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Old October 6th, 2008, 09:22 AM   #1
Chinski
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Default Test a capacitor?

Is there a way to test a starting capacitor for an electric motor? I took it off the motor and checked resistance. Starts at 0.1 ohms and slowly rises to 0.8ohms. Seems very low, maybe shorted, but I'm not even sure if that is a valid test for a capacitor. Any thoughts?
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Old October 6th, 2008, 09:28 AM   #2
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http://www.repairfaq.org/sam/captest.htm#cttes

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For small caps (like 0.01 uf or less), about all you can really test is for shorts or leakage. (However, on an analog multimeter on the high ohms scale you may see a momentary deflection when you touch the probes to the capacitor or reverse them. A DMM may not provide any indication at all.) Any capacitor that measures a few ohms or less is bad. Most should test infinite even on the highest resistance range.
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Old October 6th, 2008, 09:29 AM   #3
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Old October 6th, 2008, 09:32 AM   #4
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Quote:
Originally Posted by Chinski View Post
Is there a way to test a starting capacitor for an electric motor? I took it off the motor and checked resistance. Starts at 0.1 ohms and slowly rises to 0.8ohms. Seems very low, maybe shorted, but I'm not even sure if that is a valid test for a capacitor. Any thoughts?
If the capacitor was shorted it would be blowing fuses or most likely show physical damage. Because the capacitor resistance is changing, show the multimeter is charging it (normal), the capacitor could still be bad, but it doesn't sound like it.
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Old October 6th, 2008, 09:46 AM   #5
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Actually it was tripping the breaker. The motor hums for a few seconds then the breaker trips. Did the same thing w/o the cap in there. A guy who claimed he used to repair these motors said it might be the cap, that’s why I was trying to check it. But I didn't get the impression he completely knew what he was talking about.

I had come across that article above, but I wasn't sure if it applied to all capacitors. Sounded like they were talking about small circuit stuff. The one from the motor says 115v and something like 230-450uF (it actually has a range like that) on the cap.

I guess I'll just see how much a new one is and replace it if its only a few bucks.
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Old October 6th, 2008, 09:49 AM   #6
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Quote:
Originally Posted by Chinski View Post
Actually it was tripping the breaker. The motor hums for a few seconds then the breaker trips. Did the same thing w/o the cap in there. A guy who claimed he used to repair these motors said it might be the cap, thatís why I was trying to check it. But I didn't get the impression he completely knew what he was talking about.

I had come across that article above, but I wasn't sure if it applied to all capacitors. Sounded like they were talking about small circuit stuff. The one from the motor says 115v and something like 230-450uF (it actually has a range like that) on the cap.

I guess I'll just see how much a new one is and replace it if its only a few bucks.
i have been out of electronics stuff for a while (I was an Elect Tech in the Navy) but reading that the cap should read infinite resistance on a meter seemed to jog my memory a bit and sounds about right...
caps (unless the are bigguns) arent that expensive.

good luck
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Old October 6th, 2008, 10:11 AM   #7
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Quote:
Originally Posted by Chinski View Post
Actually it was tripping the breaker. The motor hums for a few seconds then the breaker trips. Did the same thing w/o the cap in there. A guy who claimed he used to repair these motors said it might be the cap, thatís why I was trying to check it. But I didn't get the impression he completely knew what he was talking about.

I had come across that article above, but I wasn't sure if it applied to all capacitors. Sounded like they were talking about small circuit stuff. The one from the motor says 115v and something like 230-450uF (it actually has a range like that) on the cap.

I guess I'll just see how much a new one is and replace it if its only a few bucks.
With this new information, it sure points to a bad capacitor. Checking a capacitor with a multimeter is a mixed bag, too many variables.
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Old October 6th, 2008, 11:44 AM   #8
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Go to Grainger's. I replaced both of the caps on my compressor when I had similar starting issues. I think it was $7 a cap for the exact ones I needed.
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Old October 6th, 2008, 01:51 PM   #9
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I'd say bad cap.

xABillion on the "should read infinite" thing.
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Old October 6th, 2008, 08:03 PM   #10
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well, you could always lick it...
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Old October 6th, 2008, 09:05 PM   #11
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A copule of my Flukes can check cap's, you just need the right meter. Or call up an HVAC service guy, he'll have one to check it.
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Old October 7th, 2008, 06:21 AM   #12
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You need to be able to read microfarads (uf) to tell for sure. But caps are cheap go get a new one!
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Old October 7th, 2008, 07:37 AM   #13
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Lick it, no, but I did smell it. Smelled kinda burnt. I guess I should have done that in the first place!

I couldn't find the same part number or uF range online (doesnít surprise me, the mfg of the motor no longer lists the model I have either). I found one that was slightly larger for $12. I was going to order it then I realized I should just ask the elec controls guy here at work. He had a catalogue; found another mfg that had the exact same spec cap - cost $5.
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Old October 7th, 2008, 08:46 AM   #14
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Is it a starting capacitor or a running capacitor?

Like someone said, you could probably take it to an HVAC shop and get it tested (or an electric motor shop) if you don't have a capacitor tester.

There may be a MFD rating instead of a UF rating.
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Old October 7th, 2008, 11:53 AM   #15
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GET A CAPACITOR TESTER! You can't test a cap with an ohmmeter! If you do replace it, MAKE SURE you match the capacitance value AND voltage rating so you don't damage the motor and./or blow the cap up and hurt someone.
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Old October 7th, 2008, 12:21 PM   #16
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Quote:
Originally Posted by zook < jeep View Post
GET A CAPACITOR TESTER! You can't test a cap with an ohmmeter! If you do replace it, MAKE SURE you match the capacitance value AND voltage rating so you don't damage the motor and./or blow the cap up and hurt someone.
Why couldn't you use an ohmmeter?

Sure, you won't be able to know the exact capacitance (not that you really could with a 10% (at least) cap and a capacitance meter that is probably even less accurate), but you could test its properties quite nicely with a ohmmeter. Starts off as a short, becomes an open; no matter what the capacitance.

Last edited by General Lee; October 7th, 2008 at 12:26 PM.
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Old October 7th, 2008, 01:11 PM   #17
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if its bad the top will be popped(round).
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Old October 7th, 2008, 03:49 PM   #18
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at work we test caps using and old simpson multimeter...you need to short out the cap by connecting the 2 posts with a screwdriver or something of that nature then using the ohmmeter side of the tester touch one lead to each post...you should see a momentary spike in resistance then it will slowly fade back to zero...if not...replace the cap! It sure sounds like a starting cap because 110v motors pull a huge amount of current at startup...full load amps. ohms law states that current goes up if voltage goes down...the cap is used to help keep the extra voltage there.

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Old October 7th, 2008, 04:23 PM   #19
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It is a starting cap as far as I know. That was the best function I could tell based on the manual for what appears to be the new model number of motor/pump that I have.

The cap does say 370-455MFD on it. I was pretty sure MFD = uF = microfarad?

The one I'm getting was rated very similar at 378-454uF. Still 110VAC 60Hz like the original. Different brand; original is Aero M and new is Mallory. The closest Aero M’s were either 75uF higher or 100uF lower.

The end does not look obviously bulged, but it does have a bit of corrosion on the terminals and could have a slight puff to it. There is some real slight blistering on the case, but that just may be the case itself and not from the cap's guts - hard for me to say.

Last edited by Chinski; October 7th, 2008 at 04:27 PM.
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Old October 7th, 2008, 05:03 PM   #20
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Quote:
Originally Posted by General Lee View Post
Why couldn't you use an ohmmeter?

Sure, you won't be able to know the exact capacitance (not that you really could with a 10% (at least) cap and a capacitance meter that is probably even less accurate), but you could test its properties quite nicely with a ohmmeter. Starts off as a short, becomes an open; no matter what the capacitance.
WOW! Serious? The whole purpose of a cap is it's CAPACITY to store electricity. Without knowing the proper capacitance, you have NO IDEA if it is good or not! The caps across a start winding are chosen with a certain value capacitance to overcome the charge of the opposing windings and make the motor start to turn. By testing on an ohmmeter proves nothing other than it isn't open or shorted. But if it's value is out of spec and you continue to use it, the motor still won't start!

Don't send a boy to do a man's job ... classic example.
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