Control cabinets

Control cabinets are metal enclosures housing the electrical and automation equipment responsible for a given part of an installation. They typically contain:

• PLCs (programmable logic controllers), meaning industrial computers that run the control program,
• input and output modules, to which cables from sensors and devices are connected,
• power supplies,
• electrical protection devices, such as circuit breakers and fuses,
• variable frequency drives, meaning devices that smoothly adjust motor speed by changing the frequency of the current,
• relays, contactors and terminal blocks.

A cabinet has to be designed (electrical schematics), built, wired and tested before it is delivered to site. It must also suit the conditions in which it will operate: washdown areas require water resistance, and hazardous areas with explosion risk require a special design.

Example: In a plant producing washing powders, dust from raw materials can form an explosive mixture with air. Control cabinets are therefore placed outside the hazardous area, or explosion-proof versions are used. In a meat processing plant, where the production hall is washed down with pressurised water every day, cabinets have to be sealed and are usually made of stainless steel.

Variable frequency drive example: In shampoo production, the agitator in the reactor has to run differently at different stages: faster when dissolving the ingredients and slowly at the end, so as not to entrain air into the product and cause foaming. Without a variable frequency drive, the motor would run at only one speed. With one, the controller smoothly changes the speed according to the recipe.