Wednesday, March 14, 2012

Motor Control Centre

Motor Control Centre is the device that controls various motors and drives in a process plant. MCC's purposes are starting and stopping the motors from the control room, providing protections and getting feed back from the motor side. MCC consists of a number of modules that individually controls motors. There can be different types of staring used to start motors. The most common among them is Direct On Line(DOL) starter which is used to star small capacity motors with comparatively less starting current.
A DOL starter consists of Start -stop circuit, Over Load relay (OLR) , Indicators for start, stop and trip of motors and necessary metering devices. In addition to this there shall be provision for connecting the motors to Automatic or remote operations through PLC controlled systems. Each module will have a selector for Remote-Local selection. A motor can be started or stopped from the MCC using the push buttons provided. Indications are available in the module to know the status of the motor. An over load relay is a thermocouple device that trips when the current exceeds the set value. OLRs are variable in different current ratings. According to the rating of the motor, the rating of the OLR shall be selected. Rating of the OLR is generally expressed in amperes , say for example 1.2 to 2 Amps.
Another type of starting is through Star -Delta starter. The basic principle of star delta starting is to start the motor with the windings of the motor connected in star connection and then switch over to delta connection. This is primarily used to reduce the starting current. Star delta starting is achieved through connecting both the ends of the stator windings to two contactors. The contactors, while the motor is started, first forms a star connection and after the lapse of a set time, another contactor is switched on that forms delta connection of motors. The first contactor that forms the star connection automatically switches off. Time can be set using timer.
Every motor control module is connected to PLC (Programmable Linear Control) units that is situated in the control room. Motors can be switched On or OFF from the control room using  PLCs. Motor On, OFF or trip indications also can be viewed from the PLC panels
The incoming supply to MCC is made available from a Power control centre (PCC) which is essentially  a number of Transformer and Load feeders. Feeders in the PCC have circuit breakers as main protection. One of the feeders from the PCC with the required maximum current rating of the MCC is fed to the MCC.
In a continuous process plant, since it is very important to have all machines running uninterruptedly, an uninterrupted power source to the MCC is also very important. To achieve uninterrupted power supply to all motors in the process, the Feeders in the PCC will have two different incoming power supply source which will be co-ordinated with the use of a bus coupler. One incomer will be always supply power while the other is always on stand bye. The bus coupler is used to supply power from the other incomer, if the first incomer fails. Bus coupler can act in auto mode as well as manual mode depending on the requirement of the operating personnel. In addition to the two separate incomers, some process plants deploy diesel generators also to maintain uninterrupted power in case both the incomers fail. Necessary in locking of all the three incoming source will be available in the PCC

1 comment:

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