AC Capacitor

I need help for identifying replacement capacitor for my Jun-Air silent compressor. Its started to hum and just turns off. I don't know if its short cycling or not. It seems like the capacitor is unable to turn the motor. Hope somebody point me to the right direction. I have checked the pressure switch and also isolated the motor from the tank itself and it just hum and then turn off. The capacitor has a rating of 45-60 uF, 270 volts R.M.S. max type KSLJ.

Can somebody with expertise help me towards the right direction or give me correct advice. Its easy for me to look for an electrician that specialise on this but i would like to try to fix it if its just the capacitor and will save me a lot. Thanks in advance.

Raphael

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

Thanks Simon, great help. I hope this is an easy fix as I really valued this old compressor. Jus another follow up question, do I need a motor start or motor run capacitor? Is the 450 volts to high for the 270 that I have? I appreciate your explanation, I just don't want to blow things up or fry the motor.

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

Cheers Bill, thanks for this. I will get the capacitor as Simon stated. I will post the result if it cures the problem.

+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

With the marking indicating 2 values it may be a start and run capacitors in 1 package, this could be confirmed by the wire count, 4 wires and it is dual package.

Emgee

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.

Yes it do have a venting system/valve to let air out of the system. The compressor is the silent type, same as the compressor used for refrigeration. You could barely hear it running. Its a JUN-AIR silent compressor. I have disconnected the motor from the tank itself to make sure there is no pressure locking on the system to eliminate the pressure switch and tank side. This is how I suspect the capacitor is not working. Got the specs you have suggested, lets hope that it will cure the problem. Thanks once again for the help.

regards,

Raphael

Good point folks, sorry I didn't read the first post fully and missed the possibility for a single start/run package.

May I suggest that Raphael finds the model number of the compressor and enters it into the search box on one of the spares websites such as this one. which may well give a definitive answer.

Bill

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

Hi Raphael

I was given a non-working Jun-Air model 6 compressor some time ago. I bought a new starting capacitor 70 microfarad 220 v (RS part no117-114) and I had to replace the starting relay (Jun-Air part 4523000). This relay picks up as the initial switch-on current flows through the run winding and energises the start winding via the capacitor. As the motor runs up the current through the starting relay coil decays and the relay drops out, thus de-energising the starting winding. Mine has a separate overload cutout (Jun-Air part 462000).

Hope this helps.

Dave

I should add that the capacitor is a short-time rated electrolytic (3 minutes @220V, 1 minute at 260V) specifically designed for motor starting and cost  £10 six years ago.

Edited By David Davies 8 on 26/03/2019 08:11:59

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 ac rated. That means they are rated for up to the stated RMS voltage. So, in principal one marked 230 V ac is OK however it is always a good idea to have some extra safety margin.

Russell

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

As a pedant,I should perhaps point out that 360V(p) is the theoretical minimum rating required. Mains voltage in the UK (poster may be somewhere else) should not exceed 253V RMS. That does not mean it is always less than that, mind, so a healthy surplus is good. Capacitors, these days, are likely to fail with the minimum of over-voltage spikes presented to them!

Anyhow that means that the peak (+ve) to peak (-ve) voltage change could be 720V. Peak voltage for 270 V RMS is close to 380V. but , never mind, these things are not too important as long as the device is not under-spec'ed.

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.

Hello Raphael, I note Dave Davies 8 say he bought a capacitor and start relay. Has your compressor start relay got 3 pins on it similar to a fridge start relay, if so check it before buying a new capacitor, as you have the symptoms of a faulty relay. You mentioned the hum and then turn off, which I think the overload is switching things off ,because the start winding is not being energised.due to a faulty relay. Usually the capacitance value is printed on the cap can.John

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.