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#58368 - 11/07/05 08:29 PM capacitor test  
triple  Offline
Member
Joined: Apr 2002
Posts: 184
Wisconsin
Is it possible to perform a capacitor test with a Fluke 87? If so, can mF be read or only uF?


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#58369 - 11/07/05 09:08 PM Re: capacitor test  
NJwirenut  Offline
Member
Joined: Sep 2001
Posts: 806
Bergen County, NJ
Depends on the vintage of the unit. The latest ones (model 87-V) read up to 10,000 uF (10 mF), but the older ones only go up to 10 uF, IIRC.


#58370 - 11/07/05 09:16 PM Re: capacitor test  
frank  Offline
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Joined: May 2001
Posts: 374
windsor ontario canada
Yes but i would never trust any capacitor tested with a meter and hold caps tested with a checker under great suspicion.If at all in doubt replace it.Caps can test fine but still fail under a real load and are cheap.The checker on my ampobe has been wrong twice that i know of,maybe the fluke is better.

[This message has been edited by frank (edited 11-07-2005).]


#58371 - 11/08/05 02:54 PM Re: capacitor test  
NJwirenut  Offline
Member
Joined: Sep 2001
Posts: 806
Bergen County, NJ
The Fluke isn't all that great, either.

It will find open or grossly leaky/shorted caps, but that's about it.

It only applies a tiny test voltage, so forget about a meaningful leakage test on caps rated for more than a few volts.


#58372 - 11/11/05 12:51 PM Re: capacitor test  
marcspages  Offline
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Joined: Oct 2004
Posts: 48
London, UK
I test PFC caps like this . Single phase I use the Fluke 43B, 3-phase I use the Chauvin Arnoux 8334 (not that the 8334 cannot do single phase, it's just bulkier).

M.

p.s. and I've damaged a few multimeters in my life when the cap has maintained its charge - and don't believe the bleed resistor does not burn out.


#58373 - 11/11/05 10:25 PM Re: capacitor test  
frank  Offline
Member
Joined: May 2001
Posts: 374
windsor ontario canada
I remember in college being instructed to put a 1000 ohm resistor across the terminals.But once out in the shop we were told to use our screwdrivers.I worked as a refrigeration/hvac tech for many years before becoming an industrial electrician.And in all that time never seen anyone use anything but a screwdriver on anything up to 370v 30 micro farads.I never understood why but if shorting the cap terminals caused an arc the the motor was usually bad.
Start and run caps are very cheap and the experienced guys just replaced it rather than screw around with a tester.If the costumer is paying $60 an hour and a 15 or 20 capacitor is suspect would i test it by simply installing a Known good one and trying to restart
If the tester falsely passes a capacitor and i replace a $500 compressor because of it i wouldn't sleep too well.I dealt with tons of these on a daily basis and replacing not testing was defiantly the way to go to avoid call backs.


#58374 - 11/12/05 07:10 AM Re: capacitor test  
marcspages  Offline
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Joined: Oct 2004
Posts: 48
London, UK
Frank,

I am completely with you on this one being life is far too short to bother with the single cap (especially when the prices are relatively cheap). However, just recently I walked into multiple 800kVAR banks each built out of 12.5kVAR caps, and the air may have had a distinct odour of unease if I had uttered "oh, lets just replace them all to be safe", especially as he only wanted to know if any were on their way out.

I must be an oddball (ok, ok, correction; I am an oddball) as I never use a screwdriver to discharge a cap (I don't like the chunks it takes out the side - secondly, all my screwdrivers have insulated shafts so I can't do it anyway). They say it can damage the cap when forcing it to dispense its energy so violently. I tend to use a lamp (not quite a 1000 ohms, but still a form of current limiter). I've also witnessed capacitors that manage to revive some charge sufficiently to test one's sense of humour when closing the circuit with a hand across the terminals!

I'm not a fan of motors so don't play with them if I don't have to, but could see if a winding or start-run switch was open circuit, the cap could hold a charge - but new EN rules state that such caps must have discharge resistors in them. Bang goes that test!

M.


#58375 - 11/12/05 11:18 AM Re: capacitor test  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
In electronics work, it's handy to make up a probe which contains a resistance of a few hundreds ohms and a lead terminated with a crocodile clip. You can clip to chassis and then use the probe to make sure filter caps are discharged before working on them.

The "regained" charge phenomena is quite well known. In HV power supplies it's fairly common practice to make sure the caps are discharged as described and then use a straight clip-to-clip lead to short them out.


#58376 - 11/12/05 11:12 PM Re: capacitor test  
WFO  Offline
Member
Joined: Sep 2005
Posts: 202
Cat Spring, TX
Quote:
"Yes but i would never trust any capacitor tested with a meter and hold caps tested with a checker under great suspicion.If at all in doubt replace it.Caps can test fine but still fail under a real load and are cheap."

I agree. I've seen tests on everything from 240 volts up to 14.4 KV and the tests are "suspect" at best.

After setting up an extensive test set where we monitored volts and amps across a Cap and calculated the vars to see if it was good (then watched it blow up on the pole anyway), my vote is ....when in doubt, change 'em out


#58377 - 11/13/05 01:55 AM Re: capacitor test  
frank  Offline
Member
Joined: May 2001
Posts: 374
windsor ontario canada
yes I was referring to the small run and start caps
As for the larger power factor correction caps i agree(not cheap).
We had a 10kvar cap burn on an MCC a few months back.The only way i know how to test these is to place a clamp-on meter on the cap leads while under load and the current draw should read the same on all three phases.
Anyway the cap that failed actually burnt up and tripped distribution xformer shutting down our 2000 ton chiller plant.I thought the place was burning to the ground when i walked in the door.
I disconnected the offending cap to get the pumps running so we could restart the chillers which took about an hour We tested all the caps with the clamp-on and found 4 out of 7 to had a reading of 0 amps on one lead.
The price of replacing the 4 bad caps ranging from 7.5 to 10 kvar was $2500.I had no idea that there was a tester for the larger kvar caps so i had someone else double check each motor with a power factor analyzer to see what the pf was and all four were around 80%.
These caps are only 10 years old which i would think is a short life span?How long should they last since we are on the subject?
One more thing that i found strange was that the new caps were physically twice the size of the old ones,same rating and manufacturer.If anything i thought they would be smaller?




[This message has been edited by frank (edited 11-13-2005).]



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