There are many who has answered this question before:-
Most say relays are JUST smaller brothers of contactors.
Others say:- Up to a certain amperage (30/40Amp) it is a relay and beyond that it is a contactor.
Others say:- In the construction, the relay coil sits on the side and a contactor it sits in the middle or something similar.
Then there are those that respond with sarcasm or even sneer, since it is nearly irrelevant to ask what is a relay and what is a contactor? Just accept it!
But why does the two words really exist…can I say a relay is a type of contactor and vice versa?
The truth is – all of the above answers are correct…but only in their own small way, shall we say – only in their own right.
So let me give it a bash.
Lets look at the etymology of the two words:-
Relay: “A group of people or animals engaged in a task or activity for a period of time and then replaced by a similar group”
Or my version: “The concept of passing something from one person/thing to the next generally in order to complete a demanding task”.
Another angle of meaning – and this is the one I am most interested: – “To receive or pass on” something like information…e.g. “they relayed the information to the secret service”.
Now compare this to “contactor”:
What does the word “contactor” mean – I tried the etymology – nothing on google.
So lets look at the word literally:- It is something that brings things together. It is even used in the context of chemistry when different mediums collide/connect – contactor.
There done. You don’t see it yet??
Lets look at the start of the telegraphy / telephone industry – The unit used to make the special dial code connections to your number thus relaying it to your telephone – that unit is called a “relay”, isn’t it, Thus we have a relay even till today doing similar but not the same job due to newer technologies, but its still there.
In the electricians world a relay will always be found in the “control side” of the circuit. ALWAYS!
In fact prior to PLC the switching logic in control panels was achieved by RELAYS – they can switch fast, they can switch small signals and one can then build intelligent controls around them, but even more important is the fact they come in many contact versions…in one relay, standard available, can be 4 normal open and 4 normal closed contacts!
And so it also is in the industrial complex that many “electronic modules” are called “monitoring relays” or “controlling relays” even though it is a complete control box…it takes an input signal and repurpose it for a, NOT NECESSARILY BIGGER, OUTPUT SIGNAL for the purpose of intelligent control.
On the other hand when we want to switch a huge motor – ON/OFF CONTROL – pulling hundreds of amps there is no repurposing – we just want to make and break a CONNECTION – voila the contactor.
Now I have relayed the information as best as my english allows me to, and I hope this question is forever answered.