My first RPC 5-10HP is Fitch still the design to use? I have parts galore to build it - Page 2
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  1. #21
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    Quote Originally Posted by johansen View Post
    you don't mention what voltage the .55KW motors are but if they are 208 then i would fix that issue by wiring up some buck boost transformers to drop the voltage, they won't be enough of a load to pull the 264 volts delivered down to a level that won't overheat them. if they are 240v motors then its not a problem

    this is one method to connect the transformers to get 120/208 from a 240v rpc.. but because there is no delta winding in the 16:120v transformers the neutral is useless and unbalanced loads will cause problems. https://i.imgur.com/9UtkTZj.jpg


    regarding your main 208 volt lathe.. you can instead use a single 16/32:120/240 buck boost transformer to drop the voltage from your single phase supply line to 208 instead of 240, but directly connect the third phase to the generated leg from the rpc. so the rpc sees 240 volts but your lathe sees 208 plus the generated leg. you might not need any balancing capacitors.
    Thanks for the reply. The little Hercus lathe motors are stamped 220 VAC, but just thinking, as they are direct drive, ie no contactors etc., I think I may just put individual VFDs on these.

    I have a transformer which I think is what you described, if I connect it "backwards" should do what you mentioned? Could be all I need to get this working correctly.

    Thanks!

    img_5379.jpg

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    No that transformer will not work unless you can tap the midpoint of the coil between H1 and H2

    this is how it needs to be wired
    https://i.imgur.com/KJ2xQXY.jpg

    you won't need near as much capacitance on the rpc to balance the current delivered to the motor's 3rd phase, under load

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  4. #23
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    Quote Originally Posted by johansen View Post
    No that transformer will not work unless you can tap the midpoint of the coil between H1 and H2

    this is how it needs to be wired
    https://i.imgur.com/KJ2xQXY.jpg

    you won't need near as much capacitance on the rpc to balance the current delivered to the motor's 3rd phase, under load
    Mmmm, okay, I'll have to pull the covers and see if I can access the center tap. Thank you for taking the time to draw if out for me, I understand the connections now. I'll pull out the xfmr this weekend.

    Cheers

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    You can use that transformer if you connect H3 and H4 to line one and two, H2 and H1 go to the motor. but the motor would only see a 10% reduction in voltage (which will help but it really isn't enough), and you would be applying 240vac to a transformer only intended for 228 in that configuration. The transformer's 120/240v secondary would be delivering 126v minimum (if its wound already compensated for voltage drop you'll get about 130volts out) the 120/240v secondary isn't used in this application.


    I'm somewhat doubting that you can access the midpoint of the winding between H2 and H1. if you can, that midpoint of the winding needs to be connected back to the neutral of your 120/240 service, otherwise that 208 volt winding is floating and your motor will not get 3 phase power. The 120/240v winding would then be the primary and the 208 winding the secondary.



    something like this is all you need.
    Acme T-1-13073 Buck-Boost Transformer, 1 Phase,1 kVA, 120 x 240V,16/32V Output 646826265775 | eBay

    and i see you are in canada.. you have 120/240 service correct?

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    OKay cool, I understand what you described, and will find out what xfmr taps are available. Yes Canada is 120/240 power.

    Of course, my other option is to swap motors, and I think I may have a 230/460 5HP motor that would fit....I'll continue with the xfmr route first though.

    Thanks again!

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    Default Need some help tuning my RPC

    So I have repaired the broken shift fork in my Hydrashift and have it running again on the RPC. I have gone thru again and tried tuning it for better performance but I still have an issue with phase 3.

    Here are the details of the design and measurements, maybe I am being too fussy??

    RPC 5 HP idler 230VAC 1725 RPM 12.6 FLA 230/460 wired for 230.

    NO LOAD
    Vac 271
    Vbc 260
    Vab 233
    Cp 75uF
    Cs 61uF
    Cpf 14uF
    Iline 3.5A
    Icn 12.5A

    Loaded with lathe motor 3HP 208VAC FLA 10.8
    Vac 228
    Vbc 225
    Vab 233
    Iline 8.5A
    Icn 2A as are all the other legs +/- ~

    What bothers me is the unbalance in current in the lathe motor both at startup and in run mode.

    STARTUP
    L1/L2 56A
    L3 21A

    RUNNING
    L1 8.5A
    L2 7A
    L3 6.2A

    I can't add any more Cs or Cp as Icn goes beyond 12.5A.

    Big picture scenario...from breaker panel in house, about 150' #8 to old shop, then another 110' #4 TECH to new shop, feeding RPC with 10' #8 and about 10' #12 to lathe. The only thing I can really change is within the new shop without doing a major electrical overhaul. My plan it to make the RPC feed about 3' of #8 from the breaker panel, about 4' of #8 to the lathe. That may make things better.

    The motor starts fine, about 2 seconds to full RPM, line current feeding the RPC peaks at 75A at startup.

    Is this an indication of a defective motor, either lathe or idler, or a case of too small an idler, or is it normal and should I just start making chips?

    Thanks boys!

    Cheers

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    Forgot to add the voltages at lathe startup.

    Vac and Vbc drop to about 120 then come back up to the above listed voltages, main line voltage drops to about 215.

    Thank you for any and all guidance.

    Cheers

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    Quote Originally Posted by kilohertz View Post
    Is this an indication of a defective motor, either lathe or idler, or a case of too small an idler, or is it normal and should I just start making chips?
    Doesn't sound "terrible", given that you are stuck with a less-than optimal feed situation.

    How about you just USE it for a while and observe whether - or not - it drops RPM under load, shudders, stutters, trips breakers, overheats load or idler... whatever.

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    Yes, the real question is whether or not the lathe works as it should.

    Drops at start are pretty normal, as the load motor tries to pull 5x normal current or so. Your running voltages look decent, .

    The 5HP idler and 3HP load is nominally OK. The 230V is not dropping too far, but the generated leg is dropping to about half voltage, which would be why the current in that leg is also dropping at start...reasonably normal..

    I cannot really blame it on the long feeder, since the input to the RPC is holding up reasonably. So, yes, it appears to be the generated leg dropping, which it will, because it has a motor (induction generator) driving it, which is higher impedance than the power line. You have a small phase angle issue, which is probably the thing throwing the currents off.

    You have a ton of compensation on the generated leg, which accounts for the very high no load Vac and Vbc readings. You are way above the 1.08 x Vab.......which would be 251VAC. You do not want to add more. you might be able to shift the capacitance around a bit between Cs and Cp to correct phase, but I'd not bother unless you have a reason..

    I'd leave it and use it... see if there is any reason to complain about performance. If not, keep using it.

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    Quote Originally Posted by JST View Post
    You have a ton of compensation on the generated leg, which accounts for the very high no load Vac and Vbc readings. You are way above the 1.08 x Vab.......which would be 251VAC. You do not want to add more. you might be able to shift the capacitance around a bit between Cs and Cp to correct phase, but I'd not bother unless you have a reason.
    When I was tuning this, I started out with the correct compensation, which was 61uF and 50uF and the no load voltages were perfect at the time, 258 Vac/Vbc, and 238 Vab, but when I loaded it the Vac dropped to 215 and Vbc about 210, with Vab still 235 or so, which is why I started adding Cs and Cp. Maybe I should go back to that? Easy to remove caps. I was more concerned with getting even voltages when loaded, maybe that is the wrong way to tune. Is phase more important than equal voltages?

    Thoughts?

    Cheers

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    Quote Originally Posted by kilohertz View Post
    Is phase more important than equal voltages?
    None of that is worth over-thinking or over-twiddling.

    Lot of rope or other "pony" start RPC in the world don't have ANY caps on them at all. Some have run for multiple decades that way. Idler or load, a(ny) motor's MAIN concern is whether it overheats. Or not.

    Just run it for a while and see how it works out. A body could die of old age "over-thinking" a solution back into a problem before ever making chips.

    DAMHIKT!

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    Well the RPC is a compronmise, and "tuning" the impedance out with the balance caps is another compromise.

    I think I might have left it there, but neither those readings nor the others are really a problem under load. I'd prefer the lower no load/light load voltages in order to have "normal" voltages and avoid having trouble later with anything that takes 3 phase and has electronics in it.

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    Borrow, beg rent, steal, or in final desperation, buy an oscilloscope. People go through this endless comparing voltages and guessing when a picture will settle the debate in seconds. It works best with a dual input scope. It can have a grounded common. Put one input on an incoming line and adjust the horizontal sweep to display one cycle with the zero points at the start, middle, and end. The voltage should read 120. Put the second probe on the generated leg, which should show 208 volts and 90 degree phase shift. Note that when you load the converter, the generated voltage will drop and the phase shift will increase because the drop across the capacitor increases and the phase shift by it will be greater. You can adjust the capacitor for one load, no more. Pick the one you want to use and let the others go where they will. An idler reduces the variation but cannot eliminate it unless it is infinitely large. All the start capacitor does is adjust for a large load- the inertia of the motor armature. Once you get it out of the way, the capacitor will be too large, hence the switch. If you are sufficiently adventurous, you could have multiple run capacitors and switch them in and out as the load varies.


    Bill

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    [QUOTE=kilohertz;3373370]... Is phase more important than equal voltages?

    Phase is determined by the physical phase relationships of the idler motor's
    windings. No need to worry about that issue. You could not change it if
    you wanted to.

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    Hey Bill and Jim,

    Thank you for your posts,most helpful.

    Bill, being an electronics tech most of my life, I probably have a half dozen scopes in various locations, I didn't even think to use one to look at the waveforms. I know I have a 465B in my old shop, easy to get to, my 475 is in my ham shack and the others....??? It will be good to see the phase relationship.

    As I just got in from the days work, I hadn't seen these messages until now, but I decided to go back to my original values, Cs 35uF, Cp, 67uF and what I now see is 258-258-239 unloaded, and loaded 221-221-234, everything seems happy, Icn is down to 9A, lathe motor is 8.7-8.7-5.3A and starts just as well. I am mounting the RPC on the wall tomorrow and using #8 input and output, both short runs, less than 8' each. That alone should help the startup.

    Thank you again for your input, I'll let you know what I see with the scope.

    Cheers

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    Quote Originally Posted by kilohertz View Post
    .. have a half dozen scopes.
    I've not been without at least one since 1954, nor fewer than two since 1955! Most recent is a Rigol 4-trace with memory and "smarts" - a true delight to use. Cheap, too. I paid more for four sets of US-made HV leads!

    That said, it is an RPC, not a VFD, Phase-Perfect, or DC-Drive, ALL of which have fast-rise switching artifacts, one side, the other, or even BOTH.

    Viewing RPC scope traces is about as boring as watching paint polymerize. A sine wave is a sine wave, after all.

    Jim R. has posted traces off of his "steampunk" wood-framed pony-start RPC, "right here on PM".
    Sine waves. Of course. And Jim would need a millpond and waterwheel to get any further "back to the future".


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    [QUOTE=jim rozen;3373785]
    Quote Originally Posted by kilohertz View Post
    ... Is phase more important than equal voltages?

    Phase is determined by the physical phase relationships of the idler motor's
    windings. No need to worry about that issue. You could not change it if
    you wanted to.
    You keep posting the picture of three phases all neat and even but you won't tell us how much load is on the system. The idler will attempt to keep the phase in line but it depends on how large it is relative to the load.

    In one such as is under discussion with a running capacitor, the phase will vary over a large range. The idler becomes a three phase motor fed with the wrong voltage and phase. The idler doesn't control the system unless it is huge.

    Bill

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    Quote Originally Posted by 9100 View Post
    You keep posting the picture of three phases all neat and even but you won't tell us how much load is on the system. The idler will attempt to keep the phase in line but it depends on how large it is relative to the load.
    Bill
    Not this time I didn't post it. No matter how much he was goading me. If you look back way into the dim dark recesses of history
    you will find that discussion of load that was on that system. Basically just one machine idling. Also see the same discussion
    about what happens when you mechanically load the motor being run. The phase relationships stay the same, the manufactured
    phase does droop a bit more.

    The mechanical load was applied to see what happens to the apparent electrical load being drawn by the entire system. The long and short of it is,
    an amp clamp really shows no change when you apply mechanical load. But the current being drawn by the idler shifts in phase and
    the power factor increases.

    This is why using an amp clamp meter to diagnose any features on a rotary doesn't make much sense. Most of the currents are
    typically reactive until the loads are applied. *Except* the current in the manufactured leg. This gives the odd result that the
    manufactured leg shows nearly nothing on the amp clamp meter until the mechanical load is applied. Measuring current on the utility
    legs to the load motor (unloaded) again shows nearly purely reactive current. Applying the mechanical load shows no
    change on the amp-clamp, which is phase insensitive.

    Looks strange. Works just fine.

    "Steampunk??" More like hayseed farmer.

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    [QUOTE=jim rozen;3373785]
    Quote Originally Posted by kilohertz View Post
    ... Is phase more important than equal voltages?

    Phase is determined by the physical phase relationships of the idler motor's
    windings. No need to worry about that issue. You could not change it if
    you wanted to.
    Actually... our load-motor will already have done and it may be advantageous to compensate for that.

    One does need to be mindful that Current and Voltage are BOTH involved, whether you are bothering to observe BOTH or not.

    See, for example, "power factor correction", and explain - if you can do - how it is that, Wikipedia sez:

    Where reactive loads are present, such as with capacitors or inductors, energy storage in the loads results in a phase difference between the current and voltage waveforms.
    .. whilst "we" say that phase difference, each of those two components, WILL vary with load...

    So it isn't that one "cannot" change it. Only that as a mostly-moving target it is seldom worth a great deal of BOTHER to make the attempt.

    Compromise, and make more chips. Or argue, and make fewer.

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    [QUOTE=thermite;3374083]
    Quote Originally Posted by jim rozen View Post

    Actually... our load-motor will already have done and it may be advantageous to compensate for that.

    One does need to be mindful that Current and Voltage are BOTH involved, whether you are bothering to observe BOTH or not.

    See, for example, "power factor correction", and explain - if you can do - how it is that, Wikipedia sez:



    .. whilst "we" say that phase difference, each of those two components, WILL vary with load...

    So it isn't that one "cannot" change it. Only that as a mostly-moving target it is seldom worth a great deal of BOTHER to make the attempt.

    Compromise, and make more chips. Or argue, and make fewer.
    If the voltage presented to the motor terminals is the right voltage and phase angle, the motor thinks it is running on a normal three phase line and will behave normally. It is totally unconcerned about the power factor reflected back to the line. It doesn't care about the current that may be flowing in other parts of the circuit. It only cares that it receives the correct voltage and phase and it will draw the current it wants to, at any phase angle it wants to.

    If you adjust the phase shifting capacitor for the right operation with no load, it will not deliver anything close to full power under load because the generated leg voltage will be low and the phase shift will be excessive. This is not theory but observed data. If I get time, I will set it up and make a video of it.

    Bill


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