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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 7th Feb 2020, 6:46 pm   #1
TowerRadio
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Default Start/run capacitor has lost 2/3rds of value

Our shower pump was not working, motor probably going backwards. When I checked the 10MFD cap its value had reduced to 3.5MFD. No sign of damage, not leaky but only 1/3 of its original value. Pump works fine with 10MFD replacement. Any theories as to why this would happen? Les
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Old 7th Feb 2020, 7:06 pm   #2
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Default Re: Start/run capacitor has lost 2/3rds of value

How old is it? Is it in any way heated (even residfually) by the motor or the hot water feed therough the pump?

I had to change the capacitor on the oil-pump/fan-motor of my boiler a while back. It was ten years old, but had been exposed to a lot of heat since it sits on top of the burner-assembly and is subject to 'heat-soak' from all the pipework and the burner-unit when the thermostat cycles off and there's no longer combustion-air being drawn through the outer case.

I consider a decade to be a good life under such circumstances.
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Old 7th Feb 2020, 7:18 pm   #3
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Default Re: Start/run capacitor has lost 2/3rds of value

Well I guess it is 20 years old but attached to the side of the motor by a stand-off. I could understand if it went short-circuit or open-circuit or very leaky but it still seems like a good polywhatever cap. What can I use it for? Les
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Old 7th Feb 2020, 7:27 pm   #4
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Default Re: Start/run capacitor has lost 2/3rds of value

Quote:
Originally Posted by TowerRadio View Post
.What can I use it for?
Target-practice?
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Old 7th Feb 2020, 10:17 pm   #5
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Default Re: Start/run capacitor has lost 2/3rds of value

Hi, Possibly a class X type of construction in which, if a short occurs, the dielectric in that area melts and isolates the s/c, thus reducing capacity.

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Old 7th Feb 2020, 11:38 pm   #6
TowerRadio
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Default Re: Start/run capacitor has lost 2/3rds of value

You may well be right, although there is no X anywhere in the info on the side. It is made by ICAR "Ecofill". I may pull it apart to satisfy my curiosity ( or just use it for target practice ) Les
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Old 8th Feb 2020, 12:14 am   #7
Richard
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Default Re: Start/run capacitor has lost 2/3rds of value

Modern motor run capacitors are rubbish, this happened about 15 years ago due as I understand it to changes in the regulations of what can be used in them.
In my last job repairing commercial catering equipment, for the customers who wanted 100% reliability with their equipment we used to change them during the yearly service.
In my current job repairing white goods I see them every week, low value until the motor / pump fails to start.

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Old 8th Feb 2020, 4:01 am   #8
Maarten
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Default Re: Start/run capacitor has lost 2/3rds of value

Quote:
Originally Posted by Ed_Dinning View Post
Hi, Possibly a class X type of construction in which, if a short occurs, the dielectric in that area melts and isolates the s/c, thus reducing capacity.
Motor run capacitors are usually not class X, and class X doesn't mean that it can only fail open.

You're referring to the self healing construction of many foil capacitors (class X, Y or otherwise). This will happen if it's either underdimensioned, badly constructed or there's more peaks than anticipated.

It seems plausible that that's what happened as it's quite the usual failure mode, but there's also a similar faillure mode in which the metallisation oxidises when the seal isn't 100% hermetic anymore.
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Old 8th Feb 2020, 4:34 am   #9
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Default Re: Start/run capacitor has lost 2/3rds of value

As Richard said, motor run caps from at least the last 20-25 years are just plain rubbish.
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Old 8th Feb 2020, 4:41 am   #10
Maarten
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Default Re: Start/run capacitor has lost 2/3rds of value

I agree, but were they any better before that? 20-25 years ago, polypropylene was already used, but most older ones were paper capacitors that used to fail somewhat regularly as well, though possibly less often than some of those used in radio sets.
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Old 8th Feb 2020, 8:38 am   #11
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Default Re: Start/run capacitor has lost 2/3rds of value

I've lost count of the number of times I've had to replace the run capacitor in my tumble dryer. I have a small stock of 8uF caps in a drawer.

Be careful with replacements. There are 'run' capacitors and 'start' capacitors. Run capacitors are rated for continuous use, start capacitors are rated for intermittent use since they are only used to 'start' a motor.
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