A motor control centre, or MCC, groups the power distribution, switching, protection and control needed for multiple motors. Its design follows the motor schedule and the way the process operates.
An MCC follows the process
Pumps, fans, conveyors and compressors may need different starting methods, interlocks and operating modes. Bringing their circuits together can make the interfaces and equipment easier to organise, provided the assembly is designed around those duties.
An MCC often receives a feeder from the main switchboard. It can also be part of a combined distribution and motor-control arrangement. Its name does not establish a particular fault level or mean every circuit must be identical.
Three common motor arrangements
| Arrangement | Role | Questions for the design |
|---|---|---|
| Direct-on-line starter | Switches a motor directly onto the supply. | Starting current, motor duty, protection and permitted voltage dip. |
| Soft starter | Controls the starting and stopping profile where appropriate. | Starting torque, number of starts, bypass arrangement and heat. |
| Variable speed drive | Controls motor speed and other motor functions. | Drive location, heat, EMC, harmonics, cabling and maintenance access. |
The correct method depends on the motor and driven equipment. A soft starter is not a substitute for continuous speed control. A drive also needs its own selection and integration work.
One motor circuit, several functions
A motor circuit may include a short-circuit protective device, switching equipment, overload protection and controls. The functions need to work together with the motor and supply. The selected arrangement may integrate several functions into one device or distribute them across components.
The operator also needs to know whether the motor is available, running or tripped, and what prevents a start. Those signals can come from contacts, intelligent motor protection or communications, depending on the design. Define the information required before choosing the interface.
An MCC does not remove the need for suitable field isolation and equipment-level safety arrangements. The complete installation and machine or process controls must be coordinated by the responsible project team.
Trace a start command through the control system
For an illustrative pump duty, an operator may request automatic operation at the process system. The logic checks the required permissives, then sends a command through the agreed interface to the motor starter or drive. Status returns to the control system so the operator can see the result.
The details vary: a local selector may change the control mode; a process condition may inhibit starting; a loss of communication may require a defined response. Write these behaviours in the control philosophy. A wiring drawing alone may not describe what the process should do after an interruption.
Agree which party tests each interface at the factory and which checks depend on field instruments and site equipment. Simulation can demonstrate a defined input and output, but cannot establish that an unconnected field device has been installed correctly.
At Frimley and Eastbourne, the CWS manufacturing scope included MCCs, pump/control panels and Allen-Bradley / Rockwell PLC panels. CWS carried out panel wiring, I/O termination and FAT; the project integrators handled software and site commissioning. The case study shows why those interfaces need named owners.
Where do the drives and controls go?
A drive can be accommodated in a suitable verified assembly or in a separate cabinet. The decision depends on heat, space, electromagnetic compatibility, access and the selected system. CWS commonly considers separate drive enclosures where that gives a more practical arrangement; there is no universal rule that a VSD can never be installed in an MCC.
Motor protection, local controls, interlocks, PLC I/O and communications need defined interfaces. Agree whether the manufacturer supplies hardware and wiring only or also has a specific programming and commissioning scope.
Design for the people using it
- Clear motor tags matching drawings, local controls and the process documentation.
- Access to the intended isolation points and cable terminations.
- A defined form of separation and module arrangement.
- Spare ways and electrical capacity for identified future motors.
- Suitable enclosure conditions and maintainable cooling.
- Useful status and trip information for operations.
Plan additions around the future motor duty
A spare empty compartment is useful only if the intended future equipment can fit and the wider assembly can support it. Identify likely motor ratings and starting methods, cable needs, heat, control interfaces and supply capacity. An additional VSD duty can place different demands on the arrangement from an additional direct-on-line starter.
Keep the motor tags consistent across the motor list, drawings, operator display and equipment labels. When a duty changes, update the related records together. Good identification helps the owner find the correct information long after the original project team has moved on.
See the industrial label schedule guide for a practical way to organise the identification information supplied for manufacture.
From motor list to manufacture
The 630 A industrial process MCC case study shows assigned duties, fitted spares and empty positions in a completed assembly. Use it alongside our MCC manufacturing service when discussing the motor schedule and expansion requirements for your own plant.
Provide the motor ratings, voltage, duty, starting methods, control philosophy, supply information and environment. The MCC specification checklist goes into the detail needed for procurement.
CWS manufactures motor control centres on arrangements including Simotrol and Logstrup. See the Frimley and Eastbourne project for an example of defined manufacturing and wiring scope alongside a separate systems integrator.
Questions we hear
Is an MCC also a switchboard?
Yes. It is a switchgear and controlgear assembly focused on motor circuits. The distinction is its function and contents.
Does the MCC supplier always program the PLC?
No. Hardware, wiring, software and site commissioning may belong to different parties. Agree those responsibilities in the project scope.
Can one MCC contain different starting methods?
Yes, where the selected design accommodates them. Each motor duty, protective arrangement, heat contribution and control interface must be considered.