What You Need to Know About Motor Control Centres
Distribution systems for power, especially those that are used for large industrial and commercial applications, are highly complex. In a typical industrial electricity supply, power is distributed through transformers to ensure a specific working potential then via switchgear contained in a switchboard. In certain situations, uninterruptable power supplies (UPS) are required and these can be in the form of either static or rotary UPS. Static UPS consists of a battery bank and an intelligent inverter that provides a continuous supply of power in the event of a mains fail situation.
Static UPS can be either line interactive or dual conversion type. Line interactive units monitor the power system and only switch to battery when required, dual conversion continually rectify the incoming mains and invert back to AC to provide a ‘reconstructed’ waveform at all times. Dual conversion units are more expensive but a superior UPS. Design specifics will depend on a myriad of factors, including location, industry and necessity.
Rotary UPS are generally used for short time backup to allow time for separate generators to be started. A flywheel is used and the energy stored in the flywheel maintains the load for a short time hopefully allowing the generators to take over before it runs out of energy. An improvement on this technology is the dynamic rotary UPS (DRUPS). The DRUPS has a clever assembly where the generator has two modes – conditioning mode and independent mode.
In conditioning mode, when the mains supply is within tolerance, the synchronous machine is acting as a motor, driving the main shaft and thus the accumulator, storing kinetic energy in it. When the inner rotor rotates at 1500 rpm (50 Hz) or 1800 rpm (60 Hz), the outer rotor then rotates at maximum 3000 rpm. The kinetic energy is thus stored
In independent mode–during mains failure or voltage perturbation–the stored kinetic energy is now transferred to the stato-alternator, which acts as a generator. This happens without any interruption to the user. Shortly after, the diesel engine starts and couples to the stato-alternator by latching the electromagnetic clutch. The energy is then relayed from the diesel engine to the synchronous machine and then to the loads.
Industrial and commercial applications require a number of motors, and for obvious reasons, such as efficiency and cost, it just makes sense that most or all of those motors be controlled from a central location. The best way to control all of these is through an apparatus called a motor control centre (MCC).
A motor control centre is a switchboard that generally has individual cells containing a motor starter or drive. Combination starters are just single enclosures that house the fuses and/or circuit breakers, the device that disconnects the power, and the motor starter itself.
Motor control centres are typically floor-mounted assemblies that have one or more enclosed vertical sections, buses for power distribution, and they contain the motor control units. It is all contained within a steel structure. The steel structure in which the components are housed serves two purposes: it protects the unit from exposure and damage and it protects employees from exposure to live connections.
Specific descriptions for the MCC contain a ‘form’ rating which is a description of the separation achieved within the MCC between ‘cells’. MCC’s can be built to form1 which is basically no separation though to form 4b with various levels in between.
Various other types of switchboards exist including power distribution switchboards for low voltage final distribution through to large high voltage switchboards of hundreds of thousands of volts.