Discuss compressor fault.. in the Commercial Electrical Advice area at ElectriciansForums.net



Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:  American Electrical Advice Forum

Hey all

I had a call out today to a Motorbike shop.

In the village where the shop is they had had a power cut the night before, they had come into work the next day, the power had been restored so they powed up, everything seemed to be fine exept the Compressor used by the mechanics in the workshop.

So i was called out to look at the compressor.

I have not done much 3 phase at all, nearly all domestic. but i dont want to turn any work down and want the experiance in every field.

I got to the compressor and it was a very old thing, with an isolator on the wall wich was feeding what seems to be an overload unit, (red/green start stop buttons, i could be wrong).

I sourced the fault to be the contactor inside this unit, the coil inside the contactor seemed to have got that hot it had melted the plastics surounding it, it had gone black with a slight smell. This had stoped any contact between the phases passing through it.
I had got the contactor out and manged to push in with my fingers the contactor, when i did this i put my tester across the terminals getting it to make a contact.

The photo shows the contactor

Now my question is.. What would of caused this to overheat? there would seem to be too much current down one of the phases some how causing it to get too hot and melt the plastics around the coil.

How can this be prevented from happeing again once the contactor has been replaced?

(Sorry to have gone on)


  • 28300001.jpg
    54.9 KB · Views: 28
Last edited by a moderator:
CK Tools :) The professionals choice when it comes to Electrical Tools
This official sponsor may provide discounts for members



The overload unit you describe sounds like a D.O.L (DIRECT ON LINE) starter which is used to start up small 3 phase induction motors.

Now the cables going to the compressor should have been connected to an overload which is usually connected to the bottom of the contactor and protects the contactor and associated cableing and switchgear
from overloading - getting hot and burning.

As you havn't mentioned an overload I'm guessing there wasn't one??? this could be one reason for the contactor melting. Another could be a loose connection on one of the terminals which has been arcing over a period of time causing heat.

Either way a new contactor and overload are needed and should be sized in accordance with the rating of the compressor.

I would also inspect the compressor for any signs of overheating melting etc.

Also when you re-connect the compressor and switch on, I would use a clamp meter on the phases and the neutral to check that all is balanced and one isn't pulling a lot more than another.



the DOL unit is fixed to the front of the compressor, from this is a feed to the motor on the compressor and also another flex to a small black box ontop of the compressor, (as shown in attached photo). The only difference being it doesnt have a gauge on it like the attached photo, it has a Auto & Off setting on it. Am i right in saying this is a cut out for the compressor when it reaches a certian pressure? This couldnt be related to the fault in anyway....could it???


  • 000115064.jpg
    31.4 KB · Views: 19



Yeah that little black box is exactly that, a regulator. You will prbably find that the flex going to it is connected to terminals - A + A1 on the contactor to bring it in and out when required.

I wouldn't have said this had anything to do with the fault at all.

Once you've replaced the contactor and fitted a correcly rated overload then the contactor will be protected by the overload as the cable feeding the compressor is connected into this overload and in turn the contactor i.e overload will operate and disconnect load before anything gets too hot as it is thermally biased.



It may not neccesarily be a fault with anything, the contactor is a moving part which will eventually where out and need replacing. like what has been said before replace the contactor with a new contactor/overload, and check what amps its pulling making sure its what it should be a according to the motor and that its balanced. Some isolators have fuses in them as well, worth checking them to make sure none of them are blown. :0)


Hi All


I have the ABB contactor off..

Im am struggling to find the EXACT same contactor at the same Voltage rating.
Its 415v 3p supply, and im sure the coil is 415v.. does that sound right??

there is a list of them here Rapid Electronics - Electrical & Power

But i cant find the exact same one???

Can anyone here point me in right direction ??


Hey mate.

What you've described sounds right. A 415V contactor with a 415V coil, what that means is that the coil is energised by two phases as opposed to 1 phase and neutral for a 230V one.

You don't need to get the exact same make any make will do as long as it's the correct rating.

What I would suggest is for instance, size the contactor slightly bigger than stated load of the compressor, and then size the overload to protect the cableing supplying the compressor i.e if it were a 4mm conductor then I would size the overload either the same as or slightly lower than the current carrying capacity of a 4mm but obviously higher than the load of the compressor.

Hope this helps.

Reply to compressor fault.. in the Commercial Electrical Advice area at ElectriciansForums.net