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  1. #1
    Torque1st is offline Aluminum
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    I have a 3 7,5HP 220VAC 20.2A 60Hz 1710RPM motor VOEST lathe that will probably draw 18-20A at full load due to the voltage ratings. Startup will see a ~10% increase in required current. I am planning on a few more smaller 3 machine tools (mills etc) in my one man home shop. My air compressor and welder are single phase.

    I am considering a RPC using a 10HP idler motor with friction drive 1/2HP pony motor starter with momentary start/run switch, no voltage correction caps. Connecting 1 240-250VAC line L1/L2 with size 2 contactor with 120V coil from L1 for drop out protection and E-stops. Fed thru #6AWG from a 60A breaker in a 100A 1 4-wire sub panel in my shop. The service panel for the house is 200A.

    L1 L2 and manufactured L3 connected to a 3 5-wire distribution panel with breakers for individual machines. A 40A (if possible) breaker will be used for the lathe.

    1- My question is would it be better to have a theoretical 3600RPM or 1800RPM motor for the idler? Would the 3600RPM motor have more momentum to help with starting/reversing the lathe motor? Should the idler motor be as close to theoretical speed as possible?

    2- Also has anyone ever used a relay powered from the wild/manufactured leg to drop out the main contactor if the idler motor stalls? Or would that cause too many false trips? The 60A main breaker would trip if it stalls anyway.

    Thanks for any insight! [img]smile.gif[/img]

  2. #2
    macona's Avatar
    macona is offline Diamond
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    They say a 3600 rpm motor gives a better third leg.

  3. #3
    Torque1st is offline Aluminum
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    Thanks for the info.

    I have spent hours reading old posts in this forum and have found little yet that mentions this but I could have missed it. It is kind of hard to search for.

  4. #4
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    peterh5322 is offline Diamond
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    "... no voltage correction caps ..."

    Not the best idea, and significantly more important than the decision of 1800 vs. 3600 rpm.

    Particularly as:

    1) caps vs. no caps is so easy to quantify (caps is good, no caps is bad), and

    2) 1800 vs. 3600 rpm is usually a wash, unless the lower rpm idler provides a disproportionally higher inertia (a source of potential energy) which could be could be drawn upon in a case of a momentary overload.

    Assuming you are going to use a pony motor as a starter, the 30 part of the 1-2-3-30 "rule of thumb" drops out, but the 1-2-3 part remains.

    That is 1-2-3 uF per idler motor quarter HP, thereby resulting in 40 uF for Cac (the P.F. correcting cap); and 80 uF for Ccb and 120 uF for Cab (the phase shifting and B-phase voltage control caps).

    For starters, you could split the difference and install the average, 100 uF, between each of A-B and C-B, and then "tune" from that initial point.

    The 370 VAC motor run caps from Burden are so reasonably priced I could not imagine not installing the caps.

    This search key will give you all 370 VAC motor run caps presently in Burden's inventory.

    Notice that the 50 uF caps, item 22-1111, cost only $6.99, and you need four of them for an initial stab at balancing your 10 HP RPC.

    A small price to pay for the significant benefits to be be received, I think.

  5. #5
    Torque1st is offline Aluminum
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    Thanks for the info! [img]smile.gif[/img]

    Is this right?
    2) 1800 vs. 3600 rpm is usually a wash, unless the lower rpm idler provides a disproportionally higher inertia (a source of potential energy) which could be could be drawn upon in a case of a momentary overload.
    I figured the higher RPM might help like one of those "Peak" HP bogus consumer product rated motors... -hehe! You know the type that makes 2.5 "Peak" HP from 117VAC thru a 18AWG 25' power cord...

    I initially looked for a VFD for the lathe but finding one that runs on single phase 240VAC for 5.5KW is difficult. I also knew it would not be my only 3 machine so I decided to look at a RPC.

    Since I was going to be running a number of motors with varying loads on them with the RPC I was not sure that it would be worth trying to balance the voltages with caps for any particular loading.

    Take the lathe for example. Should I put balancing caps on the load side of the lathe contactor and try to balance for no load or under some/full load?

    If I have caps on the load side of individual motor contactors should I use caps on the idler also?

    I am new to all of this RPC and voltage balancing stuff. "I Know Nothing!" as Sgt Schultz would say. I am much better at figuring bearing loads, shaft deflections, or hydraulic pump size on mechanical stuff or using a P for a machine control.

  6. #6
    gpkull is offline Cast Iron
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    not only will your voltages come in closer with caps but your amp draw will go down on your rpc. mine went from 13 to 3.8. voltage on the generated leg went from 190 to 245 when loaded

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    Torque1st , if the inspiration of your proposed setup, is the nearly identical one that I've posted with photos, let me hasten to reiterate that my draw is small, max 3HP and most frequently, my Burke horizontal, only 1HP.

    With your 7-1/2HP lathe, I would certainly dig my run capacitors out of the drawer and get a close balance before I fired up that lathe. Heck, I'm going to do it anyway just to be able to say so. Like my dad always said, "if it's worth doing, it's worth doing well."

    My excuse so far is the whine, "but I don't have an Amp clamp." So I'll follow Peters rule, (he'll probably disclaim but he's the most constant proponent of) and start with the 100uF on each line to manufactured and get serious about that Amp clamp. By the way, a simple multimeter will reveal the voltage disparity, I do have one of them.

    See ya', got some caps waiting on me out in the shop. Keep us posted and welcome aboard.

    Procrastinatin' Bob
    PS, why gamble? It's too easy to match the nameplate speed of the idler with pony. Mine, 1725 pony/3450 idler, two to one on the drive.

    Whoa gpkull, 13 amp draw down to 3.8! I'm gone, gotta' fix it.

  8. #8
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    "If I have caps on the load side of individual motor contactors should I use caps on the idler also?"

    I would.

    My implementation of this strategy would be to minimize the three-phase current into the load motor(s) and the single-phase current into the idler, this by "distributing" the caps across the load motor(s) and the idler.

    It is time-consuming, but it is the best solution, in my book.

    Of course, you would want to make sure the manufactured phase out of the idler didn't exceed 300 volts, which is the "class" which this type of device normally falls into (there is the "300 volt class" and the "600 volt class", among others).

    300 volts, BTW, is +25 percent, and normally you would design such an RPC system for no more than +10 percent, or 264 volts, unloaded, and no less than -10 percent, or 216 volts, fully loaded.

  9. #9
    peterh5322's Avatar
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    "I am new to all of this RPC and voltage balancing stuff. "I Know Nothing!" as Sgt Schultz would say."

    That's what "rules of thumb" are for ... for those of us that are "all thumbs".

  10. #10
    peterh5322's Avatar
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    "It's too easy to match the nameplate speed of the idler with pony. Mine, 1725 pony/3450 idler, two to one on the drive."

    For lowest starting current, you want the idler at 100 percent of synchronous speed, which is 3600 rpm (2-pole) or 1800 rpm (4-pole).

    So, that is 3600/3450 = 1.0435:1, or 1800/1725 = 1.0435:1, as well.

    But, 1:1, which gets the idler up to a slightly sub-synchronous speed, 0.9583, gets you most of the benefit, anyway.

  11. #11
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    Aha! so that's why my idler slightly rev's up in completing the circuit with the disconnect. Coulda' been worse, the rise is virtually instananeous, no discernable dimming lights, glad the pony run-up is only on the order of 5% slow.

    Oh, and I'm NOT kidding Peter, I very much appreciate his knowledge and attention to being precise, if there is one thing that is a universal goal in machining, it's accuracy and for that, you've got to have accurate input. Who knew about the theroetical (synchronous in 60 hertz is 60cycles per second X 60 seconds in a minute = 3600) being the goal for a 3450RPM motor? Having a tough time with that....need to get out my tach...

    Les'see, 4" on the pony, hmmm, gotta' pull that 2" idler wheel and whittle it down to 1.9166". Now I am kidding.

    Close enough Bob

  12. #12
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    "Les'see, 4" on the pony, hmmm, gotta' pull that 2" idler wheel and whittle it down to 1.9166". Now I am kidding."

    Use one of those Browning adjustable sheaves in order to get a 1:1.0435 step-up of pony to idler rpm.

    But, 1:1 is close enough, in my book.

    "Aha! so that's why my idler slightly rev's up in completing the circuit with the disconnect."

    Eureka!

    1:1.0435 is the theoretical best; 1:1 is the practical next-best.

    Of course, if your pony is 3450 and your idler is 1725, then the theoretical and practical ratios are different.

    The point is, I think, that a better understanding of the theory will enable one to implement a better actual solution.

  13. #13
    Torque1st is offline Aluminum
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    WOW! You guys are great!!!

    I don't have any run capacitors in my "junk" boxes but I am sure there will be a few at the local industrial salvage yards where I will be picking up the motors, boxes, contactors, etc.

    I have an amp probe, tachometer, and several multimeters, as well as a 4-channel O-scope with a special isolation transformer to power it. I guess on that front I will manage.

    I will adjust the friction drive for some slippage and wear to get as close to sync speed as I can. I was just going for nameplate RPM match with an ~3-5% slip factor B4.

    Peter, should I adjust the unloaded voltage to be <10% high so it won't fall as low under load? Should I use a 3600RPM motor for more momentum?

    Robert, I am not sure your system was my inspiration. I am not sure which one it might be. Post a link if you can. I have seen a number of them in my wanderings including the ones in the forum sticky.

    I have thought about hooking up an additional 3-5HP idler if necessary to help with running the lathe. It could also be used alone if I was running a smaller <2HP piece of equipment.

    I have glanced thru a few posts here, pages 40-49, so I only have 40 pages to go. I certainly didn't read all the posts tho.

    One thing I can comment on after seeing a number of system... Why do some of you paint the inside of your electrical boxes anything but gloss white??? Those dark boxes are hard on the eyes when you have to dig in them. But then maybe I am just used to industrial control enclosures which are always gloss white on the inside.

  14. #14
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    "... should I adjust the unloaded voltage to be ... high so it won't fall as low under load?"

    The larger the idler, the less variation from unloaded to loaded.

    In the extreme case (an infinitely large idler), there would be no variation from unloaded to loaded.

    At an intermediate case, there would be a 20 percent variation (+10 percent to -10 percent) from unloaded to loaded.

    At another extreme, there would be much more than a 20 percent variation from unloaded to loaded.

    From this observation comes The Law for sizing idlers: If you cannot maintain +/- 10 percent from unloaded to loaded (+/- 5 percent for "technical" loads), then your idler is too small.


    "Should I use a 3600RPM motor for more momentum?"

    Your choice, depending upon what is available locally, and cost.

    I would choose a 4-pole idler (1725 rpm) first, a 6-pole idler (1150 rpm) second, and a 2-pole idler (3450 rpm) third.

  15. #15
    Torque1st is offline Aluminum
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    Last time I was at the salvage yard they couldn't hardly give away a 3600RPM motor but that has changed now. If I remember right they are worth about $0.17/# at the scrapper for metal. If there is no advantage and costs being equal I will just get a common 1725+RPM unit. They usually have quite a selection.

    Am I correct in assuming that if the 10HP isn't quite enuf for the 7.5HP lathe that I could add another 3-5HP idler motor for extra power? Start the pony, pony up the 10HP, start the 5HP, then start the lathe? What would I need for a feed breaker to run all three motors? I will only have 100A service to the shop sub-panel and I still need lights, compressor, AC, heat, etc.

  16. #16
    peterh5322's Avatar
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    "Am I correct in assuming that if the 10HP isn't quite enuf for the 7.5HP lathe that I could add another 3-5HP idler motor for extra power?"

    Yes.


    "What would I need for a feed breaker to run all three motors?"

    Recall that the tool motor, a 7.5 HP lathe, will draw nameplate amps from the A and C phases, which are the same as L1 and L2 of the RPC system. Power for the B phase will come from the RPC system, which itself draws power from L1 and L2. I would estimate that the L1 and L2 amps of the RPC system would be 1.87 times the 7.5 HP motor's nameplate amps.

  17. #17
    Torque1st is offline Aluminum
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    So from my first post the 10+5+7.5HP system would approximately pull 21 x 1.87 = 39 amps so my 60A feed breaker would be ~OK. Not exactly 250% but does that apply in a RPC system? -Thanks!!! [img]smile.gif[/img]

    If I turned on say a 2HP mill at the same time would the RPC system draw another 7 x 1.87 = 13A or 52A?

    Is there any percentage factor from the NEC for overload that applies in these RPC systems? Or is it just basically whatever we can get to "work" without setting anything on fire or getting someone electrocuted? I absolutely hate the smell of electrical smoke.


    Did I say thanks for all the help???? [img]smile.gif[/img]

  18. #18
    Torque1st is offline Aluminum
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    Hmmmmmmmm, when I look at the full load amperage ratings for a 10HP motor I find 28A which is a lot less than 39A. Is all of that amperage generated within the B phase winding or does some come thru the caps?

  19. #19
    peterh5322's Avatar
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    10 HP would be 40 amps, give or take, thereby requiring a 50 amp feeder for 3450 rpm, or a 60 amp feeder for 1725 rpm.

    Those are single-phase amps, of course. Three-phase amps would be about 0.578 of those values.

    "Is there any percentage factor from the NEC for overload that applies in these RPC systems? Or is it just basically whatever we can get to "work" without setting anything on fire or getting someone electrocuted?"

    If you size your feeder for your idler, you'll be safe.

    6 AWG feeder, 50 amp, or 60 amp, maximum, breaker.

    8 AWG feeder, 40 amp, maximum, breaker.

    Etcetera.

    Remember, once the idler is up to speed, it is doing no work unless the load motors are also doing work.

    Of course, you have to allow for the magnetizing current in the idler, which is relatively small.

  20. #20
    Torque1st is offline Aluminum
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    From my first post I listed a planned #6AWG feed for the RPC system from a 60A breaker in the sub-panel with a 40A breaker in the 3 dist panel for the lathe which would require #8 75C wire. Looks like it will do.

    Those are single-phase amps, of course. Three-phase amps would be about 0.578 of those values.
    I wish I knew anything about polyphase electricals. Are you saying the actual current in the winding is only 0.578 of those values so I don't have to worry about the FLA rating for the 10HP idler?

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