Compressor motor keeps blowing fuses

[Mannyroad]

Hi,
I'm new on this forum and have just bought a used Clarke Airmaster 4HP 200L compressor which I am hoping to use for bodywork painting and repairs. I've had a few issues with air leaks in the unloader valve and this was down to the check valve having crap in it I reckon. The compressor has been working fine since, but the other day I replaced the pressure switch with a new 20amp rated one, like the one that was originally fitted. For the first couple of tank fills all was fine, no issues. However, suddenly the garage fuse blew. Not happened before. Re-wired the fuse (yes, I still have rewireables in the garage), checked the wiring in the pressure switch and all looked fine, switched on and the motor only turned over relatively slowly, for a few seconds, then blew the fuse again.

I've checked all the compressor wiring and connections and all seem good and secure. I can spin the pump pulley over with one finger, so there isn't much resistance on the motor. But when I fired it up again it ran for maybe 10 seconds with the motor only turning over quite slowly before blowing the fuse again. I'm at a loss. I doubt it has anything whatsoever to do with the new pressure switch. I'm guessing it is coincidence and that something else is going on. Anyone got any ideas what I can check?

 

[Hopefuldave]

If it's single-phase... Sounds like the start capacitor, when they dry out there's not enough current through 'em to give the torque needed to start up, so the motor runs very slowly and doesn't make enough back-emf (which is speed-dependent), current stays at the same level as the usual start-up surge, fuse gets warm and blows... replacing the start cap is only a few local currency units, might be a good place to start, then the centrifugal switch (if the motor has one, inside the end with the fan) or potential relay (often lurking in the junction box on top of the motor)

Dave H. (the other one)

 

[Mannyroad]

Hi Dave,

Thanks for the very quick reply. Much appreciated. I presume I need to go back to Clarke to get a starter cap. Not sure if the motor has a centrifugal switch. I took the metal motor cap off to clean up the fan which was quite dusty but couldn't see any centr. switch unless it's deeper inside the motor casing. As for a potential relay? haven't noticed one in the junction box. All that seems to be in there is the starter cap, another capacitor of some sort but the text isn't easy to read and what looks to be just a terminal block for all the wiring connections. Maybe my first port of call is Clarke's for a starter cap., unless there are other outlets for these??

Oh yes, I forgot. It is a single phase induction motor, 3kw.

 

[Turbo]

You can buy the capacitors on ebay a lot cheaper than from Clarke! All you need is the uf and volts off the old capacitor plus the physical size then start searching. You can go higher on the volts for the new one but not lower than the original.For the sake of a few £ I would replace both capacitors as the run one is probably getting tired too.

Just a thought - have you a good supply feeding the compressor. You would need a 32a circuit with good heavy cable to supply a 3kw motor, too light a supply would cause voltage drop & motor starting problems.

 

[ukracer]

You can buy the capacitors on ebay a lot cheaper than from Clarke! All you need is the uf and volts off the old capacitor plus the physical size then start searching. You can go higher on the volts for the new one but not lower than the original.For the sake of a few £ I would replace both capacitors as the run one is probably getting tired too.

Just a thought - have you a good supply feeding the compressor. You would need a 32a circuit with good heavy cable to supply a 3kw motor, too light a supply would cause voltage drop & motor starting problems.

Mine was the start capacitor.....

 

[rtbcomp]

Does it do it with tank empty or only when there is some air in the tank?

 

[Mannyroad]

Hi guys,

Thanks for all your comments. The supply is fine as it has been working great for a while with no issues. It has only just started blowing fuses as soon as they are fitted. I'm strongly leaning towards the capacitors and want to replace both. The start cap is a 200mfd 300vac unit 100 x 50mm dia. The run cap is a CBB60 55mfd 450vac unit, also 100 x 50mm. I can't seem to find anything on the net that meets these criteria, hence why I felt I had no choice but to go to Clarke.

I can get 200mfd cap but the voltage is 450vac. Does it matter if it is rated at 450vac? I can't find a 55mfd anywhere. I can get 50 or 60mfd units on Ebay for a few quid but no 55mfd. Any thoughts?

rtbcomp, the tank is empty. And the motor spins freely by hand, and the pump pulley spins fairly freely too, with only a small resistance as would be normal. hence why I think it must be the caps, though they don't show any sign of disintegration etc.

 

[carbon]

The rated voltage of 450v is more than suitable, if you can’t find a 55uf cap then the next rating up would be the way to go so 60uf rather than the 50uf

 

[the snooper]

Once it's running will it start ok each time the pressure drops, when the start cap goes the motor usually buzzes

 

[Justme]

Cbb60 are very common.

You should be tripping over them on ebay.

 

[Justme]

https://www.ebay.co.uk/sch/i.html?_....H0.Xcbb60.TRS0&_nkw=cbb60+capacitor&_sacat=0

 

[Mannyroad]

Cheers guys. ok, checking what's available it seems the 200uf 450vac start cap is available from Ebay via China etc. though I'll wait till mid next month for it. no worries, it'sw a LOT cheaper than the £25 delivered from Clarke, also delivered mid-April. The 55uf run cap is hard to find but yes, I can get a 60uf 450vac cap from Ebay for less than a tenner. I read that the run cap capacitence should match exactly or the motor runs the risk of overheating / shorter lifespan. Will the 5 uf difference we are talking about do this sort of harm? If not I'm sorted and can get one off Ebay, job done.

Snooper, I don't get as far as starting when the pressure drops because it never runs long enough to build ANY pressure in the receiver tank. It runs for maybe 10 secs before blowing the fuse. during this time the motor spins but quite slowly, and I'm not really experienced enough to understand the difference between a normal motor sound and a buzz.

 

[roofman]

Dont discount a faulty switch and poorly made unloader valve...i have been kidded in the past with cheap Asian switches from Ebay or even Machine Mart for that matter.

 

[eddie49]

I always thought that capacitors had a pretty large manufacturing tolerance, and so I would have said that 60uF is close enough to 55 ( within 10 % ). However, it looks like you are correct - a run cap needs to be exact:
http://www.dnr.louisiana.gov/assets/TAD/education/ECEP/hvac/d/d.htm
https://hvac-talk.com/vbb/showthrea...g-a-30microfarad-capacitor-with-a-35microfard
The second link above implies that new run caps from China are delivered "short" - so the 60uF that you found might be smaller anyway.....but DVMs that can measure capacitance ( accurately ) are not that common.

Edit: eBay does offer 55uF:
https://www.ebay.co.uk/itm/CBB65-Me...or-Generator-Air-Pump-Compressor/232753995375

 

[Mannyroad]

Thanks for spotting that 55uf capacitor Eddie49. I must've missed that. The only difference is it is a CBB65 not a CBB60. Does this matter? If not, I can get it ordered pronto and move forward with seeing if the caps are my issue.

And what is the CBB** reference all about anyway?

Roofman, what switch are you referring to? The centrifugal switch? Haven't found it yet, though I'm told by another member that it's behind the fan at one end of the motor.

 

[roofman]

Thanks for spotting that 55uf capacitor Eddie49. I must've missed that. The only difference is it is a CBB65 not a CBB60. Does this matter? If not, I can get it ordered pronto and move forward with seeing if the caps are my issue.

And what is the CBB** reference all about anyway?

Roofman, what switch are you referring to? The centrifugal switch? Haven't found it yet, though I'm told by another member that it's behind the fan at one end of the motor.

The main black box pressure switch

 

[the snooper]

Does your motor have 2 capacitors ? 1 start 1 run, if it only has a start capacitor that shouldn't even be connected after the initial start up, definitely not 10 seconds in, try running it with the belt off if it's belt driven of course, could be a centrifugal switch problem aswell

 

[eddie49]

Capacitors CBB60 vs. CBB65 :

60 – meaning Plastic can motor run capacitor.Broadly applied to starting and running of AC single-phase motors at 50/60Hz frequency for AC motor, electric engine, water pump, generator and high efficiency compressors of refrigerators etc.

65 – meaning P2 Anti-explosion metal capacitor can motor run capacitor.

The full scoop:

http://engr-nazmul.blogspot.com/2018/07/cbb60-and-cbb65-capacitor-difference.html

Who would have thought that there could be so much unknown detail behind something as simple as a capacitor? And how did we live before Mr. Google?

 

[ukracer]

Cheers guys. ok, checking what's available it seems the 200uf 450vac start cap is available from Ebay via China etc. though I'll wait till mid next month for it. no worries, it'sw a LOT cheaper than the £25 delivered from Clarke, also delivered mid-April. The 55uf run cap is hard to find but yes, I can get a 60uf 450vac cap from Ebay for less than a tenner. I read that the run cap capacitence should match exactly or the motor runs the risk of overheating / shorter lifespan. Will the 5 uf difference we are talking about do this sort of harm? If not I'm sorted and can get one off Ebay, job done.

Snooper, I don't get as far as starting when the pressure drops because it never runs long enough to build ANY pressure in the receiver tank. It runs for maybe 10 secs before blowing the fuse. during this time the motor spins but quite slowly, and I'm not really experienced enough to understand the difference between a normal motor sound and a buzz.

That does not sound like start capacitor as it runs.....not for long but it does. If start capacitor gets weak the motor does not turn but sits still making a humming sound. Or mine did anyway.

 

[ukracer]

The main black box pressure switch

. Not the centrifugal switch. That is at the rear of the motor and makes a connection once its spun to full speed as it throws the switch outwards.

This is the area i would be checking next. Although the leccies with confirm that direction ??? They do get stuck or lazy due to dust etc