points are usually decided when specifying an MCC.
- Voltage and fault current. The bus and each unit must be rated for the system voltage and for the available short-circuit current at that point. This is a safety-critical rating and should be confirmed by a qualified engineer.
- Starter types. The load decides between direct-on-line, reversing, reduced-voltage, soft-starter and drive units. Large motors often need more than a simple starter.
- Heat. Drives and starters generate heat. Electrical rooms need ventilation or cooling to keep equipment within its rating.
- Arc flash. Faults inside a lineup can release a large amount of energy. Some designs include arc-resistant construction, and plants typically run an arc flash study and label equipment accordingly. Related safety gear is discussed in topics on electrical safety equipment.
- Harmonics. Several drives on the same bus can affect power quality, so the design may include filtering or reactors.
- Access and clearance. Standards and local codes require working space in front of the lineup. Space should also be left for future expansion.
- Documentation. Clear labeling, wiring diagrams and unit schedules make troubleshooting easier.
Maintenance basics
Typical care includes periodic inspection by qualified personnel, cleaning of dust, checking of connections for signs of overheating (thermographic surveys are common), exercising of breakers and disconnects as the manufacturer advises, and verification of protective settings. Keeping the room clean, dry and free of corrosive air helps.
Servicing an MCC is hazardous work. It must follow the manufacturer’s instructions, the plant’s lockout and electrical safety procedures, and local regulations, and be done only by trained and qualified people using the proper protective equipment. This article gives general information, not design guidance for a specific installation.