Run caps as starting caps for rotary phase converter ???
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    Default Run caps as starting caps for rotary phase converter ???

    I am building a 10HP 230V RPC. My buddy handed me 20 run caps. They are rated at 60uf 440V. It seems that the start caps should be about 720uf. This makes me think I can gang 12 of these run caps together to get 720uf. They will be soldered together in parallel with 10AWG copper wire. There is a steel enclosure that I will put them in.

    Do you think 10AWG wire will do?

    In reading, based on 70uf per horsepower, I think that 720uf will be OK. Any comments?

    What value of resistor should I use to drain this bank of start caps? Will one large resistor work or should I use a resistor on each cap? I was thinking a single 10K at 5 or maybe 10 watts would drain the bank.

    This bank of caps will be activated with a Cutler Hammer 25 amp three phase motor starting contactor. I tied all three terminals together, using 14 AWG copper wire, so that the 3 contacts would all share the load.

    I will run the output of this system into my 3 phase panel with 8AWG wire.

    My single phase panel will feed this RPC through a 50 amp breaker.

    What say ????

    THANKS in advance. I have learned a ton of quality information here at Practical Machinist.

    Dan Miller

    .

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    Just use the number of caps that get the idler spinning (the same way every time. Some motors start very easily.

    A 2 watt resistor is enough to bleed off a charge . no need to be in a hurry.

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    What Cal said (surprise - we agree now and then!).

    Undersizing start caps just means start takes a bit longer, where "a bit" is still mighty short in HUMAN terms.

    Build TOO much of an array, yer just wasting space, enclosure cost, build-time, and material to add more points of potential failure than needs to be.

    Read; Often cheaper to buy an appropriately sized start cap and be DONE NOW than to kludge-together material, just because it was "free".

    Collecting donations of free TIME is harder, and the storage OF it for later use, harder yet.

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    Often cheaper to buy an appropriately sized start cap
    Indeed. I got my 1000 mfd for starting my 15 HP in four little "cylinders"

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    Quote Originally Posted by johnoder View Post
    Indeed. I got my 1000 mfd for starting my 15 HP in four little "cylinders"
    You've had some good years off those, as well, yah?

    Largest I've ever used, "one cylinder" was for a linear PSU, not motor starting.
    100,000 MFD, MIL-SPEC NOS salvage. Had cost Uncle Sugar serious coin in its day!
    Only 50 V rated, yet about the size of the #10 bean-tins used in "institutions" or military consolidated mess. High-density bugger. For its era, anyway.

    ISTR I made notes of what Jim had used in my "Phase Craft" for 10 HP idler out of habit. About the size of a tiny jar of sun-dried tomato & basil pesto or concentrated tomato paste is all.

    Even though "start" caps have the shorter lives, relatively, and some among us use "run" caps to extend that, I seriously doubt I'll still give a free-range statcoulomb by the time it actually comes due for a refresh!


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    You've had some good years off those, as well, yah?
    Ten years or so. But then it isn't run every day, either. I have a dozen more in a drawer with the left over bleed resistors

    They always do the same thing - kick that 364 frame into life with no messing about

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    Using all the poles of the start contactor will not give the result that you expect. The first pole to close will take the punishment. However, if you group your caps into three groups and feed each group with one contact you will then distribute the load as you intended.

    Bill

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    Quote Originally Posted by johnoder View Post
    ..the left over bleed resistors
    That's how a "Hell Box" comes into existence and expands to occupy all the human memory space available, yah? They aren't "unitary". They multiply like rodents, hide all over yer zone and easily half of Hell.

    Too cheap to buy only what we need, as usual!

    A bag of a hundred, seriously good grade, was cheaper than TEN, lower-volume source!

    And then.... the SECOND batch arrives in the post, just two weeks before we stumble across the first lot, misplaced a year or three earlier!

    Wish I could blame it on age, but - DNA dice-roll - I've been doing that for more than half a century arredy, so that would be a lie.

    Only saving grace was that electronics components didn't take up as much SPACE as lathes and mills.

    Lesson to you young 'uns out there. While there is still time.

    Be careful what SIZE you hoard for! Too late for YOU, alskdjfhg?

    Shudda took-up nanotech?


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    Quote Originally Posted by Bill Simmons View Post
    Using all the poles of the start contactor will not give the result that you expect. The first pole to close will take the punishment. However, if you group your caps into three groups and feed each group with one contact you will then distribute the load as you intended.

    Bill
    If you really want to be picky about it, connect each capacitor individually to a common terminal with all leads the same length.

    I use a bank of the large capacity electrolytics Thermite mentions in my magnetizer. They obviously were not designed to be discharged through an ignitron and a couple feet of heavy wire. I can't accurately measure the discharge current but I read the rate of voltage drop on a scope. It calculated to be about 125,000 amps. Even with everything carefully cleaned and tight, I get arcing at the terminals. Anything that gets in front of it gets magnetized.

    Bill

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    Quote Originally Posted by Lock 45 View Post
    I am building a 10HP 230V RPC. My buddy handed me 20 run caps. They are rated at 60uf 440V. It seems that the start caps should be about 720uf. This makes me think I can gang 12 of these run caps together to get 720uf. They will be soldered together in parallel with 10AWG copper wire. There is a steel enclosure that I will put them in.

    Do you think 10AWG wire will do?

    In reading, based on 70uf per horsepower, I think that 720uf will be OK. Any comments?
    The 720uF capacitor will not wear out like the typical start capacitor. Will last a long time.

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    OK......,I'll bite. Please describe the difference between a start and a run cap. All the AC motor caps I see are simple wound paper.

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    Quote Originally Posted by steve-l View Post
    OK......,I'll bite. Please describe the difference between a start and a run cap. All the AC motor caps I see are simple wound paper.
    Statcoulomb storage density, off the back of different construction and chemistry.

    The "denser" types used for start have FAR less tolerance of ripple, FAR shorter lives, are in a manner of speaking intentionally over-stressed. That works because they are utilized for such very brief exposure, each go. At little as a few seconds, seldom even two full minutes. Five to 40 seconds might be typical? "Start" is not their only use, applications are so varied I doubt there are even reliable statistics.

    A "start" cap then sits idle until next start, perhaps hours, days, or weeks later. So that works OK, space and cost wise.

    The less physically dense ones used for "run" caps must stay in the circuit the entire run time.

    The "run" caps (not their only use, either) are built and selected to handle the stress of constant reversals in AC service, constant ripple in rectified AC -> DC use. Cost per MFD is higher as well as needing higher total "package count" and greater total volume to match to any desired MFD figure.

    Capacitors have "shelf life" age limits from their chemistry as well as use-stress life limits, so:

    - a "run" cap used for starting can live for decades, even multiple decades. Much longer than the same cap in "run" service. That is still not the same as "forever", either one.

    - decently selected "start" caps are expected to be the first to fail, but even so, can ALSO last for decades.

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    When you gang 12 of the 60Uf caps in parallel to achieve calculated value of approximately 720uF, do you take into consideration of the Internal Series Resistance? When one fails in the group, will the resulting failure of the capacitor screw the other still connected? Will you know of a failure of one or more of the ganged capacitor when using the RPC?

    Do you plan to use an interlock shorting bar if access to the capacitors is attempted when capacitors are still charged?

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    Quote Originally Posted by Pathogen View Post
    When you gang 12 of the 60Uf caps in parallel to achieve calculated value of approximately 720uF, do you take into consideration of the Internal Series Resistance? When one fails in the group, will the resulting failure of the capacitor screw the other still connected? Will you know of a failure of one or more of the ganged capacitor when using the RPC?

    Do you plan to use an interlock shorting bar if access to the capacitors is attempted when capacitors are still charged?
    It isn't a particle accelerator or a rail-gun at work, here. This s**t isn't really all that complicated nor highly stressed.

    Space - and extra mounting clamps, extra wire, more terminals, and labour value to f**k with all that really ARE among good reasons to JF use a common start cap. Sharpie the date on the label and replace it on a schedule if you are paranoid or the application is especially critical.

    It will be cheaper in the end.

    What else? Oh.. one or "many", good idea to put a bleeder resistor across EACH cap, right at it's terminals. Even those - now in parallel - don't amount to much loss nor heat.

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    Quote Originally Posted by Pathogen View Post
    When you gang 12 of the 60Uf caps in parallel to achieve calculated value of approximately 720uF, do you take into consideration of the Internal Series Resistance? When one fails in the group, will the resulting failure of the capacitor screw the other still connected? Will you know of a failure of one or more of the ganged capacitor when using the RPC?

    Do you plan to use an interlock shorting bar if access to the capacitors is attempted when capacitors are still charged?
    The motor run type are good for direct connection across the line, so they are made for decently low resistance. It will not matter in the application.

    As for failures, motor run are generally set up with a fuse inside per UL.... if they short, the fuse opens and isolates that unit from the array. (obviously an "open" is not an issue for safety).

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    Quote Originally Posted by thermite View Post

    What else? Oh.. one or "many", good idea to put a bleeder resistor across EACH cap, right at it's terminals. Even those - now in parallel - don't amount to much loss nor heat.
    A useful visual

    Bleed Down Resistors - Capacitors - Product Guides

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    Quote Originally Posted by steve-l View Post
    OK......,I'll bite. Please describe the difference between a start and a run cap. All the AC motor caps I see are simple wound paper.
    If you use a start capacitor as a run type, it will explode in short order.

    A run type used as a start capacitor will last a long long time.

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    Fired it up today and it runs just fine. Made a few changes along the way.

    Start caps equal 600uf and not 720uf. At 720uf the idler jumped a bit too much. At 600uf, it takes 2 or 3 seconds but comes on smoothly.

    Switched out the 25 amp relay for a 60 amp relay to fire the start caps with.

    Still using 10AWG wire from the start caps and for the run caps.

    Run caps came out to 77uf on one leg and 94uf on the other leg.

    No load volts are 239 - 243 - 242.

    Loaded by my 5hp compressor I get 223 - 229 - 237.

    Feeding the 230v single phase through a 50 amp breaker. Will try a 40 amp breaker tomorrow.

    No smoke or fire so far.

    .

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    Quote Originally Posted by thermite View Post
    Statcoulomb storage density, off the back of different construction and chemistry.

    The "denser" types used for start have FAR less tolerance of ripple, FAR shorter lives, are in a manner of speaking intentionally over-stressed. That works because they are utilized for such very brief exposure, each go. At little as a few seconds, seldom even two full minutes. Five to 40 seconds might be typical? "Start" is not their only use, applications are so varied I doubt there are even reliable statistics.

    A "start" cap then sits idle until next start, perhaps hours, days, or weeks later. So that works OK, space and cost wise.

    The less physically dense ones used for "run" caps must stay in the circuit the entire run time.

    The "run" caps (not their only use, either) are built and selected to handle the stress of constant reversals in AC service, constant ripple in rectified AC -> DC use. Cost per MFD is higher as well as needing higher total "package count" and greater total volume to match to any desired MFD figure.

    Capacitors have "shelf life" age limits from their chemistry as well as use-stress life limits, so:

    - a "run" cap used for starting can live for decades, even multiple decades. Much longer than the same cap in "run" service. That is still not the same as "forever", either one.

    - decently selected "start" caps are expected to be the first to fail, but even so, can ALSO last for decades.
    Come on Bill, thanks for the bullshit. Please answer the very simple question. Of course there are quality differences between different caps. Just as there are differences in used materials and construction methodology. There are also huge differences between caps in surface area and frequency response, but that wasn't the question. The question is not usage either. My point is simple. Caps are NOT differentiated as start or run. There are good ones and not so good ones, but you never know when you buy them, it is a shot in dark. They work until they don't and then you buy another one. There is no difference between a run or a start cap and they are NOT sold as such either.

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    [QUOTE=steve-l;3434797]Come on Bill, thanks for the bullshit. Please answer the very simple question. Of course there are quality differences between different caps.

    *sigh* DESIGN CHOICES. Selection of components and Chemistry as to optimizing any of MANY way of implementing capacitors for different classes of application.

    What we generally chose FROM for use AS "run" and "start" come out of different classes of optimization.

    Not "quality differences."

    That exits, too - but it is a whole 'nuther issue.

    Person would think ya' never heard of phenol furferol, mylar, never hand yer hands on a vacuum capacitor nor a dozen OTHER types... whilst publishing a book on all that stuff...



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