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  1. #1
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    Default Failed Start capacitor for RPC

    I was all set up to get a few machining jobs done over the Christmas break, but my rotary phase converter wouldn't start again after I turned it off for lunch. Turned out to be a failed start capacitor and I didn't have a spare so it was game over.

    I only built the converter about 2 years ago, so a little surprised its failed so quickly. Might just be a cheap chinese capacitor issue, but thought I'd ask in case there's something else I should be looking at.

    Its a 300uF 450VAC rated start capacitor, starting a 5kW idler with 415v (from a step-up transformer). It starts in less than 2secs so its not in circuit for long.

    Any thoughts?

    Steve

  2. #2
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    Default

    capacitors can fail for no reason at all...even from lack of use...its a consumable item

  3. #3
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    Fitted the new capacitor and all running again.
    Out of interest I opened up the case of the failed one.









    At least I'm no longer guessing as to why it failed.

    Steve

  4. #4
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    Default

    Have you tried opening up the small caps Steve? Might be like those Dutch doll things.

  5. #5
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    Default

    I assume those caps are in series (and are the same value) which gives you 280uF 400V ... so not far off the marked 300uF 450V.
    Cheers.

    Vernon.
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  6. #6
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    Yes, same value and in series - with the polarities reversed. As you say - close, but still borderline IMO.

    Unless they actually test them before labelling, there's a good chance that if the electrolytics are on the low side of their tolerance the total capacitance is way out.
    Not that the capacitance is critical in this application anyway, but I'm thinking I should probably be running a higher voltage cap since I'm putting 400v on it.

    When I first opened it up it just seemed like large package filled with small dodgy components, and I've always understood that electrolytics are a no-go on AC, but apparently this back to back configuration is quite common.
    I'm still not convinced!!

    Would love to open up one of the replacements and see what that has inside, but don't want to throw away the $40

    Steve

  7. #7
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    Quote Originally Posted by OxxAndBert View Post
    ... and I've always understood that electrolytics are a no-go on AC, but apparently this back to back configuration is quite common.
    I'm still not convinced!!
    Yes it is quite normal ... that configuration is how bipolar electrolytics are made.
    Cheers.

    Vernon.
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  8. #8
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    Something I picked up on while reading about these things is that in the back-back config, the normal "double the value of the voltage when in series" (assuming 2 identical caps) doesn't apply.
    I'm really interested now to pick up a decent brand start capacitor and see what the voltage rating of the internals are, since the 200v rating in mine would seem to be VERY underdone for a 450v rated assembly.

    Ref:

    Back to back electros for start caps.JPG
    From pg9 here: http://www.cde.com/resources/catalogs/AEappGUIDE.pdf

    Steve

  9. #9
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    There you go ... I can't quite figure out why that is, but who am I to argue.
    Cheers.

    Vernon.
    __________________________________________________
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  10. #10
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    I definitely don't understand fully, but from what I gather the cap that has reverse bias basically conducts well, so has minimal voltage drop, therefore the one in forward bias sees nearly full voltage. Any more thinking than that and my brain hurts. I never could get my head around the concept that its not the electrons that move its the holes, but very comfortable with the one that "everything electrical is powered by smoke and sparks - don't let them out of the device or it won't work!!!"

    Still keen to open up a decent start cap and compare to my Shenzen Special...

    Steve

  11. #11
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    Hi Steve, Guys,

    "CapXon" was one of the people at the heart of the "bad caps" saga that started a few years ago ! I'm not all all surprised that it failed, they usually explode in a horrible smelly cloud. Note also that the ones in your picture are only 85c temperature rated rather than 110c.

    With back to back electrolytic capacitors the voltage rating is usually the lower of the two, simply because the whole half cycle voltage appears across only one of them, the other cap behaves like a diode, until the voltage reverses.

    Good quality motor capacitors are often Mylar based and are often quite heavy in comparison to electrolytic's.
    Best Regards:
    Baron J.

  12. #12
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    Thanks Baron. Interesting info.
    Looks like CapXon are exactly a high end Brand!!

    Steve

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