Start capacitor question- Clarke compressor.

Exmoor beast

Exmoor beast

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Minehead,Somerset, ENGLAND OF COURSE
I’ve been given a 90 litre Clarke industrial compressor that won’t run- but the hums when you hit the start then blows the fuse. Motor & compressor units are free to turn so I suspect the start capacitor . If you look at the pics the fitted capacitor has 90/120uf on the label- I can’t find one marked 90/120 so do I order a 90uf or a 120uf capacitor? I’ve shown the data plates if that helps, thanks.
 

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It is slightly on the large side. Normally, starter capacitors should have between 60 and 100uF per KW (IIRC), the engine has 1,1 KW it seems. 70 - 110 would therefore be ideal, while lower capacities give less torque and bigger ones can overheat the engine. I would check the coals/contacts in addition to what Hans wrote...
 
A capacitor does not need to explode, increase in size or leak to go inop.
Test if possible to determine the cause. Old school repair cafe buffs may be able to help.
Try and stay in the uf range with the appropriate voltage if you need to replace.
 
Disclaimer: “Naturally the motor and all components should be checked first, start and run windings, start switch, capacitor(s), and may as well do the bearings while it’s on the bench” 👍
But if you want DIY, first look at the windings if you can see them through any ventilation holes, if the wire bundles seem to be two different colours - reddish and black, and it smells a bit like burnt varnish, then go and buy a new motor. Otherwise try a new capacitor anywhere around 90-120uf and see if it works. Capacitors don’t last forever, they rarely explode but generally loose capacity over time. Take the belts off to disconnect the compressor before you start testing. Difficult to be certain without it being in front of me on my bench, but if original, then this is a “start” capacitor which is only in circuit for a second to kick the motor off in the right direction, then the centrifugal switch disconnects it. If you don’t want to buy a new cap, you might have one about the same uF on another machine in your workshop that you can borrow for a quick test. It must be a “Motor Start” capacitor to ensure correct voltage and specs, uF is less important for a start cap.
Important: if this doesn’t work, or if the motor runs but sounds grumbly instead of whirring, then you need an expert to dig deeper. Could be the start switch or one of the windings, and these days a new motor might be cheaper.
P.S. don’t leave the motor humming while stationary, or running and sounding like a sackful of wet arseholes, you’ve only got seconds before it burns out…
 
Cappy label, "73 35" 1973 35th week of that year manufacture, Plessey... did my apprenticeship with Plessey Australia, 1971 i predate the cappy... grin. Plessey took over Ducon capacitors out here, main factory was at Belfield. The car park when rained upon allowed all the scrapped cappies to "float" to the surface, 1000's and 1000's like dead fish.... riveting i know... had to be there,

Electrolytic cappy as motor start duty, would not be my first choice. Especially if not "polarised formed" over a long period, first applied voltage the cappy dielectric does not form properly and the chemical insulation does not form in time, need a variac to "form" the cappy, another archaic item Gerald has the notion, old blokes working on old equipment, the less aged have little direct knowledge in this era. j.

*** BTW electrolytic cappies have a nominal specification of -20 to +100% actual capacitance, over label value, more modern may be better specified, though that was the general spec in the day.
 
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Thanks for all replies- I don’t understand the 90/120uf - does this mean its rating is anywhere between 90-120? Mine is just the Start capacitor, there is no second “run “ capacitor . I’ll have a look at the motor tommorow- I’ve only given it a quick look and a spin. There is no circuit b
Andy said:
Disclaimer: “Naturally the motor and all components should be checked first, start and run windings, start switch, capacitor(s), and may as well do the bearings while it’s on the bench” 👍
But if you want DIY, first look at the windings if you can see them through any ventilation holes, if the wire bundles seem to be two different colours - reddish and black, and it smells a bit like burnt varnish, then go and buy a new motor. Otherwise try a new capacitor anywhere around 90-120uf and see if it works. Capacitors don’t last forever, they rarely explode but generally loose capacity over time. Take the belts off to disconnect the compressor before you start testing. Difficult to be certain without it being in front of me on my bench, but if original, then this is a “start” capacitor which is only in circuit for a second to kick the motor off in the right direction, then the centrifugal switch disconnects it. If you don’t want to buy a new cap, you might have one about the same uF on another machine in your workshop that you can borrow for a quick test. It must be a “Motor Start” capacitor to ensure correct voltage and specs, uF is less important for a start cap.
Important: if this doesn’t work, or if the motor runs but sounds grumbly instead of whirring, then you need an expert to dig deeper. Could be the start switch or one of the windings, and these days a new motor might be cheaper.
P.S. don’t leave the motor humming while stationary, or running and sounding like a sackful of wet arseholes, you’ve only got seconds before it burns out…
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This seems like a good course of action - I was going to give it a spin when I hit the button but taking off the belt is a better plan. I’ll try a £15 start capacitor of 100uf obviously I won’t let it hum- but I am more concerned about it sounding like a sackful of wet arseholes😳
I’ll get someone to help with the wall switch to kill the power if it doesn’t run up cleanly. …A sack full of…wet arseholes…noise…?
 
My Emco lathe did the hum and not-start thing - a new cap of the same type fixed it pronto - I'm no leccy wizard, so was happy to arrive at a successful solution. I'm sure you'll sort it with the right capacitor.
 
Hi
I had to swap out the mcb from type B to type C. Garage specific consumer.
I have an old compressor, single phase and it would sometimes start and sometimes trip the mcb.
When I researched it I came up with this that explains it better. It did blow fuses in the plug if its startup was delayed ( not something i could measure), just extremely irritating.

It since runs as it should for years since.

Conside investing in a new quality prv in case the pressure switch doesn't do its job one day.
 

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Had the same issue with my lathe. Start capacitor went tits up (didn't explode - just stopped capacitating). New cap supplied under warranty fixed it, then just after the warranty ran out the motor itself burned out. It's a Hafco lathe that came with the cheapest, nastiest Chinese motor that was apparently designed to last for the warranty period and no more. I fitted a decent Australian made motor that has heaps more torque and has run perfectly for the last 15 years or so.

Your compressor being made in England probably has a decent motor, so hopefully it's only the capacitor.

Just be mindful though that a similar failure to start can be caused by the compressor unloader valve not working. The unloader valve is part of the automatic pressure control system. When the air receiver is up to the set pressure, a pressure switch shuts off the compressor motor. At the same time it triggers the unloader valve to release the pressure from the delivery pipe between the compressor and air receiver. A non-return valve stops air flowing back from the air receiver to the compressor. If the unloader valve is working properly you'll hear a "PSSST!" noise as the motor shuts off. That's the sound of the air being dumped from the delivery pipe. The reason for its existence is so next time the motor kicks in to maintain pressure in the air receiver, it doesn't have to start against the air pressure in the system. If the unloader valve doesn't dump the air pressure from the delivery line, the compressor motor will more than likely stall as it tries to start, and will just sit there and hum, even if it has a good start capacitor.

If the motor starts OK when there's no pressure in the system, or with the belt off. Then the unloader valve may be the problem.

You mention that your compressor is blowing fuses when it fails to start. You could make life easier for yourself by fitting it with a circuit breaker that you can simply reset with the press of a button.

Just as a general note to compressor owners. The unloader valve is triggered by the switch in the pressure control unit. If you switch the compressor off at the power point while the motor is running, the unloader valve won't release. Next time you switch it on, it could still have pressure in the delivery line and fail to start. Guess how I discovered that little gem of info ... :rolleyes:
 
motoddrob said:
My pillar drill sometimes need me to turn the chuck by hand to get it running. New Capacitor?
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Yes, probably. As I said, capacitors wear out. One of the first checks I do for these symptoms is to test the capacitor(s). I’ve been surprised at how low they can get.

Dellortoman said:
failure to start can be caused by the compressor unloader valve not working
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Yes, good point 👍. If the motor starts well without a load it’d be a good thing to check.

Exmoor beast said:
does this mean its rating is anywhere between 90-120?
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Yes. Rough guide since it isn’t working for long. Although it is possible to optimise the size of the cap to minimise start current while maximising torque, for this situation we don’t need to get too carried away. The main thing is that it starts. It will sound quite rough for the first half a second, but once up to speed you should only hear the fan whirring, and maybe a little bearing noise. You may even hear a click as the centrifugal start switch opens.
But if it keeps growling after a second and tries to shake itself to pieces then you’ll need to open it up. Start switches often need fettling at this age.

I hope it works, but please be aware that without an expert testing everything it’s always possible that it’ll just go pop, although if it does I suspect you would’ve been up for a new motor anyway. As Jon says, that capacitor is almost as old as us, so very likely to be the culprit 😉
 
Exmoor beast said:
We had a diagnostic box that would run a sparking coil, you could increase the gap and get the coil hot . It also had a condenser ( capacitor ) tester- “ Here, catch!” Always made me laugh.
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I worked in mower and motorcycle repair shop after school. We had a condenser tester and I made up a blackboard duster with a concealed condenser and fine copper wires down both sides. With some graffiti on the blackboard of our least liked teacher and only one duster the experiment was set! It worked liked a charm. The duster was launched at warp speed to the back of the classroom and discreetly disposed of out the window and the original duster left on the floor. I got away with that one.
 
...while teaching elec trades ( Electrotechnolgy by nondescript name ), come the class "test equipment" of common sense challenged first year apprentices, batch of beautifully made Hitachi Japan hand crank meggers, yes it was a time back. Knew what was coming, sent the 12 or more meggers into the land of the young male "fun propelled" prospects, deliberately turned my back to them and waited, arghhh scream etc.. could not help themselves zap one another with the 500Vac, after a minute of two they had zapped one another into a semi workable room, akin to the electric chair treatment, spooked one another with ambushes from 500V must zap fellow mozzie, seemed to bring an order if not clear minded attention to the lesson unfolding. Saved me the bother of room discipline, half stuffed the lot of them.

The old charged cappy and thrown to victim gag has a problem, should the lead wires puntcture the insulating skin layer then does not take much to cause fibrilation, not a high probability agreed however is a possibility. Akin to the police using the "non lethal" Taszers, subtotaneous electric current is very dangerous, even microamps if the heart muscle is bridged. Idiotic safe public control, never mind i used to spook the highway patrol mob with their dash mounted K band radar, "known to cause infertility in males when exposed coming off the windscreen..." not long after the later models were gutter mounted i observed, should have kept quiet... let them take their chances. Overall the HP had public saftey as a goal, there was an element whom were there to subjugate riders and drivers, had my share of the latter.... random off topic ramble... birthdays the root cause... j.
 
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