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Electricians..does a single phase motor require a cap? by fierofetish
Started on: 04-03-2011 11:08 AM
Replies: 4
Last post by: heybjorn on 04-03-2011 02:13 PM
fierofetish
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Report this Post04-03-2011 11:08 AM Click Here to See the Profile for fierofetishSend a Private Message to fierofetishDirect Link to This Post
This old extractor fan has been my trusty ventilation in my workshop for years. It stopped working last year, and I just left it until now. I pulled off some old insulating stuff, and found this capacitor. The motor seems to be of the induction type, but I can't be sure. The fan runs freely when I spin it by hand with no nasty noises. So...before I toss it away...is it possibly a three-phase motor, converted to run on single phase, using the cap as the second and third phase..(or are they 2 phase motors, and the cap is the third phase )
Secondly, if it is the cap that has failed...are they cheap?
Thanks for sharing your knowledge, if you know the answer!
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MidEngineManiac
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Report this Post04-03-2011 11:26 AM Click Here to See the Profile for MidEngineManiacSend a Private Message to MidEngineManiacDirect Link to This Post
Some do, some dont...there are so many thousands of designs that its impossible to cover them all...however, a cap and/or relay never hurts in getting old ornery motors running. When I was fixing old (like real old) Stoeltings, one of my tricks was to use a compressor start cap on the fan motor. It could sometimes keep an old machine going. They just give it a little extra kick on start-up.

<edit> I dont know about price since I had a garbage-can full of them when I took over the shop, and didnt manage to use even 1/4 of them in 3 years.

[This message has been edited by MidEngineManiac (edited 04-03-2011).]

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Marvin McInnis
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Report this Post04-03-2011 11:31 AM Click Here to See the Profile for Marvin McInnisClick Here to visit Marvin McInnis's HomePageSend a Private Message to Marvin McInnisDirect Link to This Post
Single-phase induction motors will happily run equally well in either direction, but they are not self-starting unless there is a auxiliary winding provided for that purpose.

What you have is almost certainly a "split-phase" induction motor, which has two windings. One winding is connected directly to the power source, and the other is connected in series with a capacitor. The capacitor provides phase shift necessary to determine the direction the motor will start turning when power is applied. If both windings in the motor are the same size the motor will be reversible, depending upon which winding the capacitor is connected to. If the windings are not the same size, the capacitor can only be connected to the smaller "auxiliary" winding.

If the windings of your motor still have continuity (i.e. not an open circuit), then it is indeed possible that a new capacitor will restore it to service. Capacitors are a lot cheaper than motors, so it's probably worth a try. Be sure you get a proper-value "run" capacitor, which will be rated for continuous duty, not a "start" capacitor, which is only intended for intermittent service.

[This message has been edited by Marvin McInnis (edited 04-03-2011).]

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spark1
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Report this Post04-03-2011 11:56 AM Click Here to See the Profile for spark1Send a Private Message to spark1Direct Link to This Post
Many also use a centrifugal switch in series with the start cap/winding which can also cause a problem if bad.

http://dcacmotors.blogspot....phase-induction.html

[This message has been edited by spark1 (edited 04-03-2011).]

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heybjorn
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Report this Post04-03-2011 02:13 PM Click Here to See the Profile for heybjornSend a Private Message to heybjornDirect Link to This Post
No, they will happily wear a hat. If it is pink, a nice scarf will do.
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