Author Topic: Testing a motor run/start cap. How do -you- do it?  (Read 838 times)

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Offline Ed.KloonkTopic starter

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Testing a motor run/start cap. How do -you- do it?
« on: January 11, 2022, 04:13:34 am »
Just interested how everyone does it.

Seen some nasty y/t vids. One thing I don't like is the clumsy shorting out with the screwdriver.

I know it depends on what meters you have on hand and how extensively you want to test. Most of us in the field will know when the cap is dead and replace it. Then the motor works again. Chuck the old one in the garbage.
« Last Edit: January 11, 2022, 04:21:52 am by Ed.Kloonk »
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Offline floobydust

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Re: Testing a motor run/start cap. How do -you- do it?
« Reply #1 on: January 11, 2022, 04:42:22 am »
I test them like any other capacitor (LCR meter) BUT because they have mechanical failures and the pressure switch inside, they need to get banged around as well. They can go intermittent.
 
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Offline james_s

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Re: Testing a motor run/start cap. How do -you- do it?
« Reply #2 on: January 11, 2022, 04:44:34 am »
I usually use the capacitance function on my multimeter as a basic test, but the easiest thing to do is usually swap in a known good capacitor of similar value and see if the motor runs.
 
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Offline floobydust

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Re: Testing a motor run/start cap. How do -you- do it?
« Reply #3 on: January 11, 2022, 05:04:19 am »
Multimeter capacitance test is crap due to timing single-slope. Leakage current misleadingly gives a higher C reading because it's taking longer to charge up. Cheap electrolytics have a weird electrolyte that has high leakage current and gives bizarre readings, almost like dealing with a 1/2 cell.
So the capacitance reading from a multimeter is not much to go on.
 
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Offline Ed.KloonkTopic starter

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Re: Testing a motor run/start cap. How do -you- do it?
« Reply #4 on: January 11, 2022, 05:13:41 am »
I see a lot more of the CBB65 types 40-70uF so they can be unwieldy. As James says you know when they've had it..

I thought it was just me that just test jigs the things.

 :)
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Offline IanB

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Re: Testing a motor run/start cap. How do -you- do it?
« Reply #5 on: January 11, 2022, 05:27:18 am »
I'm not too bothered about shorting with a screwdriver as the capacitor should already be discharged by the motor windings.

In my case it was a simple run capacitor, and the diagnosis was firstly that the motor did not start well, and secondly the measurement of the capacitance was low.

I don't know much about start capacitors, but I suspect the diagnosis is similar.
 


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