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Old 31st March 2006, 02:10 PM   #1
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Default Is this a start or run cap

Do any of you know how to determine if a surplus cap I have is a start or a run capacitor? It is a spare unit that came with a whole house fan so I suspect that it is only a start cap. The label states...

Packard, Inc.

POC 10
10 MFD +/- 10%
370 VAC 50/60Hz
10000 AFC 70C
0004
MADE IN KOREA

SPEARINOL - 1L MAX
PROTECTED
NO. PCB'S - SH
FLAMMABLE FLUID

Under a symbol that looks like a backward "R" followed by a "U" is the number "E136401". Under a symbol that is "SA" inside of a "C" is says "C122.2NO190".

Also can start caps be used in PS filters with some type of derating or are they completely useless?

mike
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Old 31st March 2006, 02:20 PM   #2
Sherman is offline Sherman  United States
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The easiest way to tell is look at the case. If the case is shiny metal it is a run cap. If the case is plastic it is a start cap.

Start caps are made for intermittent duty and are taken out of a circuit after motor start, usually by a centrifugal switch. I wouldn't use them in a power supply. A run cap is designed for continuous duty and works great as a filter cap in a power supply.
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Old 31st March 2006, 02:36 PM   #3
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Default Re: Is this a start or run cap

Quote:
Originally posted by mashaffer
Packard, Inc.
POC 10
10 MFD +/- 10%
370 VAC 50/60Hz
10000 AFC 70C
0004
MADE IN KOREA
Hi Mike,

Run caps remain on the start winding while small motors run, have values of 3-30mf.

Start caps require a relay to remove them from start winding circuit after motors or compressors start, have values 150-250mf.
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Old 31st March 2006, 03:25 PM   #4
Tweeker is offline Tweeker  United States
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Start caps typically arent oil filled. A 10uF 370VAC oil cap is conservatively good to at least 525 VDC in PSU use.

Start caps can be used in filters, some are better than others. Pay close attention to ripple current limits, it is often this rather than direct dielectric breakdown that is the cause of failure. Some start caps have high ESR, so in addition to not liking ripple they make for poor filters anyhow.

I have some metallized polypropylene start caps that I am completely confident in for psu applications.
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Old 31st March 2006, 07:53 PM   #5
Sherman is offline Sherman  United States
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Default Re: Re: Is this a start or run cap

Quote:
Originally posted by mrlots2do

Hi Mike,

Run caps remain on the start winding while small motors run, have values of 3-30mf.

Start caps require a relay to remove them from start winding circuit after motors or compressors start, have values 150-250mf.

Higher value run caps are available. I used run caps in my last project with values of 40uF, 55uF and 100uF. I still have a couple 100uF runs on the shelf. I really like run caps in PS filters.
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Old 31st March 2006, 08:44 PM   #6
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Start caps are usually electrolytic, aren't they?
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Old 31st March 2006, 10:40 PM   #7
Sherman is offline Sherman  United States
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Quote:
Originally posted by showdown
Start caps are usually electrolytic, aren't they?

Yes, start caps are almost always 'lytics. That alone doesn't necessarily make them bad.

The main thing to remember about start caps is that they are good for their rated capacities for only several seconds. If you want to used them in a circuit such as a power supply they can't be used at anything approaching the ratings printed on them. I don't know if there is a way to reliably compute a safe rating for them.
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Old 1st April 2006, 02:01 AM   #8
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Cool! This one is in a nice shinny metal case about 1"x2"x3". I also have another cap that is specifically labeled as a start cap and it is in a plastic round case that looks like a big electrolytic.

Thanks for the wisdom guys.

mike
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Old 1st April 2006, 12:54 PM   #9
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Quote:
Originally posted by Sherman
The easiest way to tell is look at the case. If the case is shiny metal it is a run cap. If the case is plastic it is a start cap.

Start caps are made for intermittent duty and are taken out of a circuit after motor start, usually by a centrifugal switch. I wouldn't use them in a power supply. A run cap is designed for continuous duty and works great as a filter cap in a power supply.

Err, --

I have several Motor Run Caps, with White Plastic cases....(In fact, Ive never seen a metal cased Run cap, apart from a Very old American one.....)
Motor Run caps are great for making up Induction Generators out of old induction motors, thats how I have quite a collection of them left over from a generator project

I have seen many Start caps with Metal cases--Quite popular here in Europe....
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Old 1st April 2006, 04:00 PM   #10
Sherman is offline Sherman  United States
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Quote:
Originally posted by Alastair E



Err, --

I have several Motor Run Caps, with White Plastic cases....(In fact, Ive never seen a metal cased Run cap, apart from a Very old American one.....)
....

Interesting! I've never seen a plastic cased run cap or a metal cased start cap. All the run caps I've purchased (a couple dozen in the past couple years) have been new production (not NOS) and all have been metal cased. In fact motor manufacturer's (motors, not capacitors) website said that all run caps are metal cased.

Anyway it complicates things a bit. Have you seen run caps that are electrolytics?
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