Compressor Capacitor Help

[e5c0rt.mk1]

Hi,
I have got a 3HP compressor which is struggling to start. When its totally empty it takes a 10 seconds to get going, and is fine till it reaches the cut off pressure. But if it has pressure in it, it will try to start for about 5 seconds and then stop.

So i am thinking it is the start capacitor. However i have very little knowledge on capacitors.

This is the capacitor on it

This is the motor

Will this one on ebay work?

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/263204738883

Thanks Owen

 

[rtbcomp]

Does this happen the first time you use it for the day, or even when it's warmed up?

 

[e5c0rt.mk1]

It does it all the time, wether its stone cold or warmed up.

 

[carbon]

Not sure with a couple of wires taped to the side it would be the original one

 

[e5c0rt.mk1]

There is another capacitor on the side but it think this is a run as its only got 5uf written on it.

 

[carbon]

How are they wired? If they are connected in parallel then the capacitance is added together. Would have thought 5uf was to low as a run capacitor and 30uf was also to low as a start capacitor. Perhaps a single 35uf capacitor would be a better option if existing two are wired in parallel

 

[Anguz]

Mostcompressors have an unload valve so no matter how much air is in the tank the compressor starts unloaded.... maybe yours does not unload properly

 

[Alan Reynolds]

I'm with Anguz on this. If it is completely empty, does it start ok? If so, usually if you follow the pipe from the cylinder head to the air receiver (tank) you will see a coupling which usually has a removable cap. Undo the cap and you should find a spring and washer. Clean it all out and try it out. The washer may have disintegrated but you can just fit a new valve. as they don't usually sell parts only. I made my own washer on my ancient compressor. The unloader valve stops the pressure feeding back into the cylinder and stopping the piston from being able to work as it's pushing against full pressure.

 

[e5c0rt.mk1]

Well, ive just been out to check that the unloader valve is working, and the compressor started up great when empty.

Then when it was full the unloader worked, like it should.

So i let a bit of air out and the compressor started up perfect, it didnt struggle at all.

I then repeated that multiple times and it fired up perfect every time.

Could a capacitor work one minute, and not the next? Or is it more likley to be a dodgy connection somewhere.

 

[carbon]

Never seen a capacitor that lost capacitance then regained it later, usually loose capacitance so instead of being 30uf get to 20uf and cause issues with motor starting, drawing more current or fail completely

 

[e5c0rt.mk1]

I'll check all the connections tomorrow, hopefully its just a dodgy connection

 

[dannyp]

i expect that one cap is XX.5 uf i have never seen a 5 uf capacitor on a 3 hp motor yet.

my clarke just runs singel 60 uf capacitors. pressure washer pump has 70 uf ( start* ) and 40 uf ( run* ) * well i think there meant to be but it works better the other way round ( but there is big question mark over that motor fully expecting magic smoke at any time ). most twin capacitor motors seem to be in the 40-50 uf range for run. or singel caps the 50-60 uf range

 

[e5c0rt.mk1]

Went out this morning to try it, and it struggled to start again.

So ive checked every connnection from the plug to the motor and everything seems fine.

So my next thought was the centrifugal switch, i then took the motor apart to find out it hasnt got one.

I'm no expert on motors, but i always thought a single phase motor had to have one?

This is the way the motor is wired up.

Is it worth buying a couple of capacitors to totally rule them out?

Thanks again

 

[Seadog]

As someone thought, earlier in the post, they are wired in parallel. So you just need to buy one 35uF capacitor.

 

[dannyp]

some motors have a starter switch / relay in them. it basicly connects the start cap in for a few seconds when power starts flowing through it, then drops it out again.

thats a singel capacitor motor id put a new 35uf cap on it or if not avalible a 40uf (very common )

 

[e5c0rt.mk1]

Thanks for the replies !!

I'll order a 40uf capacitor off ebay and try it, hopefully it will sort it out.

 

[rtbcomp]

... they don't usually sell parts only. I made my own washer on my ancient compressor. The unloader valve stops the pressure feeding back into the cylinder and stopping the piston from being able to work as it's pushing against full pressure.

For future reference that bit of rubber is called a pastel or pastille and they are available.

http://www.fps-compressors.co.uk/pastels_and_springs.html

To be completely accurate, it's the non-return valve (NRV) that prevents pressure feeding back to the pump, the unloader valve empties the pipe between the NRV and pump (the delivery pipe). When the motor is switched off either by pressure or operating the button on the pressure switch you should get a short hiss. No hiss and the unloader isn't working, continuous stream of air and the NRV is leaking. Both conditions will stall the motor on starting up.

If you switch off at the mains whilst the pump is running, turn the knob on the pressure switch to "off" to empty the delivery pipe before switching the mains back on.

I have a SIP compressor which insists on an empty tank when starting from cold.

 

[carbon]

Think in my old single phase compressor with a similar motor the capacitor was a 40uf. 35uf are available but being a slightly higher value doesn’t seem to make much difference. Have seen some motors that have one value listed on the data plate but the manufacturer of the machine fitted a bigger one to improve starting. But assume that increase only works up to a certain point

 

[normspanners]

The other thing is thick oil, I know I bang on about it, but if tank is empty and cold it is still easier to start, than at cut in pressure (or any pressure) and cold coz the motor is still struggling to crank the compressor, then the pressure kicks in as the delivery pipe is soon filled with pressure.
Also are you running it off an extension ? the volt drop can easily be 40 volts at start up, fine when warm NOT fine when everything is at its limit.
surprising what a piece of 2.5 arctic or 4mm will make, at startup.

 

[Alan Reynolds]

I couldn't find the right pastile size for my valve but I'll always make something instead of buying it anyway. Right, so when it was left overnight, it struggled. You released some air and it worked. Doesn't that point to air leaking back and pressurising the cylinder? I used to run a compressor on a generator and the compressor didn't like starting when it was fairly full because I ran the generator on a low rpm and the voltage was below 200 volts. If I let the air out quite a bit then the compressor wouls start fine on 200 volts. I can't see it being electrical or has my senility completely kicked in?