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  1. #1
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    Default Balancing thoughts?

    So I have been working on my 7.5 HP RPC. Got it starting and running well. It's an old GE motor that was very noisy on first start up. I replaced the bearings with new and now it is still a bit loud, but tolerable. I won't forget to turn it off.

    Anyway I "think" that I am on the right track as far as balance is concerned. But of course have a couple questions.

    Here is some of the figures that I got,

    Cp Cs Cpf Vac Vab Vbc Ia Ib Ic L1A L2A

    100 50 0 240 247 249 10 13 9 5 5

    100 90 0 240 245 261 7 16 13 4 4

    100 90 20 241 246 262 7 16 13 3.8 3.8



    My question is on the change in the Cs between 50 and 90 on Vbc.

    From the instructions that I followed I am looking for,

    Adjust Cp & Cs until Vbc~=1.10 Vac, Vab~= 1.05 Vac and Icn does not exceed motor nameplate FLA.


    So it seems that 261Vbc is better, but 249 is a more balanced figure between the three legs.

    Am I getting there? I'm thinking that I have it figured, but not sure which is best. I might be completely wrong!


    Sorry about the way this looks, damn forum keeps formatting my post. I had the figures spread out so that they are understandable.

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    You are supposed to balance when the RPC is running with a load motor.

    No one will know what your subscripts for capacitors mean. I could guess that Cpf means a power factor capacitor. I would rather not guess.

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    Quote Originally Posted by rons View Post
    You are supposed to balance when the RPC is running with a load motor.

    No one will know what your subscripts for capacitors mean. I could guess that Cpf means a power factor capacitor. I would rather not guess.
    Thank you rons for the reply.

    I do have to question your remarks though. This is my first RPC build admittedly and obviously. So in this effort I have done a reasonable (probably un-reasonable) amount of research. From all that I have read you have to start somewhere. Yes it would be best to balance with a load, but you have to start somewhere.

    As far as the "subscripts for capacitors" the nomenclature that I used Cp,Cs,Cpf is used by me only from what I have learned from the many pages that I have read. One for instance is the well respected and often recommended Fitch Converter.


    https://www.practicalmachinist.com/FitchWConverter.pdf

    As well as many others that I found. Took me a bit of study to understand what those terms meant. But it was in there often enough that I figured it out. Now that I have figured those terms out I don't know a better way to describe the values that I have found and make them understandable to others that may be willing to help. I used the terms that I thought were needed. If there are better terms tell me, I'd like to be able to give the information needed to get the advice that I ask for.

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    Don’t over think it. I built a Fitch design RPC and ran it for years and years without ever worrying about balancing. It always worked fine. I’ve since upgraded to a Phase Perfect which is the cat’s meow but I recognize may not be in everyone’s budget.

    If you’re going to balance it must be done under load. No load device = waste of time.


    Sent from my iPhone using Tapatalk

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    YES you need to balance (if you bother) under load.

    BUT, tou want to chack the balance AGAIN at NO LOAD..... The reason being that a good balance at the loaded condition MAY be sending the no-load, or "light load" voltage up to the sky. You do not want the light load or no load voltage to be much if any over 10% high, so that any other equipment connected to the RPC does not get small motors like coolant pumps etc, fried.

    That's a very good argument for the autoformer method of balancing instead of the "power factor correction" method with capacitors. The autotransformer will never give a boost larger than its turns ratio allows. The capacitors can give you a no load/light load boost that is way high, close to 300V.

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    Remove Cpf. Then with your first set of numbers with Cp & Cs at 100 & 50 connect a load machine. Those values will probably have to be increased.
    My 5Hp converter used 90 and 50. My 2Hp converter used 50 and 27.5. Both with a ratio = .55

    Scaling down a 5Hp to 2Hp converter

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    Thank you rons. I will do that. I was wondering if that Cpf capacitor was needed. It didn't change my voltages much and only slightly changed the Line In amps. I like the first set of numbers better than the others also.

    I'll do as you suggest and load test. Trouble is that my biggest load at this point is my 1 1/2 HP Bridgeport. I also have a surface grinder, but I think that it is only 1 1/2 HP also.

    I built a 7 1/2 HP RPC because that is what I had to start with. The reason for the balancing questions is that at this point I have everything mounted to a piece of plywood. I'd like to get things reasonably in the finished order before drilling and mounting in my enclosure. As you probably know the enclosure is the most expensive part of the project.

    But as well as that I would really like to have a better understanding of how these things work.


    Thanks guys for the guidance.

    I'm now looking for a 5HP 3 phase motor as my next build. I have a better understanding of the way RPC's function and I would like to quiet down the idler motor.

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    Quote Originally Posted by JST View Post
    YES you need to balance (if you bother) under load.

    BUT, tou want to chack the balance AGAIN at NO LOAD..... The reason being that a good balance at the loaded condition MAY be sending the no-load, or "light load" voltage up to the sky. You do not want the light load or no load voltage to be much if any over 10% high, so that any other equipment connected to the RPC does not get small motors like coolant pumps etc, fried.

    That's a very good argument for the autoformer method of balancing instead of the "power factor correction" method with capacitors. The autotransformer will never give a boost larger than its turns ratio allows. The capacitors can give you a no load/light load boost that is way high, close to 300V.
    First I have heard of autoformer method. Any links where I could start?

    Never mind I did a Google, led me back to PM and I have plenty to read on the autotformer method. So far it looks good for a single load, balanced for that and not used for others. Not looking like a good route for me where I always like to leave room for something else.

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    Actually, it should be good for ANY load.... Do not be confused by some other converter that is based on an autoformer...... those are not for machinery, decent for irrigation pumps, no good for mills and lathes. NOT what I mean,

    What I mean is boosting the manufactured leg with an autotransformer instead of the capacitors. Much more predictable and controlled than the balance caps.

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    What do you think of these numbers?

    Unloaded

    Cp Cs Cpf Vac Vab Vbc Ia Ib Ic L1A L2A

    140 50 20 242 257 259 12.5 17.9 9.6 4.2 4.2



    Loaded


    Cp Cs Cpf Vac Vab Vbc Ia Ib Ic L1A L2A

    140 50 20 241 243 244 10.2 11.8 9.9 7.3 7.8



    Stupid formatting! I wish I could stop that. It looks good until I post.

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    Put the data in something that will export it as a JPEG and post it as a picture.

    Bill

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    If I have llined up he numbers and labels right, the balance loaded seems very good, voltage and current (if the current is what I think it is). The balance UNloaded is not an issue......

    Balance is needed because a voltage imbalance coes not let the motor develop full power. You need to de-rate. Do good loaded balance is needed. Unloaded, you are not drawing much power, so the issue of balance and possible de-rating does not come up.

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    Thank you JST. That is what I was hoping for. I appreciate the explanation that you give as the reasoning. Not just a good or bad answer. Very helpful.

    I will figure out how to make the data into a JPEG next time.


    I got my Surface grinder running for the first time. I waited until I got my RPC running for it because it has two mag starters and I didn't figure that my static converter would be good for them. And a VFD wouldn't work with the mag's.

    So Yay! Now I can decide how to wrap this RPC up and put it to use.

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    Quote Originally Posted by RonRock View Post
    Loaded


    Cp.....Cs...Cpf...Vac....Vab....Vbc......Ia....... .Ib........Ic.....L1A.....L2A

    140...50...20....241...243....244...10.2.....11.8. ..9.9....7.3.....7.8
    This is how I do it.

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    Quote Originally Posted by RonRock View Post
    ..decide how to wrap this RPC up
    On which score.. I was looking to fab a bracket to hold a length of pipe, capped, over the unused but madly rotating shaft of the idler. So as to not get my baggy socks, droopy undershorts, shop apron - wotever - caught up in it..



    Off-the-shelf wire-mesh wastebasket proved cheap and easy.

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    Quote Originally Posted by thermite View Post
    So as to not get my baggy socks, droopy undershorts, shop apron - wotever - caught up in it..
    Put it in the crawl space. Or did you not get the memo?

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    Quote Originally Posted by jim rozen View Post
    Put it in the crawl space. Or did you not get the memo?
    "In the fullness of time.." it will bunk-in with the MEP-803a in a shed, back garden, since said shed is meant to be sound-deadened, and #4 THHNW Copper connected anyway. Tripping over 100 feet, each of five colours already bought.

    The Phase-Perfects are meant to be hoisted to the garage/annex attic to camp-out on fire-resistant cement board & sheet steel. One has the fancy display, but BFD.

    Just gots to get my procrastination list re-sorted, move a wall and migrate to new conduit to swing it all over to the new load centres instead of "temporary-for-years-already" fat yellow SO cords.

    Y'all have seen THAT kind of movie arredy, yah?

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    I think I'll chop the shaft off this motor. It will never be used for anything other than an RPC. Not like me to limit myself like that, but in this instance I think that is what I'll do. Easy to say, we'll see. But I know what the spinning shaft could do. In my assembly activities I had the motor running without the fan at one point. Much quieter but I don't know how long the motor would last without the cooling. So I put the fan on left the shroud off, still quieter but very dangerous to go stumbling around with a motor spinning a fan blade on the shop floor. I put the shroud back on and it will stay.

    Motor noise is defiantly more of an issue than I expected.

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    Quote Originally Posted by RonRock View Post
    Motor noise is defiantly more of an issue than I expected.
    "Defiant" motors were definitely a distraction. When I could still detect much of anything less definitive than a distinctive Ducati.


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    Spell check. Gotta pat attention.


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