Air compressor start run capacitor

[roys]

My mate has an air compressor 240V so has a 200uf start and 30uf run capacitor on it.

The compressor had sat for a couple of years in unused as my mate had said it died on him.

I had a look at it last year tidied up the electrics on it, fitted a new starter, tested the motor changed the capacitors and it ran well.

Last month he called me back to it "just humming on start up" changed start capacitor all good again.

This morning exactly the same, so I am about to order another start capacitor, am I just ordering rubbish capacitors or can vibration be an issue? as there is a fair bit of vibes from it when it runs, thinking it can't be doing the capacitors any good, I could mount the capacitors on the wall behind.

Anyone had this problem before?

 

[Tony S]

How are you testing the capacitors?

Have you checked the centrifugal speed switch?

Have a look at

https://talk.electricianforum.co.uk/knowledgebase/electric-motors-and-generators/single-phase-motors/single-phase-motors-r52/

 

[roys]

Hi Tony

Used to strip down and service many CF switches, must hold hands up and say I haven't stripped this motor down for a couple of reasons, with the belts of the bearings felt smooth, fitted new capacitors and it worked perfectly.

I am thinking it is a combination of high vibes and cheap capacitors of eBay.

 

[roys]

Hi Tony

Used to strip down and service many CF switches, must hold hands up and say I haven't stripped this motor down for a couple of reasons, with the belts of the bearings felt smooth, fitted new capacitors and it worked perfectly.

I am thinking it is a combination of high vibes and cheap capacitors of eBay.

Right Tony having a cup of tea just now, and what you said is playing on my mind, right going across to strip the motor just to rule out CF switch.

 

[Tony S]

Roy, can you take some pictures of the switch please? I could use them in knowledgebase.

 

[roys]

Just back in from checking the switch, looks like the day it came out of the GEC factory, spotless it is.

Wish I had read your post re pics before I went over Tony.

I did take one pic just to show my mate how clean it was, I will try and get it off my old scabby phone but I think it will fall well below the standard of picture you need for your knowledge base.

 

[roys]

 

[Tony S]

Thanks for the photograph, it will go in to KB.

So the switch is OK. Now the most likely culprit is the start capacitor. Before changing it test it first with a Megger on the 250V setting. It will test low to start with but will rise to several MΩ if it is any good. (Don’t get bitten by it, it bloody hurts.)

 

[roys]

Start capacitor was always my prime suspect.

Wish I had a quid for every time I have been buzzed by a capacitor, one of the "games" in the workshop from a time long gone by, was to throw a charged up capacitor at your mate while shouting catch, oh how we laughed.

 

[OnOff]

Charge it up and whack a screwdriver across it, spark, happy days!  :coat

 

[Marvo]

Check the discharge voltage rating on your replacement caps with motor manufacturers original spec. Also check the air unloader valve on the pressure switch, if it's sticking the motor might be trying to start under high head pres and having prolonged LRA condition. If the compressor's chewing start caps I'd replace the pressure switch as a matter of course.

 

[roys]

Thanks Marvo, never thought of that, just to clarify for my brain please Marvo, how is the head pressure reduced for starting?

Check the discharge voltage rating on your replacement caps with motor manufacturers original spec. Also check the air unloader valve on the pressure switch, if it's sticking the motor might be trying to start under high head pres and having prolonged LRA condition. If the compressor's chewing start caps I'd replace the pressure switch as a matter of course.

it will be an old pressure switch that is on it as it is a 30 year compressor.

 

[Marvo]

The pressure switch that stops and starts the motor has a small unloader valve built into it that dumps the air pressure between the compressor and the check valve on the tank. By doing this it makes less load on start-up because it's not starting under full head pressure. Usually the unloader is an integral part of the pressure switch with small hobby compressors, larger units have a separate unloader valve. If the unloader is working you hear a loud hiss every time the compressor reaches its set pressure and stops running. Sometimes the unloader will leak because of debris or wear in the needle valve and sometimes it can jam closed. If it's an intermittant problem the chances of catching it jamming are slim so just replace the pressure switch anyway, they're not very expensive.

 

[Marvo]

If it is an unloader problem and you want to try fix it here's a video

 

[apprentice87]

There are different sorts of capacitors mind.... The one sort are suitable as "start" caps, as they are only in circuit for a little while. You need one that is rated as a "run" cap, this will be a better job and the end of your troubles i suspect.

http://www.remco.co.uk/products/capacitors

john..

 

[roys]

The pressure switch that stops and starts the motor has a small unloader valve built into it that dumps the air pressure between the compressor and the check valve on the tank. By doing this it makes less load on start-up because it's not starting under full head pressure. Usually the unloader is an integral part of the pressure switch with small hobby compressors, larger units have a separate unloader valve. If the unloader is working you hear a loud hiss every time the compressor reaches its set pressure and stops running. Sometimes the unloader will leak because of debris or wear in the needle valve and sometimes it can jam closed. If it's an intermittant problem the chances of catching it jamming are slim so just replace the pressure switch anyway, they're not very expensive.

Thanks Marvo, think you might be onto something there, it would certainly explain why it is eating the start capacitors.

 

[roys]

Thanks John, the capacitors I have fitted should be fit for purpose, although I might have been unlucky with the ones I have bought.

We shall see, going to look at the unloaded, then I might be back to the capacitors

There are different sorts of capacitors mind.... The one sort are suitable as "start" caps, as they are only in circuit for a little while. You need one that is rated as a "run" cap, this will be a better job and the end of your troubles i suspect.

http://www.remco.co.uk/products/capacitors

john..

 

[Andy™]

Thanks Marvo, think you might be onto something there, it would certainly explain why it is eating the start capacitors.

could be. most smaller compressors just wont run or blow the 13a fuse if it tries to start under pressure

 

[Marvo]

Blowing the fuse could be a symptom of a failed offloader valve but with fractional horse motors and small single phase motors their internal impedance is relatively high under locked rotor so it might become a lottery whether the cap or the fuse surrenders first.....especially with cheap caps off EBay

You could always fit a 5 or 10 Amp fuse, that way if the fault reoccurs it might not cost you another capacitor.

 

[roys]

Just for clarification and I appear to have omitted the motor rating, from memory it's rating 2.2kW or 3HP, wee bit bigger than a hobby more but still just single phase, my neighbour uses it for primarily his DA sander and spray painting.