AC motor capacitor

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VARISTORS
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AC motor capacitor

Post by VARISTORS » Sun Nov 22, 2009 4:01 pm

How does one test a 105-126 MFD 110 V AC 60 Hz cap? Its off a very old drill press motor.

dyarker
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Re: AC motor capacitor

Post by dyarker » Sun Nov 22, 2009 8:04 pm

Quick and dirty check ... with an Ohm meter.

Easier with analog meter, but assuming DMM -

Cap disconnected from everything for safety. Short the cap leads leads for a few seconds to ensure it is discharged. Put Ohm meter leads on cap leads. Digits should change for a few seconds, "K" indicator should come on, then "M" indicator. Digits should stabilize at a couple MegOhms to infinity. Reverse the Ohm meter leads, digits should change for about twice as long as first time to stabilize.

If the cap passes this test, then it is worth while trying capacitance and voltage tests.

Cheers,
Dale Y

richfloe
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Re: AC motor capacitor

Post by richfloe » Mon Nov 23, 2009 2:15 pm

You can test the capacitor to determine its capacitance.

You need:
1. The capacitor (of course!)
2. A low voltage AC source (you could use 120vac but I dont recommend it!)
3. A meter (or two meters) to measure AC volts and AC amps.

The low voltage AC source can be simple transformer like a doorbell transformer or anything that puts out AC, prefer nothing over 30 volts for safety, maybe up to an amp.

Simply connect the capacitor to the AC source, measure the current flow thru the capacitor (by placing the ammeter in series with one of the capacitor connections) and also measure the voltage across the capacitor. (if you have two meters you can do both measurements at once or just do two measurements with the one meter, dont forget to check the setting so you dont blow the fuse or smoke the meter by having it set for amps when you are trying to measure the voltage)

Insert your voltage and amperage readings in to the magic formula:

uF = 2652 x amps / volts

and see if the answer falls between 105 and 126 uF. A hair low wouldnt bother me.

The factor 2652 is correct for 60hz. 3183 for 50hz.

Example: you measure 12v and 0.5 amps
2652 x 0.5 / 12 = 110.5 uF cap is ok.

Let us know how you make out.

Rich

VARISTORS
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Re: AC motor capacitor

Post by VARISTORS » Tue Nov 24, 2009 5:38 pm

Thanks, tried both methods. Obtained o.1 A, and 19.3 V all AC calculated to 13.74 micro farads not the desired 100 or so microfarads needed. Jack,

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MrAl
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Re: AC motor capacitor

Post by MrAl » Wed Nov 25, 2009 6:40 am

VARISTORS wrote:Thanks, tried both methods. Obtained o.1 A, and 19.3 V all AC calculated to 13.74 micro farads not the desired 100 or so microfarads needed. Jack,
Hi there,

I get about the same so yeah it sounds like the cap is beat. Have to buy a new one i guess.

I can suggest another test if you have any resistors laying around. The other test does not
require measuring the current.
LEDs vs Bulbs, LEDs are winning.

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