Testing motor capacitors

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25,160
Location
Upstate NY
I pulled a starting capacitor from my well pump and used two meters that can measure capacitance. The capacitor sparked when I shorted it to remove the charge prior to testing. Other similar capacitors within 10 seconds show the capacitance and stay at that reading. When I tested the suspect one, it first showed too high a capacitance, then just continued to count down well below the capacitance on the label. Two meters do the same thing. I assume the capacitor is either bad or going bad. I will replace it, but am curious.
 
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483
Location
TX
How exactly did you measure capacitance. My meter measures uF (microfarads). When you go across the poles it gives a solid reading. Back in the old days we used ohm meters. Go across the poles and watch it bleed down, reverse look for the kick and watch it bleed down again. Did the capacitor have a resistor across the poles?
 
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3,067
Location
Western S.C.
Had the oil leaked out? Some starting capacitors must be properly oriented to avoid leaks. If they leak, they'll fail. When I worked for a power tool manufacturer, we had some drill press returns due to that cause. KJSmith's ohmmeter method will show an open connection, but won't reveal a breakdown at operating voltage.
 
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483
Location
TX
Originally Posted by Donald
The meter measures in uF which is a unit of capacitance.
Right. I have not run across one that does as you describe. If it has the resistor you are supposed to remove ( cut ) it off. Usually just one side so you can reconnect it. Replacing would be my first choice also.
 
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2,169
Location
Saskatchewan, Canada
Does one (or both) of your meters measure tan δ? You can compare the value directly against the data sheet provided by the manufacturer; if it's within spec, the cap is good if it passes the other usual tests (physical check for bulging, evidence of leaks, high temperature rise in use, etc). But you will need a proper capacitance meter (an LCR meter), not the capacitance setting on a DVM, to perform the test. LCR = Inductance, Capacitance, Resistance
 

Donald

Thread starter
Messages
25,160
Location
Upstate NY
Originally Posted by Johnny2Bad
Does one (or both) of your meters measure tan δ? You can compare the value directly against the data sheet provided by the manufacturer; if it's within spec, the cap is good if it passes the other usual tests (physical check for bulging, evidence of leaks, high temperature rise in use, etc). But you will need a proper capacitance meter (an LCR meter), not the capacitance setting on a DVM, to perform the test. LCR = Inductance, Capacitance, Resistance
My meters are both DVM with a capacitance setting. I am guessing The $15 for a new capacitor is less than buying an LCR meter. The capacitor case looked new, no leaks or bulging. No resistor.
 
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10,451
Location
Cincinnati, OH, USA
In my experience (with my Fluke 16) it should hold value-if the little button by the terminals is swollen, bulging, or broken the electrolyte has boiled off or leaked out. A start cap can be bad & look good, wouldn't hurt to take your meter & capacitor to a motor shop and see what your meter indicates on a new one. I usually have so many around that I just throw on a new one of the same (or close) rating and see what happens.
 
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1,555
Location
TX, USA
Yes, better replace it if the capacitance is lower than labeled value. I have used Amrad. Their warranty is good also for 5 years. I have 1 failed and they replace it right away with a return label to send it back. A little pricy but not that much.
 
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2,431
Location
Toronto, Canada
Motor capacitors can be easily tested by applying 120 volts across it and measuring the current. For 120V and 60Hz, the formula is microfarad rating of capacitor = 22.1 x amps thru capacitor. A 22.1 microfarad capacitor will conduct exactly one amp, hence the prevalence of 22 microfarad capacitors. The current is directly proportional to the farad rating, so this is gives you a quick way to calculate the microfarads. For example, a capacitor conducting 3 amps is 66.3 microfarads. For large capacitors I apply lower AC voltages, like 12V, to avoid excessive current through the capacitor and the consequent risk of damage. The calculation has to be modified for the lower voltage.
 
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5,698
Location
Baltimore, Maryland, USA
If I had a well, I would keep a spare capacitor around for it. Having written that, do you have something else around that you can borrow a cap from to see if that makes your well work? Plenty of YouTube videos on how to test a capacitor using an ohm meter
 
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