Compressor only works for one cycle

I have an ABAC twin cylinder 3HP compressor (3000rpm). When I fire it up from cold, it runs and gets up to 120-130 psi and then turns off with a pop and a hiss - all sounds good.

Once I've used a bit of air and it gets down to whatever the set point is (i've adjusted it up and down between 50-80psi lately) it fires back up, but seems to run out of oomph as it passes 90psi. It will start to struggle and more often than not, as soon as the revs drop below a certain point, it rapidly declines and stalls with and angry buzzing sound.

I just wondered if anyone had any input or experiance before I start pulling it all to bits - is there perhaps anything pneumatic or electrical that it could be?

Edited By Russ B on 25/08/2016 13:58:59

Edited By Russ B on 25/08/2016 13:59:55

Are the fins on the top of the cylinder getting hot to touch? Sounds like a thermal problem rather than electrical. Heat soak may be causing loss of clearance somewhere in the compressor.

Martin

Probably the reed valves not sealing properly due to getting hot or there cracked

they are at least quite easy to strip the heads off and check

Abac actually sell spare parts as well

Trevor

Does the motor actually get to full speed on restart? If not it may be starting on load due to back leakage and then struggling because the start windings have not got it up to speed quickly enough?

Try starting from very low pressure, or zero, immediately after it gets to the stalling situation. That would determine if it is the compressor getting tight or the motor not delivering full torque.

Hi Russ,
Does the compressor have an unloader valve ? If it does then it may not be working correctly. If it does not then fitting one may help. This suggestion is an extension to "not done it yet's" comments. An unloader valve is fitted between the compressor and a non return valve at the inlet of the air receiver. The unloader valve bleads off the high pressure air in the pipe between the compressor and the non return valve. This means that when the motor tries to start it less load on it as the compressor does not have a high pressure on it's output. Have a look at this link for more information on unloader valves.

Les.

I doubt it is an unloader valve problem as it is starting up and pumping from 50 psi to 90 psi. Anyway, the 'pop and hiss' mentioned in first post suggests it's got one.

It's strange that it works from cold, but not second time. Is it still hot when it starts the second time, and so is getting hotter second time round?

Hi Duncan,
An unloader valve is not needed when starting with no pressure in the air receiver. An unloader makes the compessor think it is working into an emply receiver.

Les.

Try running it up to full pressure then turn it off at the power switch and dump the air in the tank and then turn it back on .

If it runs ok you have a back pressure problem that is stalling the motor .

If there is no change i would look at the motor capacitors or check if the pump is seizing .

You didn't mention if it is single phase or 3 phase ?

Ian.

thanks, I'll give this a try tomorrow and maybe borrow the FLIR camera from work over the weekend (hours of fun to be had with that thing - its quite literally a whole new perspective on EVERYTHING!

Don't try to complicate things. You do not need an infrared thermometer. Just check as I said earlier. A rev counter would be more appropriate.

Not relevant. It starts OK at 50 psi, then gves up the ghost at 90 psi, when presumably it has been running for a while. How can this be down to the presence or not of an unloader valve?

I wonder if the check valve may be jamming shut at the end of the first cycle and binding up so it won't permit proper air flow ?

Just a thought .

P.s how old is it ?

Ian

Hi Duncan,
Good point. I now realise I had not read the original post properly. I was thinking it had problems straight away when it tried to restart.
XD 351,
If the check valve had jammed shut you would not see the pressure start to rise again as the gauge will be on the receiver side of the check valve. It would be nice to see the readings of a pressure gauge between the compressor outlet and the check valve. (In case the check valve was partially blocked.) It would also be interesting to monitor the current taken by the compressor and compare the readings between the first time it charges the receiver as it gets to about 90 PSI and the same after a restart.

Les.

Hi there, Russ,

I hope that you find the solution to your problem soon, and that the fix is not costly.

But I'd like to put in a plea that you come back and tell us all what the problem was and how you fixed it. It can be a touch frustrating when threads are inconclusive.

Best regards,

Swarf, Mostly!

Firstly get a regulator and stop messing with the electrical stop start switch to vary the pressure they are meant to be set just under max pressure at the factory and then left alone. you have been cycling the motor twice as often than it should. These motors are not normally rated for constant running and can cook.

The check the fan is on the right way round some times they go on backwards and errrr suck!

Hi Russ,

Does the compressor require oil or is it of the oil-less type? I'm wondering if it's starting to seize when it gets warm/hot?

Ed.

How can this be down to the presence or not of an unloader valve?

'Presence or not' or not working properly?

The very simple check/test of seeing if it works from zero pressure, immediately after showing signs of stalling, will indicate whether the motor is actually restarting correctly after a high pressure restart.

Very simple, easy to do and informative. The job needs analysing as to whether the problem is due to the motor not fully speeding or the compressor tightening up.

The motor is well oversized for the job. My 10cfm,   @10 Bar, compressor has a 2HP motor, but only 1425rpm (so better starting characteristics)

Guessing is no good at all. Actual checking , to isolate the problem, is the first priority!.

Edited By not done it yet on 27/08/2016 07:21:05

Had similar problem with an Atlas Copco KE2 16 cu ft / min vee twin compressor when using enough air to require restarts at fairly frequent intervals, maybe 25 minutes or less. Turned out to be due to a partially blocked bleed down orifice on the controller leaving too much pressure in the outlet manifold so the capacitor start / capacitor run motor couldn't come properly up to speed before the backpressure got too high. Effectively motor ended up running at reduced power.

Was told by a motor guru that cap start / cap run motors can settle at a reduced speed / reduced power run point if they aren't given time to get right up to speed before coming under full load. Don't understand motors well enought to know if this really is the case of if its an old guru's tale to sidestep pesky questions but it certainly fits with what I've seen in other cases where capacitor start / capacitor run motors have struggled. Especially the economy range types. In similar circumstances an ordinary capacitor rum motor will keep dropping back into, or never get out of, start mode but a capacitor start capacitor / run motor has a bit more low down punch.

When sorted the Atals Copco outlet manifold bled down to atmospheric in about 3 minutes. Its volume was around 1/4 cu ft so I imagine providing a similar ratio of bled down volume to compressor output would endure that the motor will always come properly up to speed.

Clive

Dave H, Fan on backwards ? Apart from a mounting issue, there wouldn't be any difference would there? A Left Hand fan is a LH fan whichever way around (same for a Rh fan). Or have I blundered again.
BobH

Just a quick update as I'm replying from my phone.

This morning from clock cold, with 50psi in the tank (it was 90 last night?) it struggled around 100psi and cut out. And then refuse to restart. I could spin the compressor with a single finger by its fan and could hear the reed valves popping so it was compressing.

I removed (very carefully) one of the pipes between the cylinders (there are no check valaves between the 2) and restarted it, and nothing, the motor was just making angry grunting sounds while the fan wobbled as it attempted to move. Feeling brave I gave the fan a good spin with a stick to get it moving and see if it was stuck, but it didn't, and it wasn't.

It feels very much like a motor issue. It only has one 40uF cap on top, I've no idea how to test it but I think I saw a capacitor testing machine at work (I've no idea what to do with it but I'll ask).

I'll check the cap and then pull the motor apart if needed. The motor is part of the crankcase so I can't just change it. I'm hoping it's not also the crank but I've a feeling it almost certainly is - looks like a complete strip will be needed to look inside it.