Compressor blowing fuses

[NathanPrince]

Hi guys, got a 2.5hp 150L belt driven compressor here that wouldn't start, after stripping down the pressure switch and freeing up the stuck diaphram that holds the switch in the off position when the compressors up to MAX psi it switches on and runs lovely.

It gets to 120psi and auto shuts off as it should, when the compressor then reaches 70psi and the motor kicks back in the fuse blows. Cant seem to think whats doing it. Its fitted with a 13amp fuse in the plug as suggested on the motor. Could it be a short somewhere on the motor or a similar problem to the Clarke welders where they come with a 13amp plug but recommend a larger one?


Any help would be appreciated as its holding up my restoration on my Capri

Thanks

 

[NathanPrince]

Also if your using an air tool and keep the compressor from going into auto shut down it will run all day long!

 

[Bill Edwards]

Do you hear a hiss when the compressor shuts off, either when at the required pressure or when you press the stop?

It should have an unloader valve which releases the air in the pipe between the pump and the tank so the motor isn't having to start under load.

 

[NathanPrince]

No I don't hear a hiss, it used to before it stopped working but now it just builds up and shuts off. It's the little 13mm bolt on a block that it should hiss from isnt it?

Even when it used to hiss though it still used to blow the fuses occasionally.

 

[Bill Edwards]

I'm not sure, but that's the problem and it won't work properly without the unloader valve releasing the pressure.

It might be that you're asking a bit too much from a 13a fuse anyway if it was blowing before, but it must have the unloader valve to ever stand a chance. Try stripping it down if you can and get it working, or you can buy new pressure switch units.

 

[NathanPrince]

I'll see if I can get the bolt out and see whats behind it, the actual switch is now working perfectly but the unloader thing in on an elbow coming in/going out of the "engine"

Would you try a larger fuse and see if that helps too?

 

[Bill Edwards]

The pipe that goes from the compressor goes to the tank through a non return valve. It'll have a small pipe running from this valve back to the pressure switch. The pipe is on the pump side of the non return valve so that when vented that pipe will remain empty.

As to the fuse rating, a sparky would generally be the person to advise. If the unit says 13a then I couldn't suggest trying anything bigger.
But I do know that it is not uncommon for compressors to state that they need a larger supply, such as 16a. For these, the correct solution is to wire it in without a fuse but to a trip switch for the correct rating - and usually a 'Type C', which are more suited to the sudden current draw of compressors.

When I say wire it in without a fuse, this means ditching the 13a plug as you aren't supposed to put nails in place of the fuse. It means through a heavier duty round, blue plug/socket, or wired straight to the trip switch.

 

[rtbcomp]

I'd sort the non-hiss out first. A 2.5 hp compressor should be fine on a 13A plug, but if you use it on an extension lead you could hit problems with fuse-blowing.

 

[NathanPrince]

Saying that before I used it on an extension but put a much longer lead on it to reach the mains today. Maybe that would of solved it if it wasn't for the non-hissing! Vicious circle of problems! I'll check it out tomorrow and see if I can get it hissing again.

Thanks for the help

 

[NathanPrince]

It looks as though the fibre washer behind the bolt has expanded loads, could possibly be stopping the air from coming out, will try getting it undone tomorrow and sticking a new washer in

 

[NathanPrince]

Got the pressure relief valve hissing again today but still blows the fuse on startup, Don't wanna be wiring any new circuits up for a 16A plug so any other ideas?

 

[Bill Edwards]

So it's venting the air in the pipe as soon as the compressor builds up to pressure and shuts off?
If it's fully venting and there is no load on the pump (if in doubt, slacken the pipe to prove it's empty) then that only leaves a motor problem, stiff compressor (normal problem is thick oil, but it's doing it while warm isn't it?) or a poor power supply - voltage drop.

Some suggest removing the belt to check everything turns freely, and even to try it without the belt on, on the basis if it still blows it's the motor.
Hopefully someone will come and see something I've missed!

 

[ford34/2]

Guess its time to remove the belts, run the motor without any load on it whatsoever, 21/2 Hp should run on a 13 amp, or double check the pipe after the valve hisses,double check there is no pressure in the pipe, the compressor will not like starting "under load", eg pressure in that pipe is sufficient to to put a considerable starting load on the supply, its only a split second but quite frightening if you can check the amperage its pulling for the split second startup. Clamp on ammeter,
Does the compressor start up ok whan the tank is empty????? it then fills the tank to SWP, cuts off, unlader valve hisses, but then it tries to start when pressure drops and it blows a fuse. If it does you still appear to have an unloader valve problem.

Stan

EDIT::::Sorry Bill, we both sem to be answering at the same time and you beat me to it.

 

[NathanPrince]

Yeah pretty much as you described ford34/2, starts fine, gets to pressure, shuts off, hisses, tries to restart at 70psi and blows the fuse.

The bolt that I had to take out of the valve block did have some rust dust buildup in the holes where it vents the air from, wonder if the return hose is blocked with the same stuff causing it to start under load like you say?

I'll whip that off tomorrow to check, then if thats not the problem I'll whip the belt off and try again. Gotta be something simple somewhere!

Cheers for the help guys

 

[the snooper]

once it stops its not hissing continuously is it ? if it is you may have a faulty non return valve aswell

 

[NathanPrince]

It does yeah but I thought it was supposed to?

 

[mr migwire]

this guys page is pretty helpful,
http://www.about-air-compressors.com/tank-check-valve.html

 

[ford34/2]

sounds like the Motor is trying to start with 70 psi against it, when the unloader valve operates all you will hear is a momentry
ssssssshhh sound when the compressor stops. maybe a couple of seconds. At the same time, a check-valve makes sure that the whole air receiver (tank) and all the air in it does not flow out and empty the tank, all the unloader valve does is empty this small pipe.

Why? if this pipe is under pressure, When the compressor starts up again, the electric motor will not be strong enough to turn the compressor pump.

The output of the piston compressor is connected through a pipe to the air receiver. When the compressor stops, only the pressure in this pipe is blown off to 0 bar (atmosphere). the tank stays at SWP and you should not hear any more air escaping other than anything you are using, when the pressure drops to 70 the motor starts up, the unloader valve operates the pipe fills and it goes on into the tank.

Stan

 

[NathanPrince]

Ah I get it now, so my valve is knackered because its leaking air all the time, would a universal item fit because my compressors quite old and I cant find any bits for it anywhere?

That would cause it to keep tripping out then I take it, I'll have to strip that bit of pipework that goes from the compressor to the tank tomorrow and check it all out.

Cheers

 

[ford34/2]

I would double check the valve, they are very simple in their operation, and you may find them easy to service, also give a thought to maybe checking instructions on You Tube, probably search Unloader valve. compressor unloader valve.

Stan