Compressor 3hp motor DOL starter question

Styleruk - Would like to post a picture but apparently jpg is not recognised so I can't be bothered to figure that out.

pc1966 provided a top tip which I tried and it worked for me too: Post the image to google pictures and then insert a link in a forum message to that google image. To insert a link in a message use the chain button in the toolbar immediately above.

Google Photos - All your photos organized and easy to find - https://www.google.com/photos/about/

Sorry, bit late to the party, but we too have a compressor - 3hp in our home workshop, and it may be worth explaining what I fitted for my husband.

Machine Mart are not electrically qualified, and merely sell the cheapest things at the price their customer's want - so electrically, fitting a cheap motor via a simple switch on a 13A outlet will work. - However, in time this will breakdown at which point they can sell you more cheap parts! - win win!

In our case we had issues - once I found out how it was wired and so on - I did the changes below! (Husband buys stuff, plugs it in - only mentioned to me when it goes wrong!)

The standard arrangement is that the Line comes in through the pressure switch, then direct to the motor - standard run and start capacitors in the motor.

So the little pressure switch makes against the full start current ~50-60A, and once up to pressure breaks the highly inductive full load current of the motor ~10-12A - it doesn't last long!

So, I provided a double pole isolator (Local isolation) with a much higher capacity than strictly needed, and a contactor/overload (DOL), rather importantly I wired the coil to the starter from Line - through the pressure switch, to the coil (other side to neutral) to ensure that it is the starter that makes and breaks the motor current - NOT the pressure switch which is how it is wired as standard by MM.

The isolator is sized to be able to break FLC, but operationally we open this only once the motor has stopped.

It actually starts better than via the pressure switch (I think the high start current lifts the pressure switch contacts a little making a high contact resistance ).

One other issue was that the capacitors in the top of the motor get incredibly hot, as a result they do fail - noticeable once you have a proper overload - for technical reasons the run capacitor reduces the current seen by the overload (Known as power factor correction) once it fails the current goes from 10A to around 15A - this is not protected for by a simple fuse!
(after this I went round to a friend of my husband's, his compressor was just plugged in and destroying plugs and socket outlets repeatedly - his run capacitor was also shot, so had been running for months at 15A or so!

In our case we replaced the capacitors with the same size (electrically), but higher quality (and about 3-4x the physical size) - in an external enclosure. Shug just bought the replacement ones from MM - tiny - they fit inside the motor terminal box, we have not had any issues over the past several years, he had to replace them a couple of times!
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Oops, just realised I lied about the compressor wiring!

After some time I added a timer - so the control circuit is -
Line - Via the contacts on the timer, then through the pressure switch, to the coil (other side to neutral).

Because the Muppet would leave it switched on and it would fire up in the early hours of the morning!!!!

Now it only works 08:00 - 21:00 and we don't annoy the neighbours (well not with this anyway!)

OK, been a while, but I did promise a picture. This is the old girl finished up. the compressor did a grand job and held up nicely spraying, this is my first attempt at spraying, so not perfect, but suits the level of practical classic she is.
The weakest part of the spraying was damp. I bought an inline air dryer, cool piece of kit and suited my budget (second hand of course), but could not do long runs of spraying, however, it did dry the air perfectly.
Now, I do have an issue. Not used the compressor for about 4wks due to holiday etc. Went to turn it on and it tripped the main board.
I checked...
Took belt off pump, same issue
Socket; all checks out good
DOL; all looks secure and good
Auto shut off junction; all good.
Checked starter capacitor, measures 270µf, rated as 200....guess that's OK?
Not yet checked the running capacitor, maybe that's the issue...doubt it though.

I've ran out of things to check, could it be the actual motor faulty? It spins OK, so it's not jammed.
Any thoughts? I'll have another crack at it this weekend, wife don't like it when the house trips and she is left in darkness. So any suggestions (apart from pay an expert), is welcome.

styleruk said:

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View attachment 112073

OK, been a while, but I did promise a picture. This is the old girl finished up. the compressor did a grand job and held up nicely spraying, this is my first attempt at spraying, so not perfect, but suits the level of practical classic she is.
The weakest part of the spraying was damp. I bought an inline air dryer, cool piece of kit and suited my budget (second hand of course), but could not do long runs of spraying, however, it did dry the air perfectly.
Now, I do have an issue. Not used the compressor for about 4wks due to holiday etc. Went to turn it on and it tripped the main board.
I checked...
Took belt off pump, same issue
Socket; all checks out good
DOL; all looks secure and good
Auto shut off junction; all good.
Checked starter capacitor, measures 270µf, rated as 200....guess that's OK?
Not yet checked the running capacitor, maybe that's the issue...doubt it though.

I've ran out of things to check, could it be the actual motor faulty? It spins OK, so it's not jammed.
Any thoughts? I'll have another crack at it this weekend, wife don't like it when the house trips and she is left in darkness. So any suggestions (apart from pay an expert), is welcome.

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The car looks lovely!

DPG said:

The car looks lovely!

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Thank you, if you look closely though (which my paint supplier did), it's not perfect. Don't think I'd spray an exotic classic but next up is an old 84 Land rover.

styleruk said:

Not used the compressor for about 4wks due to holiday etc. Went to turn it on and it tripped the main board.

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What is tripping, an RCD or MCB?

styleruk said:

Any thoughts? I'll have another crack at it this weekend, wife don't like it when the house trips and she is left in darkness. So any suggestions (apart from pay an expert), is welcome.

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If lights are going as well, sounds like the RCD. Generally that implies an insulation fault, might be hardware really failed or water in somewhere.

Also at what point is it tripping, the moment the supply is connected, or when you try to start the motor running? That could tell you something about where the fault it.

If it is a hard short somewhere then a multimeter might reveal it, but often you need a high voltage (250V or 500V DC) insulation tester to really find out what is happening as far as 230V AC is concerned.

pc1966 said:

What is tripping, an RCD or MCB?

If lights are going as well, sounds like the RCD. Generally that implies an insulation fault, might be hardware really failed or water in somewhere.

Also at what point is it tripping, the moment the supply is connected, or when you try to start the motor running? That could tell you something about where the fault it.

If it is a hard short somewhere then a multimeter might reveal it, but often you need a high voltage (250V or 500V DC) insulation tester to really find out what is happening as far as 230V AC is concerned.

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RCD trips.
It could be water ingress, there is a small hole in the roof nearby and in extreme weather, like we had when I was on holiday, water may have got in there, never happened before but there is a small chance this could have happened. It seems dry, maybe I should take the motor off and have a good look at it. Pic attached showing the wiring at the top, I took this so I could zoom in on my phone and have a closer look at the wires. (start cap taken off)

If an electric motor has been unused for a while and cools down to low temperature its windings can absorb moisture from the air drawn inside the motor casing. Obviously more of a problem during damp conditions like you mentioned. One should not attempt to dry the motor by running it which can damage the windings irrevocably. Instead try and blow warm air on the motor from a fan heater so that its body temperature is elevated to say 40- 60C and maintained there for say 8 hours. Avoid blowing warm air on the capacitors.

Wait, I tested the caps again with my shonky electrical tester. The running cap rated at 50µf measures exactly 50µf. The starter cap rated at 200µf measures 260-274µf. What does this mean? Does this mean the cap is buggered? How does a cap fail, does the farad reading go up or down? To me that seems a lot out.

styleruk said:

Wait, I tested the caps again with my shonky electrical tester. The running cap rated at 50µf measures exactly 50µf. The starter cap rated at 200µf measures 260-274µf. What does this mean? Does this mean the cap is buggered? How does a cap fail, does the farad reading go up or down? To me that seems a lot out.

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A capacitor can still read correctly as far as the capacity (uF) value goes, but can still be duff. Electrolytic capacitor tend to dry out and their ESR (effective series resistance) increases. The ESR can be measured with the right meter, but it's cheaper to try a replacement normally.

It is not uncommon for the sort of capacitor used for motor starting to be 20% or 25% tolerance, so it might still be in-spec, however, if you have any reason to doubt it then replacement might be cheaper as @DPG suggested.

Out-of-spec capacitance won't trip your RCD though, that would require leakage to earth in some way. Insulation fault is most obvious, but sparking and a bit of filter capacitance, etc, can cause intermittent issues. Ideally get someone with an insulation tester to check for L+N to E leakage at 250V or 500V DC.

pc1966 said:

It is not uncommon for the sort of capacitor used for motor starting to be 20% or 25% tolerance, so it might still be in-spec, however, if you have any reason to doubt it then replacement might be cheaper as @DPG suggested.

Out-of-spec capacitance won't trip your RCD though, that would require leakage to earth in some way. Insulation fault is most obvious, but sparking and a bit of filter capacitance, etc, can cause intermittent issues. Ideally get someone with an insulation tester to check for L+N to E leakage at 250V or 500V DC.

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That's a good idea, I think this weekend I'm going to wire direct from the motor to a plug and test if it starts, just to check that the DOL or socket is not causing an issue, at least I'll see if the motor spins up, then I know it's not the caps or motor and can re-look at my wiring at the wall. For all I know the Clarkes DOL has copped out.
One small thing I noticed, first it tripped the RCD at the house and everything went off (reason why I'll only test at the weekend and not in the evening, wife is getting irritated). But after I tinkered and rewired all the socket connections, it then triped the garage isolator (at the house board), and not the RCD, that tells me something different that I only partly understand. Which is why I'm thinking of bypassing the DOL and wall socket for now just to test.

Another question I have, where is a good place to buy a replacement starter Capacitor. The internet is awash with wrong products.

Before I plug direct to the motor, I checked continuity across live/N from the motor and it shows 0 resistance, is that right? More worryingly, earth to L/N was ~500.

styleruk said:

Before I plug direct to the motor, I checked continuity across live/N from the motor and it shows 0 resistance, is that right? More worryingly, earth to L/N was ~500.

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