Repair.IT – Air Compressor

Some months ago, I needed to paint the wooden deck in my backyard – yes, the one that I built – did I write about it? Anyway, the oil that I used was starting to wear off. A colleague from work suggested that I use a spray gun to make the job go faster which I thought at the time was a great idea. I had already been asking around for a new oil to buy, but the final word was that since we had already used the Cutek oil, that we should continue with that oil.

My colleague had lent the air compressor to another colleague, who in due course finished with his work, and I was able to get the compressor, a long coil of air hose, and a spray gun attachment. On the next available weekend, I hauled everything to the backyard, connected the power to the compressor, and switched on. At this point all I heard was a slight clicking sound coming from the compressor, so switched off. Switched on again, and still the clicking sound, which doesn’t quite sound like a motor turning, but more like a stalled motor.

I shut the power off, then could see the rotor through the grill and could turn the rotor with a long screwdriver, so that means that the motor hadn’t siezed up.  After a bit of head scratching, I decided to open up the cover to expose the motor. An air compressor is essentially a motor that turns a compressor that pushes air into a container until it bursts – or actually, until a pressure is reached whereby the motor is switched off before bursting point.

The motor is just an average ac motor, usually one that is either a capacitor-start motor or a capacitor-run motor – which means that if it doesn’t work, usually it is because a capacitor has failed. So essentially, this capacitor failed between its last job and in transport to my home – what luck. So, opening the cover should allow me to see the capacitor, remove it for checking, and then obtain a replacement.

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As Murphy would have it, part of this was easy and also not easy. The capacitor had some hard black plastic foam glued to it, to stop it from rattling against the case, but the foam had hardened to be almost rock-like – it had dented the side of the capacitor, and as luck would have it, has also covered the part that shows what size the capacitor should be. I measured the capacitance and it was very small, almost unmeasurable whereas I would expect something in the 10-30 uF.

I then used a hacksaw to cut the black foam, and eventually exposed the label enough to show that it is a 35uF capacitor, which was also not an easy one to find. After some time, I decided that I could get a 30uF and a 4.5uF and connect them in parallel, to get 34.5uF which should be close enough, since most capacitances are +-5%.

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Here is my replacement capacitors wired together.

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And now, here they are mounted on the motor frame, which fortuitously had mounting points for two capacitors – great.  Once wired in, and cover replaced – switch on, and the sound of… a loud compressor running, ok – so pressure is not increasing and air coming out the bottom – the drain bolt needs to be screwed out to close the drain.  Try again – and finally, pressure increasing – and eventually, yes – it stops.  Great – another Repair.IT done and can get back to spraying some decking oil.

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