Hi Raphael – certainly those symptoms sound like it is the capacitor that is the problem – I assume the shut down after failing to start is the overload trip popping.

A replacement shouldn't be too expensive – here's one such , I just put "motor start capacitor" into the search string on ebay and this was the first entry. You need to scroll down the page and select the uF value to suit.

The voltage rating looks like it is way over the top, but this isn't so. In fact the 270 volts RMS rating is approximately equivalent to the 450 volts of the advert, as this is a peak to peak value.

Try this link as a starting point – you may wish to find an equivalent located closer to you to keep the carriage charges down.

Motor start capacitor – select 50uF

It doesn't generally matter too much to get the value spot on the same as the original – the actual uF value of the new one will be subject to a wide tolerance – typically +/- 20% at least.

Good luck, do let us know if it cures the problem.

Best rgds Simon

As far as I'm aware, there's no difference between Start and Run capacitors, other than the value required for a given motor. i.e. for a motor which requires separate Start and Run capacitors, their actual capacitance might be different, so it's important to connect the right one in the right part of the circuit.
Re the 415v rating being greater than your 270v, it really doesn't matter, provided that the capacitor has a higher voltage rating then the circuit in which it's connected. You could happily use a 1000v rated one, but it would be more expensive and physically larger.

Bill

+1 from me, Bill got there first.

Just to be sure though, you have checked the compressor is running freely and is not trying to start against the stored back pressure from the air in the tank? Hopefully there is some sort of decompressor or venting system so that the motor only has to accelerate the compressor internals up to speed. After the motor has got going then the compressor starts pumping and generating back pressure.

If the stored back pressure is on the compressor piston when the motor is stopped then that can make it stall, though usually it will at least try to start.

I'm not familiar with the compressor you have, but most of them have some sort of decompressor controlled by the pressure switch. There is usually a non return valve located at the pressure tank, and some sort of bleed off valve which allows the air trapped between the compressor outlet and the tank non-return valve to bleed away when the pressure switch shuts off the motor. Often the bleed off valve is part of the pressure switch mechanism.

But a capacitor is first suspect and a cheap fix. It worked for my shower pump, which exhibited exactly the symptoms you describe.

Best rgds Simon

If you have motor start capacitor there should be a resistor fitted across it. If not then it is probably a motor run capacitor so make sure that you specify it as a 'motor run' item.

Raphael

Jun-Air are still going and have an excellent website with lots of pdf format manuals available for download **LINK** . Most of the manuals include parts lists and component diagrams with things like capacitors properly specified, not just a part number.

If the manual for your model is their it will almost certainly have the capacitor value. The manual for the one we had in the lab many years ago did have capacitor value.

Clive

I have replaced two of these AC capacitors, one on my air compressor, and the second on a friends lawnmower.

Both were fairly cheap, £6 or so.

I got them from the local company that does motor rewinds. Take along the dud, although they all seem to look alike, in terms of size, so that you get an exact replacement.

They need to be 450 V rating, because the normal UK supply is 230 V R M S, which means that the peak voltage is much higher, about 330, from memory.

Howard

Start capacitors are short term rated as they should only be in circuit for the very few seconds it takes the motor top run up to speed. As with a lot of electronics and electrical components things that only run for a short time can be theoretically overloaded because the magic smoke doesn't have time to get out.

Run capacitors operate all the time the motor is running so need to be continuous rated. You also need to pay attention to how hot things get. Electrical and electronic components don't do well if they get too warm so you may have to buy higher quality, more expensive, components rated for high temperature use if the capacitor is enclosed.

On affordable equipment motor capacitors can be considered consumable. Typical design life will be 10 to 20 years under reasonable conditions. Practical life is a classic "how long is the piece of string question" as its so dependant on how much you use it and how warm the environment is. I have motor capacitors over 30 years old that still do their job just fine but if I have to change one after 15 years I'd not grumble.

Was seriously annoyed by the one on a Draper belt and disk sander that gave out after only 3 years tho'. Motor and capacitor enclosed in the base, theoretically good for keeping dust out but guaranteed that things would get too warm if run for more than a couple or three minutes at time. In practice the base box was pretty pants at keeping dust out anyway so both motor and capacitor got insulated by being covered in dust helping them get hotter faster.

Clive

Motor capacitors are normally AC rated so take the peak voltage into account. a 270V AC capacitor will be fine on UK mains.

Are you sure the compressor has not seized? they normally run in an oil bath. Most Jun-Air silents have a band clamp on the housing so you can take the top off and make sure it's rurning by hand. The relay is also suspect..

Robert G8RPI.

Good comment John, I must admit when I was on the fridge tools it was standard practice to replace the complete starter kit, relays, capacitors etc, when one item had failed.

It wasn't worth the cost risk to skimp on the starter kit compared with blowing the compressor.

Raphael

I believe that SB42 is specified for older JunAir compressors so maybe this :-**LINK** will do for yours.

Not sure what older means in this context. Ours had a sticker on the side giving oil grade but what it was I don't know as its over 20 years since I last saw it, probably new around mid 1980's.

Nothing super magic about the oil. Its just a long life synthetic. Remember many of the JunAir range are intended to be on all day everyday with a pretty high duty cycle. If you're on occasional home shop uses slightly less than ideal oil probably won't matter as the compressor just won't clock up the hours.

Clive

PS According to the JUnAir MDS data sheet it looks like S27 viscosity is 20 cSt nominally equivalent to ISO 20 at 40°C and 95 SUS at 100C.  Quite thin.

Edited By Clive Foster on 26/03/2019 16:17:30

Hello Raphael, good evening again.

As I said before I don't know this particular machine, but I'd be leery of using motor oil in any compressor because of the detergent content. Compressors suffer from condensation by virtue of the water in the compressed air coalescing on the internals; this water gets mixed into the oil and emulsifies if the oil has a detergent content. You actually want an oil which does the opposite and is hydrophobic, i.e. it rejects the water, not dissolves it. Of course – just like a car engine – if you get water mixed in the oil it not only goes a nasty grey sludgy treacle but it also ruins its lubricity properties.

I spent a lot of time in a previous life playing with Atlas Copco industrial compressors, mostly reciprocating, some screw compressors. The reciprocating compressors normally used an oil called POA which stands for poly alpha olefin, and which has the long life lubricating and water rejection characteristics. Others will hopefully tell us if this is a suitable lubricant for your application, if so it's available in the after market quite readily. It's not cheap but it certainly isn't as expensive as the prices you mentioned above.

We used to take this oil and discard it, which sounds a bit barmy but we used the compressors in the food industry, injecting compressed air directly into the food process. This meant that the compressor oil had to be food safe, so we actually used an oil made by Rocol called FoodLube. Again I don't feel confident to say definitively that this is suitable for your compressor, but I'd be happy to give it a go if it were mine.

Hope this helps, I'm a bit reluctant to recommend an oil, but I'm pretty certain straight motor oil isn't compatible with your compressor.

Best rgds Simon