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Start capacitor voltage rating?

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  • Start capacitor voltage rating?

    I have a 2 hp single phase motor that I was planning to use for my horizontal band saw project. A recently as a couple of weeks ago I tested it out to make sure I had the wiring correct for the rotation direction I wanted - it worked fine. Today I had it on the test bench to double check before testing with my control circuitry. It started very slowly, then KA-BOOM! the magic smoke dramatically departed the start capacitor. (Now I know why they house those things inside a metal shield! )

    The wiring allows 120 or 240 volt hook up. I have always used 240. The (now dead) capacitor says '400 mfd 125Volts". As far as I know this was the original cap the motor was manufactured with. (Yes, 30 years old, more or less.) I expect the voltage across the start cap will not be the same as the line voltage, but I would have thought that if anything the rating would be something a couple of times greater rather than half. I understand that a start cap is not rated for continuous duty, and can self destruct if left in the circuit too long.

    So.
    1) If this is a reasonable voltage rating for a 240 volt motor, why? What am I missing? If not, what should it be?
    2) I will check the centrifugal switch, but what else might has caused this or should I be looking for before I simple replace this start cap? I'd hate to replace the cap only to find the motor had some other terminal issue.
    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

  • #2
    Motor start capacitors are electrolytic, and after 30 years the electrolyte may dry out. If it has not been used in a long time, the aluminum oxide dielectric may "deform" and get thin spots that may arc through and cause destruction. Such capacitors may need to be "reformed" by slowly applying an increasing voltage, while observing current, looking for "spikes" which indicate weak spots, which may or may not heal.
    http://pauleschoen.com/pix/PM08_P76_P54.png
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

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    • #3
      They do wear out. After a certain number of starts, usually in the 10,000 or so area, they may fail, especially with a slow start. The slow start may be a result of drying causing the microfarad capacity to be lower than marked.

      120VAC is an appropriate rating, since in most cases the 240V connection is made across one of the two series windings when the motor is connected for 240V, so the 120V is still good.
      CNC machines only go through the motions

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      • #4
        capacitors may need to be "reformed" by slowly applying an increasing voltage
        Perhaps I should not be, but I was a bit surprised since it seemed to be fine quite recently. But yes, it could well be that it just went beyond it's "best before date".
        I will assume that if the centrifugal switch and bearings are good (no physical reason for it to have trouble starting or leaving the start cap connected for too long) that a replacement cap is all that is needed, and go from there.
        "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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        • #5
          I/m not sure that the AC type "start" capacitors CAN be reformed effectively. They are composed of two standard DC capacitors in series, but opposed, i.e. + to +, with the two negative ends available to connect into the circuit

          Reforming is applying a DC voltage at low current capacity. With an AC capacitor, "forming" one of the two parts is only possible by putting current through the other half "the wrong way". That actually tends to "un-form" one half as the other is formed.

          That happens naturally when the part is used with AC, of course, but that is short term and reverses at the AC frequency, as opposed to being in-place for a much longer time.
          CNC machines only go through the motions

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