Discuss Relays and Contactors in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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axel132132

Hey,

I don't quite understand Relays and Contactors previously ive worked for a company and if we ever needed a contactor they just said heres a contactor wire it (For example we used contactors for the air conditioning switching)

I know how to wire them and that but i dont UNDERSTAND why they are used and when to use them as i am now self employed it may be worth having the knowledge! if anyone can help as to why they are used, when they are used and what they are used for.

Thanks alot
 
They are mainly used to to allow a low current signal to switch larger currents. Allows one signal to switch multiple circuits (depending on number of contacts) or a 24VDC signal can switch a 415V 3ph supply, Before the advent of PLC's they were used to create "relay logic" circuits.
Plenty more uses/applications.
 
Probably the vast majority of contacters i have encountered have been used to switch on motors. They are used to switch the high current a motor demands. The start, stop and control of the motor is usually done by "signals" from control devices or buttons, which have a very low voltage applied across them making it more reliable and safer.
When the supply fails and then returns, a contacter with appropriate controls ensure the motor does not start without manual intervention which makes it much safer.

Vast majority of relays i have come across are part of an control strategy to enable signals to be logically manipulated to give the desired output, sometimes they are used to enable a small signal to switch a large load but nowadays plcs have largely replaced relays but contactors live on.....

If you have been fitting them, then you can at least follow the drawing to see what they are switching - but look at what is making the contactor or relay energise, this is the real story about why they are there
 

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