Compressor only works for one cycle

Also I noted on the thermal imaging camera the motor was around 85degress but was 100 at the top - I'm thinking it's possibly a winding issue or perhaps just the start winding not dropping out (I don't know if this is that kind of motor - I don't really do electrics beyond what I have to to get by)

Before pulling the motor apart, I'd replace the motor start/run cap....Cheap enough, around £7 from the world supermarket....

It does sound like a cap issue....

Yes but it needs to blow air over the cylinders. fitted back to front it sucks air over the cyliners and nowhere as efficient at cooling

Dave H , Would it ??

Fittred back to front it would still blow in the same direction but depending on blade shape it might be not as good

Ahh! I'll get me coat

Before dismantling the compressor, either replace or have the START capacitor tested. These capacitors are high value AC electrolytic and are only in use on start up, they give the motor more grunt on start up. Once started the motor continues to run on the run capacitor only, the run capacitors are usually very reliable. A friend had a similar but not the same make of compressor and his fault was the unloader valve.John

Capacitors usually work or don't. Here we have the case where the failure is not apparent when starting with an empty tank and cold compressor/motor, but only when starting against a part filled tank after one cycle.

The simple question is whether it will run normally, for one cycle, if started from an empty tank but not completely from 'cold'.

Plenty of guesses but no diagnosis whatsoever so far. A bit like a doctor diagnosing an ailment with only one piece of information, like 'I don't feel well'!

Dont agree......capacitors can lose their capacitance, electrolytics dry up......

I have plenty of experience of 'faulty' capacitors having worked in the brown goods/electronics industry.....

Folks, as per the last post on the first page (although I admitt, it wasn't a clear as I was rambling in the moment using my phone to reply)

I have completely eliminated the starting against load by disconnecting the compressed air pipes from the heads so both cylinders are just venting to atmosphere and for the first time ever, it's completely failed to start under 0 load - even when I prodded the fan blade and gave it a good spin, it still didn't go.

So now it won't start at all. And before it would start and either start fine and run well once, then start stalling at high pressures,

So I'm a bit confused - and my motor only appears to have one capacitor, unless there is one inside it somewhere.

Thanks for all the help so far, I'll get this capacitor change as its a cheap and easy thing to do. Perhaps we have 2 faults combined here, maybe it does stall at high pressure, and it's damaged the motor?

Edited By Russ B on 28/08/2016 14:22:42

Your motor is probably an SPC type( Split Phase Capacitor ) which will only have 1 capacitor in series with the start winding.....

If your motor isnt starting at all now, then most likely is the cap has insufficient value to produce the required phase shift for starting....

Like I said before, and as you indicate, replacement will rule it out..

> even when I prodded the fan blade and gave it a good spin, it still didn't go.

Sorry, but that suggests it isn't the start capacitor that's the problem, you can usually start a motor with a duff capacitor or centrifugal switch by spinning it.

If you have a multimeter and know how to use it safely, check power is getting to the motor, you may have a sticky pressure switch.

neil

Well the cap didn't change anything, but considering how easy it was to change, it was well worth a shot.

I'll get the meter on to it. Neil, If I check the voltage at the capacitor, I presume that is sufficient? I doubt the mass of that rather large motor would be able to heat up to over 100 degrees if it weren't getting the full 10amps - but my fingers are crossed non the less.

I doubt the mass of that rather large motor would be able to heat up to over 100 degrees...

I would not be surprised and would not be taking chances with an expensive rewind job, or replacement possibly looming nearer.

Think here starting current - three or more times the running current. Thermal losses are a function I f the suare of the current (partly why number of restarts per unit time is restricted on many motors).

Think here that the motor efficiency is appalling at lower speeds (not producig the expected mechanical output, is it?).

Think here that the cooling air flow is severly reduced at slow speed operation (likely a cubic function).

Think here that it only needs two wires, in contact, to rise above the windigs enamelling melting point before it may well fail catastrophically.

I would say you are pushing your luck by continuing to run the motor in an unsatisfactory mode of operation. it may have either thermal overloads set conservatively or a winding thermister trip circuit (both of which are in doubt).

Do you have a tachometer or are you still relying on thermal IR as your technology aid?

Not done it yet,

Thanks, that has opened my eyes a lot but it doesn't change anything, I wasn't running the compressor, I have only run it for the purposes of diagnostics, and now it doesn't run at all.

I have an optical tacho but I wasnt savvy enough to use it, I just assumed it was running up to pressure quickly enough as it was able to fill the 100 litre tank to 9 bar very quickly first run, and only started steadying up seemly on the 2nd/3rd fill. Obviously that performance rapidly diminished as per the descriptions in this thread.

I will sell the whole things as spares repairs and clearly describe the fault. I'm having a clear out and this had to go. I naturally wanted to fix it to maximise its value but I'm also equally happy to sell it as scrap if that is what it is - if the buyer gets what they expected/wanted I'm happy. If I sold something I wasn't 100% happy with it be playing on my mind for ages.