Motor control contactor type

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. 

Comparison of Switchgear and Contactor-Type Control Frequently switchgear is used for motor control, particularly for large high-voltage motors. Switchgear (Fig. 29-6) must be used for motors... 

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. 

It is therefore necessary to provide a sensitive method for detecting earth fault currents. The most common method is to provide a core balance current transformer at the circuit breaker or contactor. This current transformer has a current or turns ratio, which is independent of the ratios used by the transformers connected in the three-phase conductors. This is because a particular level of current is to be detected rather than a fraction or multiple of the stator load current. The switchgear manufactnrer will normally recommend the ratio of the core balance transformer and the matching relay. The relay will be either instantaneous 50 N or an inverse time 51 N type depending upon whether the motor is controlled by a circuit breaker or a contactor. 

When the start button is pressed, current will flow from the brown phase through the control circuit and contactor coil to the grey phase which energizes the contactor coil and the contacts close, connecting the three-phase supply to the motor, as can be seen in Fig. 3.29. If the start button is released the control circuit is maintained by the hold on contact. If the stop button is pressed or the overload coils operate, the control circuit is broken and the contactor drops out, breaking the supply to the load. Once the supply is interrupted, the supply to the motor can only be reconnected by pressing the start button. Therefore, this type of arrangement also provides no-volt protection. 

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