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 Post subject: Can Motor Start/Run Capcitors be Used as Filter Capacitors?
PostPosted: Jul Mon 17, 2017 12:55 am 
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I have run into a little bit of a problem here. I have a phonograph that I'm replacing the filter capacitor on. A standard polarized filter capacitor for this is a bit too much money. I was wondering if you can use the cheap non-polarized electric motor start/run capacitors in place of the polarized filter capacitors.

Thanks


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 Post subject: Re: Can Motor Start/Run Capcitors be Used as Filter Capacito
PostPosted: Jul Mon 17, 2017 1:04 am 
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Understand that start caps and run caps are two VERY different products.

Run caps may be of decent quality, designed for continuous use over long periods of time.

Start caps on the other hand are the cheapest things that can be called capacitors.

I would not recommend using either as filter caps, although that might be possible.
IMO you're always better off using a capacitor for its designed purpose.

- Leigh

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 Post subject: Re: Can Motor Start/Run Capcitors be Used as Filter Capacito
PostPosted: Jul Mon 17, 2017 1:09 am 
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jimjimmunster wrote:
...A standard polarized filter capacitor for this is a bit too much money....
Thanks
Huh? Three 33pf/450 should not be over the price of a couple of Big Macs and a shake??? Or even a large Fish & Chips at $12.99... Check out Sal's >>>>

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 Post subject: Re: Can Motor Start/Run Capcitors be Used as Filter Capacito
PostPosted: Jul Mon 17, 2017 1:14 am 
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Agree with above. While you can use AC rated capacitors run/start or even oil filled. Why do that when the real correct capacitors are cheap enough. See Sal's corner at right of page and replace filters with correct values.


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 Post subject: Re: Can Motor Start/Run Capcitors be Used as Filter Capacito
PostPosted: Jul Mon 17, 2017 11:10 am 
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What are the values you need, is it a can type?


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 Post subject: Re: Can Motor Start/Run Capcitors be Used as Filter Capacito
PostPosted: Jul Mon 17, 2017 2:32 pm 
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Water mostly under the bridge, but......

If you are looking for a part at someplace like Mouser or Digikey, plug in the basic numbers you need...eg 20uF at 450 volts....AND several values that are close. So you might enter a value from 25 to 25, and voltage 400 to 500.
Next, sort by price.
Finally, start with the cheapest, and review other specs to see what matters to you.

At Sal's or JustRadios, it's much easier cuz there's less data......:)

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 Post subject: Re: Can Motor Start/Run Capcitors be Used as Filter Capacito
PostPosted: Jul Mon 17, 2017 3:11 pm 
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Typical motor start caps are non-polarized electrolytics designed for very intermittent duty. They often have poor power factor, which is fine for motors but not good in power supply filters in electronics (it results in excessive hum). Not advisable.

Motor run capacitors, on the other hand are usually plastic film types, or in older motors, paper in oil. They were designed for constant operation and will work as filter caps within their ratings. But unless your air conditioner or fan crapped out in the summer heat and you salvaged the caps out of it, why would you use a bigger, heavier, and more expensive cap than you have to?

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 Post subject: Re: Can Motor Start/Run Capcitors be Used as Filter Capacito
PostPosted: Jul Mon 17, 2017 3:25 pm 
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Note: Motor start capacitors only have a 3 sec duty cycle
Old motor run capacitors may leak oil change the value If taking out oil type and they upside-down typical have loss oil

Dave

jimjimmunster wrote:
I have run into a little bit of a problem here. I have a phonograph that I'm replacing the filter capacitor on. A standard polarized filter capacitor for this is a bit too much money. I was wondering if you can use the cheap non-polarized electric motor start/run capacitors in place of the polarized filter capacitors.

Thanks


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 Post subject: Re: Can Motor Start/Run Capcitors be Used as Filter Capacito
PostPosted: Jul Mon 17, 2017 5:08 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
When you say motor run capacitors, some types that might be considered
are ceiling fan capacitors (with flexible leads) and component rated (to be used in
overarching approval) appliance parts.

Such types may have multiple rating agency stamps.

When you use any AC capacitor connected to the AC line, beware of any circuit
configuration that might situate the capacitor in series with an iron core inductor.

Such connections can produce thousands of volts step-up, or sub harmonic
instability depending on core saturation.

While such exotic possibilities may seem remote to radio folks, down in the ugly
side of electric power, things that can go bang big-time, are commonly found.
Attachment:
Ceiling fan capacitors   Note the integral safety discharge resistors.jpg
Ceiling fan capacitors Note the integral safety discharge resistors.jpg [ 95.1 KiB | Viewed 632 times ]

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 Post subject: Re: Can Motor Start/Run Capcitors be Used as Filter Capacito
PostPosted: Jul Thu 20, 2017 3:16 am 
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Location: Brooklyn, NY 11217
Audio guys (like the Vinyl Savor guy) use oil motor run caps on big amplifiers all the time. I have rarely heard any of them complain of failure.

Shawn


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