Motor control protection

Overload Protection Overload relays lor protecting motor insulation against excessive temperature are located either in the motor control or in the motor itsell. The most common method is to use thermal overciirrent relays in the starter. These relays have heating characteristics similar to those ol the motor which they are intended to protect. Either motor current or a current proportional to motorline current passes through the relays so that relay heating is comparable to motor heating. 

High-voltage contactor-type motor controls depend on power fuses for short-circuit protection. The fuses are coordinated with the overload relays to protect the motor circuit over the full range of fault conditions from overload conditions to solid maximum-current short circmts. 

Figure 12.46 A simple power diagram for a 6.6 kV motor control with an isolator, contactor and a motor protection relay...

Lythall, R.T., AC Motor Control (on earth fault protection and thermistor protection). 

The engineer providing the initial design of major facilities is faced with the decision of providing a motor control center building or indi id-ual (usually rack-mounted) motor starters and corresponding branch circuit protection devices. For installations using only several motors it fre-... 

This chapter introduces the basic items of design and specification for the principal systems and components of an electrical industrial installation. Electrical supply systems are discussed with regard to interface with the supply authorities and the characteristics. Salient features of switchgear, transformers, protection systems, power factor correction, motor control equipment and standby supplies are identified and discussed together with reference to the relevant codes of practice and standards. The equipment and systems described are appropriate to industrial plant installations operating at typically 11 kV with supply capacities of around 20MVA. 

In operation containers constructed of microwave-transparent materials, (e.g. quartz or fluoropolymers), are used to hold multiple samples inside the ultraCLAVE . The interior of the stainless steel vessel is protected by a titanium nitride or multi-layer PTFE plasma coating for complete acid and chemical resistance. Sample containers may be open or covered by a lid. After the samples are loaded (manually or robotically) the ultraCLAVE cover is lowered into place by an electric motor controlled from the system s PC. The vessel closure is engaged and secured in place to seal the ultraCLAVE for high pressure operation. 

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 -... 

Four main units are used in the IMCS, which are the motor control unit (MCU), the feeder control unit (FCU), the circuit breaker control unit (CBCU) and the central control unit (CCU). A MCU is a microprocessor (micro-computer) based module which has integrated control, monitoring, protection functions, and a communication interface for the motor starter. An FCU is very similar to a MCU and interfaces communication for the plain feeder contactor or circuit breaker. A CBCU is also similar to a MCU but is used for incomers, interconnectors and busbar section circuit breakers. A CCU provides the facility to communicate simultaneously with MCUs, FCUs, CBCU, a distributed control system (DCS), system control and data acquisition (SCADA) and other digital information systems. Other discrete devices such as special protective relays can also be addressed by the CCU provided the software and porting systems are compatible. 

In switchboards that supply motors the protection, control and measurement functions will be the most varied for the motor circuits. Generator incomers would be the next lower in complexity. The following functions are typical of all but the lowest ratings of motors -... 

This is also known as pressnrisation or continuous dilution. It is mainly applied to large motors, control panels, display panels, and occasionally special purpose generators. Type p protection is suitable for Zone 1 and Zone 2 areas. 

This method is commonly used in the oil industry because of the predominance of steelwork in a typical installation. In this method the power system is earthed separately from the equipment frames and cnbicles. The star point at the sonrce is the only common earthing point. The lEG standard reqnires the circuit to be protected by a residual earth fault current device at the switchboard or motor control centre, where the consumer is located in a Zone 1 area. 

The system receives its main power from a circuit breaker (6) or contactor in the upstream switchboard or motor control centre. This switchgear will contain the main power protective relays... 

If there is a large number of motors to restart (or also called reaccelerate) then there will be a large surge of main feeder current if they all start at the same time. This will cause a severe volt-drop at the common busbars, which will cause the run-up times to be extended and the possibility of the overcurrent protection relay at the main feeder circuit breaker to trip. Similarly if there is a group of motor control centres all in the same state of restarting their motors, then their common busbar e.g. main generator switchboard, will experience a significant volt-drop which may not be acceptable. 

No notable technical incident troubled the operation of the plant in 1996. Five unscheduled shutdowns, four of which resulted in the automatic activation of the protection systems with motorized control rod insertion (rapid shutdown), marked the operating periods of the reactor. These shutdowns, that were followed by a rapid restart, lasted only a total of about twenty days. 

SOLID-STATE VARIABLE-SPEED AC MOTOR CONTROLS CONTROL OF DC MOTORS DC Motor Starting and Braking Speed Control DC Drive Characteristics PROTECTION OF MOTOR CONTROLLERS Electrical Enclosure Types and Specifications Motor Overload (Running) Protection Other Types of Motor Protection Short-Circuit Protection Protection of Solid-State Motor Controls COMBINATION STARTERS Interrupting Rating... 

Current-responsive protector Devices that inclnde time-lag fuses, magnetic relays, and thermal relays (normally located in the motor or between the motor and controller) that provide a degree of protection to the motor, motor control apparatns, and branch-circuit conductors from overloads or failures to start. 

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