Relays and Contactors

How Relays and Contactors Work


Relays and contactors are electromagnetically actuated switches with spring-operated return. When a voltage is applied to the coil of the electromagnet an electromagnetic field is induced by the flow of current. This causes the sliding armature to be attracted by the coil core so that the relay contacts are closed or opened depending on how they are arranged. If the current flowing through the coil is interrupted, a spring ensures that the armature is restored to its rest position. The following animation illustrates how this works.




One or more contacts can be switched by a single relay coil. The contacts of the relay are depicted either separately or in relation to the electromagnetic drive, i.e. the coil. The following diagram shows the latter case for a relay (K1) with two NC contactors and one NO contactor.



Relay contacts are labelled according to a certain system: each first digit of a contact designation serves to number the existing contact paths or circuits. The second digit specifies respectively whether we are dealing with an NO, NC or a changeover contactor.

In modern circuit diagrams coils and relay contacts are mostly drawn separate from each other, as this leads to more clarity in the drafting of circuit diagrams. The following graphic shows the example of a lamp P that is indirectly switched on or off via a pushbutton S and a relay K. Diagram a) shows the contacts depicted directly at the position of the coil, diagram b) shows a separated depiction (the red arrow serves here merely to illustrate the relationship and is not part of the circuit diagram!). The circuit diagram on the left (1) depicts the control circuit whle the circuit diagram on the right (2) shows the main circuit.



ontactors operate according to the same principle as relays, but they have two interruption points per contact as well as arc quenching chambers. For that reason they can switch higher currents than a relay.

Applications in Pneumatics

In pneumatics relays and contactors are used primarily for the following applications:

For signal multiplication (switching several circuits simultaneously)
For signal conversion (switching circuits with higher voltage)
For signal time delays
To combine information (to form logical switching operations using more than one relay)
To isolate the control circuit from the main circuit






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