DIY full size air compressor - am I mad?

[ooo000ooo]

I have a diy compressor in the garage my dad built probably 30 years ago from a big gas tank. It's provided air for air guns and stuff flawlessly up until a couple of months ago when it refused to start (just makes a hum and the electric motor doesn't turn) I suspect it's probably just the brushes have given up after all these years.
Anyone give me a clue where to start trouble shooting it?

[mikey k]

MDMA . said:

To me, it's a horrible solution to a problem that doesn't exist.

Good luck building something. I'd just buy it.

[mikey k]

ooo000ooo said:

I have a diy compressor in the garage my dad built probably 30 years ago from a big gas tank. It's provided air for air guns and stuff flawlessly up until a couple of months ago when it refused to start (just makes a hum and the electric motor doesn't turn) I suspect it's probably just the brushes have given up after all these years.
Anyone give me a clue where to start trouble shooting it?

Uncouple the pump unit from the motor and depressurise it
Try to start it
Does the motor run? Yes = seizes pump No = dead motor
Does the pump unit turn by hand Yes = valves not opening or starting against pressure

[battered]

I echo the comments re "don't build it if you can buy it". Secondhand compressors are not that expensive. My Dad had one he sold a couple of years ago - 15cfm FAD, max pressure ~150psi. This is the biggest you can run on a 13A domestic supply, it consumes 3kW/4hp. It will do air tools, spraying, all you need. I think it went for £300-350. How much cheaper is your bodge-up by the time you've finished?

[Chainsaw Rebuild]

Good luck op - this is part madness and part excellent. If it goes well you will be due a pint or two as recognition of your achievements. If it goes badly perhaps we will have a whip round to help fund the repairs. For the hole the explosion blew in the side of your house.

[DrDeAtH]

ooo000ooo said:

I have a diy compressor in the garage my dad built probably 30 years ago from a big gas tank. It's provided air for air guns and stuff flawlessly up until a couple of months ago when it refused to start (just makes a hum and the electric motor doesn't turn) I suspect it's probably just the brushes have given up after all these years.
Anyone give me a clue where to start trouble shooting it?

If it's 230v then it's probably the motor start capacitor

[ooo000ooo]

DrDeAtH said:

ooo000ooo said:

I have a diy compressor in the garage my dad built probably 30 years ago from a big gas tank. It's provided air for air guns and stuff flawlessly up until a couple of months ago when it refused to start (just makes a hum and the electric motor doesn't turn) I suspect it's probably just the brushes have given up after all these years.
Anyone give me a clue where to start trouble shooting it?

If it's 230v then it's probably the motor start capacitor[/quote
It's 230. It turns over ok by hand. Is the capacitor the cylinder thing to the left of the motor?

[battered]

Yes. The capacitor shifts the phase for the field coils, the stator. This means the field always leads the motor and the motor always goes the right way. Remember the 3 finger thing in school? It's that all over again. No cap means no phase shift so no twist.

[bearman68]

I have a cheap 50 litre one here. It has a leak in the supply pipework between the compressor and the tank. It's a Wolf one. You can have it if you want. It's sat here doing nothing. (No I won't deliver it)

[battered]

Take his arm off. Bear man, if he says no and you live between Leeds and SE London, may I have 2nd dibs?

[Mr2Mike]

battered said:

Yes. The capacitor shifts the phase for the field coils, the stator. This means the field always leads the motor and the motor always goes the right way. Remember the 3 finger thing in school? It's that all over again. No cap means no phase shift so no twist.

If it's just a start capacitor then there will likely be a centrifugal switch in the motor that switches out the cap above a certain speed. The contacts on the switches can get burnt so that cap remains out of circuit all the time.

[battered]

That's interesting. So if there phase shift cap is switched out then how is the phase shifted to be ahead of the rotor? Or is it the case that beyond a certain speed the rotor's speed ensures the phase difference, because the physical rotation is chasing the moving wave? My AC theory isn't strong enough to reason this through, though I do imagine that the phase shift required is not the same at 100 rpm as it as at 4000 rpm.

[Chainsaw Rebuild]

Bearman, may I have 3rd dibs (location dependant of course) please?

[battered]

I think Bearman needs an auction!

[Mr2Mike]

battered said:

That's interesting. So if there phase shift cap is switched out then how is the phase shifted to be ahead of the rotor? Or is it the case that beyond a certain speed the rotor's speed ensures the phase difference, because the physical rotation is chasing the moving wave? My AC theory isn't strong enough to reason this through, though I do imagine that the phase shift required is not the same at 100 rpm as it as at 4000 rpm.

You are correct, there is no rotating field when the rotor is not turning which is why some method of starting them is required. Once running the slip (rotor rpm lower that stator magnetic field rpm) generates the rotating magnetic field.

There's a good explanation here.

[Petrolhead_Rich]

Sounds like a fantastically stupid over-engineered way of seriously maiming yourself....

Gas Tank - Absolutely not, no drain valve to empty moisture, not necessarily rated to the pressure, no relief valve, oh and they remain property of the company who supplied them!

Engine - Not designed to pump/compress air in the manner you are planning, will likely fail in spectacular fashion due to lack of lubrication & exceeding design limits (Engines are not designed to run upto 9 bar, pumping air!)

cost - what you're suggesting in pipework, valves, fking around etc will easily exceed the cost of a used 3ph compressor, which will be more reliable and less likely to kill/seriously injure you in the process!!!

In short - it's a bloody stupid idea, respect compressed air, it is extremely dangerous!

[GreenV8S]

Petrolhead_Rich said:

(Engines are not designed to run upto 9 bar, pumping air!)

There's a lot of sense in what you say, but I was surprised by this particular comment. Internal combustion engines routinely see cylinder pressures far in excess of 9 bar, and are basically air pumps at the end of the day.

[Mr2Mike]

GreenV8S said:

There's a lot of sense in what you say, but I was surprised by this particular comment. Internal combustion engines routinely see cylinder pressures far in excess of 9 bar, and are basically air pumps at the end of the day.

An engine is massively over-built compared to an air compressor. However an exhaust valve is unlikely to be able to block 9 bar in a reverse direction.

[Peanut Gallery]

Mad - yes - but I fully agree with the idea!

Instead of trying to use the exhaust valve to push the air out, I would remove the cam shaft effect from both valves, and replace the valve springs with a much lighter weight jobbie, then screw a non return valve into the spark plug hole.

Randomly, I help start an old diesel steam roller occasionally. The donkey engine is sort of what you are looking at, but this one is petrol driven. The inlet valve on the compressor side is just a weak spring holding what I assume to be a normal shaped valve closed, to start the engine (and keep your thumbs) you use a small clip to hold the valve open so the piston does not build up any pressure. When running, (and you have your breath back again) remove clip, the suction pulls the valve open, the spring closes it, the pressure holds it closed and pushes up to 200 psi across to the cylinder.

Another item to look into would be to open an outlet before the pressure tank when the engine is running up to speed, so the engine is not trying to do much compression until it is up to speed.

[LunarOne]

OP did you ever get anywhere with this? This evening at my local Sainsbury's petrol station I saw a Clarke Industrial air compressor standing in a Hippo bag, clearly awaiting collection. I checked at the petrol station to see if there was any chance I could have it, and they said they couldn't see why not but to ask the manager in the main store. So I went and asked and they said I was welcome to it. So I came home and got my A4 Avant which it turns out is only just big enough for the compressor. Apparently it works fine, but was removed when they renewed the air machine and the car wash a couple of weeks ago. I haven't been able to test it as it's got a 3-phase motor and I don't have a 3-phase supply, but I did hear of ways to convert the motor to use a single phase supply, or to change the motor for a single phase one. I'm not sure of the exact specs as it was dark, but here are some photos of the unit, now standing in my garage. My current compressor is a 25l Wolf Air and when using it with air tools the compressor fires up after just a few seconds and I've been hankering after a bigger vertical compressor for ages. From what I can see online, it's 14CFM and has a 150 litre receiver. A great freebie if I can get it going and maybe worth something to someone even if I can't!