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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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7th Feb 2020, 6:46 pm | #1 |
Hexode
Join Date: Mar 2008
Location: Bristol, UK.
Posts: 388
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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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7th Feb 2020, 7:06 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
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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. |
7th Feb 2020, 7:18 pm | #3 |
Hexode
Join Date: Mar 2008
Location: Bristol, UK.
Posts: 388
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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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7th Feb 2020, 7:27 pm | #4 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
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Re: Start/run capacitor has lost 2/3rds of value
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7th Feb 2020, 10:17 pm | #5 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,172
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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.
Ed |
7th Feb 2020, 11:38 pm | #6 |
Hexode
Join Date: Mar 2008
Location: Bristol, UK.
Posts: 388
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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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8th Feb 2020, 12:14 am | #7 |
Heptode
Join Date: Feb 2003
Location: Garnant, near Ammanford, South Wales, UK.
Posts: 657
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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. Richard.
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8th Feb 2020, 4:01 am | #8 | |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
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Re: Start/run capacitor has lost 2/3rds of value
Quote:
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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8th Feb 2020, 4:34 am | #9 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
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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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8th Feb 2020, 4:41 am | #10 |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
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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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8th Feb 2020, 8:38 am | #11 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
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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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