contactors and relays

[ecowarrior]

Is there a definitive difference between a contactor and a relay?
I appreciate contactors do the heavy end of stuff but I am after some 10amp ones to sit on din rails and can source both - just wondered what the pro's n cons might be?

 

[JohnW2]

Is there a definitive difference between a contactor and a relay?

Having often asked the question myself, it seems to me that there is no 'definitive difference' but, rather, a lot of debate and discussion about terminology! At the extremes, there are some things which most people would agree are (small) relays and other things which most people would agree are contactors, but in-between, things get less clear. Many contactors tend to be constructed so that additional contacts and/or interlocks can be added - which is something rarely,if ever, seen with something called a 'relay'.

In the final analysis, they are all (solid-state ones aside) just electromechanical 'switches' - so what really matters is the specification of the contacts in relation to one's application - I'm sure, for example, that you could find some heavy-duty 'relays' with 'superior' contact specifications as compared with some small 'contactors'.

That's how it seems to me, anyway.

Kind Regards, John

 

[ericmark]

What a great answer.
I have also asked the question and where around the 5 amp mark goes it is hard to decide as to which label to attach.
What I would say is in the main relay refers to non inductive load where contactor is often not a power factor of unity.
So if pressed then I would say with a unity power factor it's a relay but if not unity then a contactor.
Would be interested to see another definition.

 

[ecowarrior]

Yes John - good answer thanks - so on the specification then I imagine a relay that operates 500 times a day needs to be different in its mechanical properties as well as its ability to not build up carbon deposits on the terminals ( my application being heating would be at the very low end of the scale as they are controlling largish pumps so only switch a few times a day.)
What terminology or 'units' does one look for to find quality / suitability apart from a brand name ?

 

[Iggifer]

And contactors are generally noisier. At least in my experience. Relays tend to make a barely audible click, where as contactors make a definite clunk when they pull in.

Obviously the noise increases with the size of the contractors. Can be useful for troubleshooting but a little inconvenient in a domestic setting.

I've got a little 20amp DIN contactor that is the enclosed type rather than the big ones you can push in the contacts yourself, and that still clunks

Perhaps not so much of an indepth answer, but something to consider.

 

[RF Lighting]

I was of the understanding that relays are used for control, and contactors are used for load switching.

I'm not sure there's really any difference in the real world though.

 

[JohnW2]

I was of the understanding that relays are used for control, and contactors are used for load switching. I'm not sure there's really any difference in the real world though.

I'm not exactly sure what the difference is between 'control' and 'load switching'. There are certainly things called relays rated to switch pretty high currents - does that not count as 'load switching'? As you say, I really don't think there is any meaningful (or consistent) difference between the two words.

Kind Regards, John

 

[Lectrician]

IMO, contactors have an AC rating (AC1 - AC3), designed for switching loads. Relays may have ratings upto 10amp, but they have not been given an AC1 - AC3 type load rating.

I would go with RF - Use relays for control purposes, and contactors to switch the final load.

 

[bernardgreen]

Genrally the distinction above between control by relays and power by contactors is correct.

Relays tend to be "as is" and non repairable where as most contactors can be configured and / or repaired with coils or contact sets being replacable.

This is because relays are used in well defined and stable circuits and seldom need repair where as contactors are used with loads that while defined may in fault conditions ( stalled motor, burnout motor ) lead to damage to the contactor.

 

[ban-all-sheds]

most contactors can be configured and / or repaired with coils or contact sets being replacable.

Even the DIN rail mounted ones which are 1 module wide and have moulded cases the same shape as an MCB?

 

[ricicle]

most contactors can be configured and / or repaired with coils or contact sets being replacable.

Even the DIN rail mounted ones which are 1 module wide and have moulded cases the same shape as an MCB?

 

[bernardgreen]

OK OK I am a few years out of date.....with an old man's memories of big and chunky clunky contactors.

It seems in these modern days the manufacturers are going for "disposable" rather than repairable when making contactors.

 

[JohnW2]

IMO, contactors have an AC rating (AC1 - AC3), designed for switching loads. Relays may have ratings upto 10amp, but they have not been given an AC1 - AC3 type load rating.

That's very sensible, and probably corresponds generally with how most people think about it, but there is far from universal consistency. If you look around, you'll find 'relays' with 100A or higher AC3 ratings, and 'contactors' rated at 5A or less. Just to add to the confusion, some manufacturers offer things they call 'relay contactors'!

As I have said, and as you are implying, what matters is the rating of the contacts, regardless of what one chooses to call the device!

Kind Regards, John

 

[JohnW2]

OK OK I am a few years out of date.....with an old man's memories of big and chunky clunky contactors.

Albeit not AC rated, I wouldn't mind betting that 'relays' like this one go 'clunk'

Kind Regards, John

 

[stillp]

I think Lectrician is nearest, although many relays have a utilization category such as AC-1 or AC-3 as well.
Here's the definition of a contactor from BS EN 60947-4-1:
mechanical switching device having only one position of rest, operated otherwise than by hand, capable of making, carrying and breaking currents under normal circuit conditions including operating overload conditions
NOTE 1 Contactors may be designated according to the method by which the force for closing the main contacts
is provided.
NOTE 2 The term "operated otherwise than by hand" means that the device is intended to be controlled and kept
in working position from one or more external supplies.
NOTE 3 In French, a contactor the main contacts of which are closed in the position of rest is usually called a
"rupteur". The word "rupteur" has no equivalent in the English language.
NOTE 4 A contactor is usually intended to operate frequently.
However the corresponding standard for relays (60947-5-1) doesn't have a definition!
The IEC online glossary is down at the moment, so I'll check for a definition tomorrow.