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  1. #1
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    Default Converter repair

    I was asked to trouble shoot an old 30 hp commercially built Phase converter that a friend picked up an auction. There were eight 60 uf run caps and six 408 uf start caps which is about right in my calculations. However someone had disconnected 4 of the run capacitors and taped the leads off. I connected all of the run caps and installed simple start station for him. It starts and runs well and with a small 10 hp load the voltage variance is about 6%. I am puzzled why someone would eliminate run capacitors. Must have been a balancing maneuver. You think.
    Last edited by Don Gitzel; 05-25-2020 at 08:19 AM. Reason: Additional info

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    Quote Originally Posted by Don Gitzel View Post
    I was asked to trouble shoot an old 30 hp commercially built Phase converter that a friend picked up an auction. There were eight 60 uf run caps and six 408 uf start caps which is about right in my calculations. However someone had disconnected 4 of the run capacitors and taped the leads off. I connected all of the run caps and installed simple start station for him. It starts and runs well and with a small 10 hp load the voltage variance is about 6%. I am puzzled why someone would eliminate run capacitors. Must have been a balancing maneuver. You think.

    Since installations and power requirements will vary, it's likely that removal of run caps was done for balancing. Keep in mind that you can run an RPC with no capacitors whatsoever if you have a pony start arrangement. I understand that Jim Rozen has been doing just that for many years.

    jin-rozens-pony-start-setup.jpg

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    The run caps are dependent on the load to some extent, so as long as it runs the load and does not have excess voltage unloaded, the value is not critical, and as noted the run caps are not even required. Usually, operating without run caps is better with an over-sized idler, which Jim Rozen has.

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    Sometimes too much capacitance will make a small motor run hot. maybe they were running a small motor with it.

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    This large idler motor was probably, in the past, being used to run smaller load motors. The previous owner probably realized that
    he could get better voltage balance with smaller run capacitance, and used a voltmeter to check the balance as he reduced the
    capacitance. The goal is to get the line-to-line voltage balance within about five percent, while the load motor is connected and running.

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    Great feedback guys. Thanks a lot.

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    Quote Originally Posted by Don Gitzel View Post
    I was asked to trouble shoot an old 30 hp commercially built Phase converter that a friend picked up an auction. There were eight 60 uf run caps and six 408 uf start caps which is about right in my calculations. However someone had disconnected 4 of the run capacitors and taped the leads off. I connected all of the run caps and installed simple start station for him. It starts and runs well and with a small 10 hp load the voltage variance is about 6%. I am puzzled why someone would eliminate run capacitors. Must have been a balancing maneuver. You think.
    A few years back I ran into a situation where several small motors refused to run with all the run caps installed.
    I broke the ones removed into 2 groups and installed discharge resistors and a 30 Amp DP switch to connect them when he needed to run the large air compressor for sandblasting. The switch was installed with a lock to make sure it was never switched on when running.

    Bill

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    Further to the 'enlightenment' as to wisdom of different balance/run caps for a mix of very large and very small motors.

    There are two OTHER ways to "get to the end".

    Take the hitandmiss illustration, for example:

    - If the "extra" caps were to be installed on the heavy load motor, the lesser value ONLY were to be left within the RPC controller, there would no longer be a need for the switch or abiity to lock it out. Selecting the heavier load motor would bring the needed heavier balance caps onto line with it, "automagically".

    Whether this is universally a 'good idea" or not = "DOTS". "Depends On The Circumstances". But if the shop environment is "stable" it could remove an "Oh, dammit, I FORGOT to flip that damned SWITCH, AGAIN!" nuisance.

    Page Two:

    The reverse. Sort-off. An ACCEPTABLE extra layer of complexity, but not necessarily onerous. Multi-idler RPC.

    Put the appropriate caps idler-motor side of the contactor for each IDLER, not in the master control. Goal is that however the array is optioned as to whom is "online" or NOT? The "right" (mostly) caps come with that idler.

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    Not sure of the original application for the man who bought this converter, but the additional bonus for multi (small/large) converter
    setup is that if the 30 is not needed, and a smaller one can be run, then the parasitic losses for the larger converter can be avoided.

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    Quote Originally Posted by Don Gitzel View Post
    There were eight 60 uf run caps and six 408 uf start caps which is about right in my calculations. However someone had disconnected 4 of the run capacitors and taped the leads off.
    If the eight run caps had been all connected in parallel then that means one equivalent run cap equal to 480uf. So does that mean the balance was so great.
    I doubt it. You could do better.

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    Maybe I’m a bit thick but I don’t get your point???

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    I don't get it either.

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    Quote Originally Posted by Don Gitzel View Post
    Maybe I’m a bit thick but I don’t get your point???
    Don't even try. There generally ISN'T one!

    It ain't YOU that's "thick" in this case. Chronic issue.

    Not to worry. He'll wander off and polish a fender-washer or sumthin' to TiG into a knock-out hole soon enough ...


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    Phew 😅. Thanks

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    Quote Originally Posted by Don Gitzel View Post
    Maybe I’m a bit thick but I don’t get your point???

    The best I can see is that he (correctly) added up the 8 60 uF caps and says that is one 480 uF capacitor.

    He is questioning if that is an appropriate value, and I think it is worth questioning.... It' a good deal to hang on one line to the generated leg. But for that size idler, perhaps not so odd, it is however, about a 5.5 ohm impedance, drawing 43 amps (reactive) at 240V.

    You may not need that much, but 6% is not bad for voltage variation, you could just leave it at that.

    If you want to try it, I would actually disconnect all of them, check performance, and add back just enough of them to get the needed performance with the various loads used. "Performance" being "enough" voltage/current under load, and not too much voltage at light load, measured from the two "through" leads to the generated lead.

    You can also put half of what is needed from one incoming lead to the generated, and the other half from the other to the generated as a starting point. Then shift them around if you need to in order to fine tune performance.

    Having all from one incoming lead to the generated lead may not be optimal, it affects phase somewhat. Usually the idler has the phase about perfect to begin with. Splitting them makes them more "phase neutral" but they still work.

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    Quote Originally Posted by Don Gitzel View Post
    Phew ��. Thanks
    Not to worry.

    FOURTH idler picked up today, here, BTW.

    Second NOS "Iowa-class" seriously heavy (237 lbs, Avoir..) Reliance Dutymaster / E-Master motor as has arrived with fan-shroud damage! But otherwise really iS "as new". Got's some "bodywork" to do.

    NB: Not really going to run a four-player one. "The Plan" is to get back down to just three.

    But I'll go ahead and wire all four... then decide WHICH three about a year out.

    AFTER I have a far better experience base as to what best fits my specific loads.

    2CW

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    Quote Originally Posted by Don Gitzel View Post
    Maybe I’m a bit thick but I don’t get your point???
    A RPC that is balanced will have two run capacitors. Each cap connected from one line to the generated leg.

    I suggest that you go seek some termite help. Gives him something to do.

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    Quote Originally Posted by rons View Post
    A RPC that is balanced will have two run capacitors. Each cap connected from one line to the generated leg.
    ...
    Not always. Suggest he peruse the Fitch Williams 'how I did it' discussion.

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    You guys are way too smart for me. I read an engineering report years ago that a good start is applying 17 uf of run capacitance and 90 uf of run capacitance per hp of idler size. I divide the Run Capacitors into two and wire one Bank from L1 and The other from L2. Over the years this has worked for me. It is very simple however I am blown away at the knowledge available on this site. I am receiving a real education.

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