Motor control centre

MCC - motor control centre. For details refer to Chapter 1.4. 

These may receive power from one or more sources of supplies and distribute them to different load centres, which may be a motor control centre (MCC) or a distribution board (DB), as illustrated in Figures 13.2... 

Figure 13.2 Details of a draw-out motor control centre (MCC) (Courtesy ECS)...

When there is only one process line and one MCC alone is adequate to control the entire process, it is possible to combine the PCC and the MCC into one unit to save on space and cost. The assembly may now be called a PMCC (power-cum-motor control centre). 

Recommended practices for seismic gualitlcation of class IE equipment for nuclear power generating stations (NPGS) For qualifying class IE motor control centres for nuclear power generating stations (NPGS)... 

Outgoing switching device for motor control centres Fuses for Motor Control Centre Outgoing Circuits... 

All switchboards and motor control centre names, bns-section nnmbers, line voltages, number of phases, number of wires, frequency, busbar continuous current rating. 

Switchboards and motor control centre name and tag number. 

Interlocking and controls that affect motor control centres and switchboards, generator controls, control panels, local and remote stations, mimic panels, SCAD A, computer networking, displays in the CCR and other locations. 

Assuming 100% voltage at the switchboard or motor control centre, the volt-drop at the motor terminals should not exceed the following guidelines -... 

Switchgear tends to be operated infrequently, whereas motor control centres operate frequently as required by the process that uses the motor. Apart from the incomers and busbar section circuit breakers, the motor control centres are designed with contactors and fuses (or some types of moulded case circuit breakers in low voltage equipment) that will interrupt fault currents within a fraction of a cycle of AC current. Circuit breakers need several cycles of fault current to flow before interruption is complete. Consequently the components within a circuit breaker must withstand the higher forces and heat produced when several complete cycles of fault current flow. 

Switchgear is available up to at least 400 kV, whereas motor control centres are only designed for voltages up to approximately 15 kV because this is the normal limit for high voltage motors. 

The maximnm valne of 4000 A for low voltage bnsbars roughly corresponds to the secondary cnrrent of a fnlly loaded 2500 kVA transformer. 2500 kVA is often chosen as the hmit for transformers that feed motor control centres because the fault current that they allow through is typically near to the limit that the mannfactnrers can normally supply, e.g. 80 kA symmetrical rms current. A 2500 kVA transformer with a 6% leakage impedance and a 400 V secondary winding will pass approximately 60 kA of fanlt cnrrent. If the MCC feeds mostly motors then they will collectively contribute some fault current in addition to that from the transformer, see IEC60363 clause 4 and IEC60909 clause... 

The earthing (grounding) bnsbar is separate from the neutral busbar, and is nsed to earth all conductors that need to be earthed as well as the metallic frame and casing of the switchboard or motor control centre. The earthing bnsbar is made of high-grade copper and is usually located at the front or rear of the enclosnre at gronnd level. 

Switchboards and motor control centres are generally required to operate continuously at temperatures above 15°C, for example when the switchroom air conditioning fails or the ambient temperature is exceptionally high. However, switchboards are usually manufactured to meet the requirements of an ambient temperature of 40°C, see for example, 1EC60439 clause 6.1.1 ambient air temperature . 

A good rule-of-thumb guide is to assume that between 15% and 25% extra capacity will be required. Hence the chosen rating will be 115% to 125% of the best-known estimate at the early design stage. This requirement also applies to power transformers and their main cables or overhead power lines, and to outgoing feeder cables to auxiliary switchboards and motor control centres. It does not usually apply to individual motor consumers, see Chapter 1. 

Outgoing Switching Device for Motor Control Centres... 

Motor control centres and some switchboards nse contactors as the freqnently operated switching device for individnal ontgoing loads up to about 400 amps. The figure of 400 amps is about the limit of fuse and contactor design capability. See Chapter 8 for a discnssion on fuses. Contactors and their accompanying fnses shonld be used where ever possible because -... 

FUSES FOR MOTOR CONTROL CENTRE OUTGOING CIRCUITS... 

Most switchboards and motor control centres are fitted with a variety of electrical and mechanical safety interlocking devices. Their purposes are to protect against for example -... 

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