N2 Electrical Trade Theory|The Easy Way! 35 2.6.3 Contactors


Contactors are, just like relays, electromagnetic switches, except that they are larger than relays as they are used to control larger currents, fig. 2.9.


Tey usually have a main set of contacts which control the main circuit, such as the supply to a three-phase electric motor, and an auxiliary set of contacts which carries the current of the single-phase control circuit.


Te following table compares disconnectors, relays and contactors with regard to various factors as indicated by the headings:


Comparison Disconnectors


Disconnectors, relays and contactors Relays


Construction A disconnector is a switching device which is capable of opening contacts wide enough to isolate a circuit to comply with regulations. Some can be locked in the open position.


Operation Function Use Switched manually.


To open contacts wide enough to isolate a circuit.


To isolate a circuit in order for repairs and maintenance to be carried out.


Small electromagnetic switch with one or more sets of contacts.


Contactors


Large electromagnetic switch with one or more sets of contacts.


By means of a coil (solenoid) and can be operated remotely.


To open and close contacts in order to open or close a circuit.


Te primary circuit (coil) controls a secondary circuit (contacts), e.g. a relay controlling the headlamps of a car and the tripping relay found in earth leakage devices.


By means of a coil (solenoid) and can be operated remotely.


To open and close contacts in order to open or close a circuit.


Te primary circuit (coil) controls a secondary circuit (contacts), e.g. a contactor used for the direct starting of a large motor and forward and reverse contactors used on electric cranes.


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