air compressor faulty

[ritchiehulse]

hi just wondering if anyone can help me

i have a air compressor, the type used for spray guns, air tools etc that is no longer working.

I opened the case as I could smell burning and found the capacitor was all melted and blown, it was a 25mf so I changed that and the compressor fired up again worked for about ten minutes then stopped again.

If i now turn it on with the new capacitor it will do nothing but hum unless i spin the fan on the motor myself then it will spin but very slowly, if i disconnect the new capacitor it will continue to make the same noise. If i was to put the old cap back on the motor will spin but very slowly.

It is a single phase 230v compressor

I have tried to test the new capacitor to see if it is working correct but please correct me if i have tested it incorrectly, i only have a megger multi meter with digital display and not the bar.
I put the the capacitor on voltage test which was 0v this indicates short circuit, I then turned to ohms where i would hear the click to indicate continuity but would not get a reading, turn back to voltage test where i would get 4v and would slowly discharge until the reading was back to 0v

any help would be very much appreciated, thanks richi

 

[grantr37]

was the motor run capacitor (shunt capacitor) replaced with one of an identical physical size, as well as voltage rating? I take it that it was a 25 microfarad? is it a white one or a yellow one? also if it is a non electrolytic, you may have connected it the wrong way around, if it was electrolytic you would know as it would pop again....could be that the windings in the motor have finished....the laquer/varnish wears through and causes problems with the windings, reduces magnetic field strength etc...

if you can put a photo of the capacitor on here I can look up a suitable new one somewhere cheap and put the details on here...

 

[Knobhead]

Instead of trying to fix the compressor piecemeal, get an electrician that understands motors. Most domestic electricians don’t. OK a capacitor can fail but 9 times out of 10 something’s caused it. If it’s a relatively small motor a replacement could work out cheapest.

The statement by grantr37 proves my point. A capacitor can’t be connected the wrong way on AC motors.

 

[telectrix]

possibly been powered with an extension lead, burnt out a winding. had same thing happen with a clarke compressor some time ago when i lent it to a muppet. new motor is the answer.

 

[ritchiehulse]

was the motor run capacitor (shunt capacitor) replaced with one of an identical physical size, as well as voltage rating? I take it that it was a 25 microfarad? is it a white one or a yellow one? also if it is a non electrolytic, you may have connected it the wrong way around, if it was electrolytic you would know as it would pop again....could be that the windings in the motor have finished....the laquer/varnish wears through and causes problems with the windings, reduces magnetic field strength etc...

if you can put a photo of the capacitor on here I can look up a suitable new one somewhere cheap and put the details on here...

i didnt know they had to be the same size though maybe thats my problem

 

[ritchiehulse]

with response to tony it surely cant hurt to have a try to fix it or even diagnose the problem, seems silly giving up straight away.

if the windings are burnt out how do you find out?

 

[telectrix]

continuity test then IR test. also, if burnt there will be a smell which ain't caused by farting.

 

[ritchiehulse]

there is no burnt smell, continuity test reads 4.98 between L-N, IR test gives 0.00 between L-N and 99.9 between L-E and N-E

 

[telectrix]

can you separate the windings at the motor terminals? if so continuity on both and IR between them.

 

[Marvo]

......the compressor fired up again worked for about ten minutes then stopped again.

Just out of interest when the compressor ran for ten minutes was this constant running or did it reach full pressure, stop, then try to restart after the pressure dropped? Did you check the running current before it went bang again?

I'm wondering about a faulty dump or offloader valve causing it to try and restart under head pressure. This would certainly explain the cap damage and everything else tests okay.

 

[ritchiehulse]

Just out of interest when the compressor ran for ten minutes was this constant running or did it reach full pressure, stop, then try to restart after the pressure dropped? Did you check the running current before it went bang again?

I'm wondering about a faulty dump or offloader valve causing it to try and restart under head pressure. This would certainly explain the cap damage and everything else tests okay.

yes ran constantly dont think it reached full pressure but was running constant as it was powering a sander, but then again could have reached full pressure and cut out. As it only has a 50ltr tank would only power a sander for ten ish seconds then would need to power up again, then it just started to run slowly then cut out

no i did not check the running current as when i changed the cap it started straight away so i just assumed it was the faulty cap

telectrix i am unable to get to the windings at the motor tried to remove the casing but would mean pulling the motor to complete bits to get at it

 

[Knobhead]

continuity test then IR test. also, if burnt there will be a smell which ain't caused by farting.

Just to give you a chuckle.
I had a large DC motor burn out. New engineer asked our team leader what tests I’d done. Team leader, non, Tony turned green when he took the brush cover off and then slammed it back on again.
It’s the worst smell in the world, you can taste it!

 

[Knobhead]

What make of compressor is it? Model number would be useful.

 

[ritchiehulse]

nu tool model no.NAG950

 

[ritchiehulse]

guess im out of luck and will try a re tool place

 

[Marvo]

The first thing I would do is fit another replacement cap and check the start and run current of the motor using an ammeter and the on-line votage using your favorite voltmeter. Compare the results with the currents/voltage stamped on the motor info plate.

There could be lots of reasons a motor capacitor would prematurely fail. They would include;

• Low supply voltage
• High supply voltage
• Air dump valve failure causing starting under head pressure
• Poor wiring termination causing arcing
• Pressure switch faulty causing contactor chatter
• General motor short cycling for whatever reason
• Belts overtightened
• Bearing lubrication failure and overheating
• Compressor head assembly mechanical problems.

I haven't had time to check out your exact model so some of these might not be applicable.Bottom line is this is the kind of problem that will only be found by getting hands on with a tester whilst the machine is running. There are mechanical and electrical hazards involved, even when the power is off (especially with capacitors), if you're not comfortable with working on live machinery I would suggest you get a service technician in to help. .