Single phase motor capacitors

Hi all,

About a month ago I noticed the sound of arcing coming from my lathes motor when I started it. I took it off today and couldn't see any scorch marks around the centrifugal switch plate where the contact is.

I checked the capacitors and neither of them seemed to have loose or eroded terminals but they did show signs of heating on the insulating sleeve around the terminals.

Put it back together and onto the lathe, still arcing. I tried the slowest and fastest speeds. Little bit of arcing on the slowest speed and lots on the fastest on startup. On fastest speed I could easily smell the problem too.

I looked over the bed at the motor and noticed flashes seemed to be coming from the smaller (physical size) capacitor.

I assume I wouldn't see arcing from the centrifugal switch as it is totally enclosed.

Is the arcing because the capacitor is failing, or could it be caused by the centrifugal switch making the capacitor arc?

Can I just remove the capacitor and short the leads or something to see if the arcing goes away before replacing it?

Interestingly (to me) the milling machine motor ALSO has a problem with a capacitor. In its case the arcing has burnt through the eye terminal so it is a C section and can be loose, probably making the problem even worse.

Both machines are about 6 years old and both motors are dual capacitor motors. On the lathe I assume the starting capacitor is the problem. Is this normal?

Regards, David.

I had similar issues a couple of years ago!

I just got the info off the capacitors and got some off Ebay. When you change them, remove them and short them across a filament light bulb, quite impressive how much power they store!

Jim

David

Replacements required, make sure you get the same/similar uf rating but important to be 400v working.

Emgee

Agree with emgee. You may have been a tad unlucky that they 'only' lasted 6 years. The poor things lead a hard-life though and it's not unusual for motor capacitors to fail.

As JimmyB noted capacitors store charge; fully charged they pack quite a wallop. Best to short them out before dismantling. They're not outrageously dangerous but an expected shock isn't a good way to find you have a dodgy ticker. As they're scrap, I'd use an old screwdriver or a length of insulted wire to render them safe rather than Jimmy's much better bulb method.

Slightly odd that two machines have failed at about the same time. Do you have anything nearby, like a welder, that might be spiking your supply? Otherwise many things put spikes on the mains, not least lightning.

Dave

The milling machine one could have been gone for a long time - I never heard any sparking from it and only noticed because I thought I had to remove the terminal box to remove the motor from the top of the gearbox.

I’m surprised the lathe one looks so ‘normal’ but perhaps it hasn’t had time to get scarred up yet. I don’t often run at top speed which is when it arcs most.

It doesn’t sound like anyone suspects the centrifugal switch so I’ll get some new caps and see what happens.

Thanks for the advice guys.

The capacitor on the 1/2 HP motor of my saw bench,which I built some 50 years ago, had to be replaced once,perhaps 20 odd years ago. I have no idea how old the motor was when I bought it,but it was far from new,so the capacitor would have done a fair amount of work by the time I replaced it and it did not have an easy time since.

Perhaps I was just lucky,or perhaps these units are not made as well as they were,like so many other things.

I might add that the motor in question was never intended for woodwork since it was not TEFC,but despite heavy use,it,s still going strong. Must be at least 60 years old by now.

If the centrifugal switch is arcing when the motor is up to speed then there is something wrong with it. I suspect the contacts are not opening properly. The only time I would expect to see a spark from the centrifugal switch is when the contact open just before the motor is up to speed.

Les.

Get a 460 V rated name brand capacitor from a proper distributor, CPC / Farnell et al, rather than going for E-Bay bargains. Objectively the price differential isn't great, maybe £5, so its not worth the risk and hassle of being penny wise and possibly pound foolish.

Early failure of capacitors on Far East import motors is often due to them being primarily designed for American market use on 110 volts. Actual safe voltage rating of such devices tends to be around 300 volts so they don't immediately go pop if accidentally connected to the 220 V high load connection found in American house power installations alongside the 110 V provided for ordinary loads. As such they will survive for some time under our higher voltages but eventually succumb.

My view of the "good old stuff" thing is that back in the day design / operating characteristics weren't as well understood and the production process was less well controlled. Hence the design was conservative by modern standards and the quality scatter rather greater. So a decent range between good ones and bad ones coming out of the factory. All the bad ones died years ago so only the good ones have survived giving a skewed view of quality.

Bit like antiques. Only the good stuff got kept, cheap things that Mr & Mrs Ordinary Folk had for everyday use fell apart and was binned or burnt.

Getting back to motors true consumer priced consumer quality tools and other large motor using devices are a relatively recent phenomena. Certainly within my lifetime (will be 64 come July). Hence the fraction of a penny per unit price trimming process only useful with mass production along with the all to often associated quality reduction is also fairly recent. Back in the day proper motors were generally industrial related items where decent reliable lifetime was more important than absolute minimal first cost. Which did spread over to consumer items for a while. Pulled one over the very nice square cased Hoover motors out of Mums first washing machine when that died with an unfixable leak. No doubts about that being industrial quality. Similarly there are couple of two speed, solenoid change, gearbox equipped motors out of slightly later decent quality washers under the bench. Been too good to bin for maybe 40 years by now. Maybe the open case and one or two other details aren't full industrial quality but no obvious corners cut.

Clive.

This tread was very helpful when the bandsaw I inherited from my dad refused to start. Would run without load however. It was over 30 yrs old so I took off the capacitor and checked its size, 12 microfarads. Found a suitable one on ebay which came in 2 days for £7.50. Bandsaw back in business. The old one lasted a long time. I hope the new one will too! This tread gave me the courage to do the swap so thanks to all above for the advice.