Sparks Electrical News March 2022

MCCs AND MOTOR PROTECTION

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When did you last think about your motor control centres? W hat keeps a plant manager awake at night? Chances are, it’s whether their production processes are going to be reli- able and consistent so that both output and quality will be

Additionally, modern MCCs are designed to be easily maintained. A compartmentalised ‘Form 4’ construction with withdrawable buckets or assemblies, for example, means that, should a problem occur, a replacement can be fitted in seconds. The ‘intelligent’ aspect of theMCC can take care of the configuration and overload settings automatically. MCCs are fundamental in any installation that uses electric motors. That means that nearly every factory in the world relies on them. They provide the electrical power and protection necessary for safe and reliable operation of the motors in your plant. When did you last think about them? And if MCCs were smarter, would you sleep a little easier?

works, it’s easy for it to simply be taken for granted. An expectation that just grows with each passing month or year. But of course, nothing lasts forever and when the inevitable failure does come, it’s at best an inconvenience and at worst catastrophic. It needn’t be like this. MCCs can be intelligent. They can be integrated through EtherNet/IP into your Connected Enterprise and they can provide important information on the health of not just themselves but also of the motors and machines they feed. They can provide trends for energy use – often in much more granular detail than conventional energy monitoring. They can even provide advance information before a trip condition actually occurs. Intelligent MCCs can be a principal enabler for predictive maintenance on a plant-wide scale!

maintained. And where do managers look for reassurance that everything is working as it should be? Probably it’s in the heart of the operation – the automation control room. Having access to this data and information is, of course, reassuring, but is there still critical equipment that managers cannot see? Unseen equipment that, if it failed, would create a major headache (or worse) in production loss and may prove difficult to repair quickly? Most plants rely heavily on electric motors. Whether fixed or variable speed, they provide the motive force that transports, cuts,

Enquiries: www.rockwellautomation.com

mixes, pumps, cools, doses, aerates, packages and drives all manner of processes that products need to undergo before they are ready for dispatch and sale. While the demand control of these motors comes from the programmable automation controller (PAC), the grunt work, the actual switching of the electrical load, is usually through a motor control centre (MCC). Frequently, MCCs are not located in the same area as the automation control equipment, but rather in plant rooms or switch rooms. And they quietly (or not so quietly) get on with their work. Their reliability – which is usually very impressive – being their worst enemy. Why? Well, when something just gets on and As with any electromechanical relay technology, these have stationary electromagnetic coils that (upon a command from a pushbutton, limit switch, timer, float switch, or other relay) force together two circuits. These circuits include input power contacts and a mating carrier that (once closed together) allow current flow into the motor wind- ings. One variation on this design is a combination starter, which includes the magnetic action as well as some way of disconnecting electrical power when needed … either with a fuse, breaker or mo- tor circuit switch. Wye-delta motor starting (one type of reduced- inrush system) sends full-line voltage to the mo- tor’s wye windings during start up – though volt- age across each motor winding is reduced by the inverse of the square root of three (57,7%) which is why this arrangement is sometimes (rather inex- actly) called reduced-voltage starting. Then a cir- cuit (usually with a contactor for each phase, over- load relay, timer, and mechanical interlock) switches the motor input to supply full-line voltage into its delta windings. Part-winding motor starting – used in conjunc- tion with the specialty dual-voltage motors men- tioned above – applies across-the-line voltage to only one part (half or two thirds) of the motor wind- ings (typically nine or twelve) upon start. Then, once a set time has passed or set voltage is detected, a relay or timer springs into action and commands that the rest of the windings be added and sup- plied power as well. Acceleration may be irregular, but part-winding motor starting impedance has no effect on starting torque … and allows low-torque starts that are useful for pumps, fans, and blow- ers. Like wye-delta starting, part-winding starting is a type of reduced-inrush system and delivers a diminished full-line voltage upon motor start up – but doesn’t technically qualify as reduced-voltage starting. Full-voltage reversible starting leverages how induction motors change rotation direction upon reversal of any two power leads. Reversing starter systems simply incorporate a pair of mirrored con- tactors complemented by interlocking subcompo- nents to allow run-forward and run-reverse condi- tions. Quicker rotational-direction reversals can be made with plugging, which is the temporary power- ing of both circuits. matically triggered motor starting is sometimes called magnetic starting for the electromechanical contactors that are core to this design.

Enquiries: www.motioncontroltips.com

SPARKS ELECTRICAL NEWS

MARCH 2022