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Topic Title: Testing Capacitors
Topic Summary: Using an Insulation/Continuity tester
Created On: 07 May 2010 01:48 PM
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 07 May 2010 01:48 PM
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Avatar for TwinBrown.
TwinBrown

Posts: 26
Joined: 26 February 2010

Hi,
Can anyone provide details/tips on testing a run capacitor single phase, it is being used on a motor in a dishwasher and i suspect this may be the problem.
Any helpful suggestions welcome.
Kind regards.
 07 May 2010 02:08 PM
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Avatar for TwinBrown.
TwinBrown

Posts: 26
Joined: 26 February 2010

Its ok guys,
Removed the capacitor from my oil fired burner which had the same rating and fitted to dishwasher motor and off it went perfectly, so as i suspected it was the run capacitor.
Now do i want heating or dishwashing facilities? think the wife will want the dishwasher sorted 1st.
Ok, im off to find a 4 uF.
 07 May 2010 04:28 PM
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Avatar for potential.
potential

Posts: 1257
Joined: 01 February 2007

Substitution is usually the best test.
Doesn't have to be exactly the same value, just the same voltage (or more) and ripple.
If testing using a high voltage please remember to discharge the capacitor.
the voltage held within it can be lethal for a long period if left undischarged.
Not all meters and not all capacitors have a parallel discharge resistor and sometimes they are broken.
 07 May 2010 04:58 PM
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intrinsic4225B

Posts: 1623
Joined: 30 September 2004

I would propose that the 0.5 J or so which could potentially be stored in a 4uF capacitor tested at 500V is unlikely to be lethal to a person coming into contact with it, although it might lead to secondary injury if one were to be startled by its discharge!
 07 May 2010 05:26 PM
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slittle

Posts: 3482
Joined: 22 November 2007

Reminds me of a game to play with a group of apprentices (or students).
Involves a megger, a large capacitor and a game of catch

Stu
 07 May 2010 05:39 PM
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alancapon

Posts: 5759
Joined: 27 December 2005

We played the same game at Poly. In the electrical lab, there was a 210v dc supply to each bench for some of the experiments, so we didn't need the megger.

Regards,

Alan.
 08 May 2010 02:49 AM
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ebee

Posts: 5672
Joined: 02 December 2004

"Catch the Capacitor" the well known game for all the family.
Fun innit?


-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 10 May 2010 09:58 AM
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broadgage

Posts: 1279
Joined: 07 August 2007

As suggested above, substituting a known good capacitor for a doubtful one is a simple means of testing.
Another crude test (presuming a mains rated capacitor) is to connect it to the mains via an incandescent lamp.
If the lamp lights at full brightness, then either the capacitor is short circuit and useless, or the lamp is not drawing enough current, try a larger lamp.

If the lamp lights not all, then either the capacitor is open circuit and useless, or the lamp is too big, try a smaller one.

If with a suitable wattage lamp, the lamp lights dimly, then the capacitor is probably fine.
 10 May 2010 01:33 PM
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Avatar for gkenyon.
gkenyon

Posts: 4478
Joined: 06 May 2002

But caution with the substitution method.

There is a failure mode with single-phase capacitor-run motor systems that can damage the replacement capacitor due to overvoltage.

When you've fitted the new capacitor, best to check that the voltage it's operating at is within spec, under various conditions of motor load.

The overvoltage can be caused by various things, but normally a higher-than-usual impedance somewhere in the run winding (or connections), or the supply to the motor.


Alternatively, if you have this failure mode, just wait until the capacitor goes again - might be 2 weeks, might be 2 years !

-------------------------
Eur Ing Graham Kenyon CEng MIET TechIOSH
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