30 HP Rotary Phase Converter
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    Question 30 HP Rotary Phase Converter

    Hi all, I'm a relative newbie to this forum and looking for some help.

    I recently purchased an older Fadal VMC 3016, wired 3 Phase. I've looked all over for the transformer to convert it to single phase, but have come up empty or looking at $3000+ pricetag.

    I then turned my attention to an RPC to run this and any other machine I may find. I found a reasonably priced 30HP motor in great condition and have studied different solutions, but few go into details I can understand or to match the size of this motor.

    One of the latest concerns I've had is the inrush of current to start this beast and whether my load center will support it. I have been toying with using a VFD to start up the idler, then switch to my single phase power with the created wild leg.

    If anyone can help with this idea or other options, it would be greatly appreciated.

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    use an idler to spool up the rpc.

    after the idler comes up to speed, connect the rpc to the mains power.

    There is a potential problem others have reported: the idler can generate its own voltage if you leave the balance capacitors connected to the idler motor while turning it without mains power applied. This can cause a surge current capable of blowing breakers if they are connected out of phase.

    one solution is to spin up the idler motor without any capacitors connected to it. (you can leave the capacitors connected to the mains but not connected to the idler motor, both capacitors will be in series until the 3rd leg of the idler is connected)

    so you need 2, 3 phase disconnects. one to connect the idler motor to the mains and the balance capacitors.

    one to connect the idler to the downstream 3 phase loads.

    another solution is don't use capacitors and instead use a transformer to boost the voltage generated by the rpc. for a 240v system without capacitors the voltage generated by the idler will be about 20 volts low. either a 12/24 or a 16/32v boost transformer can be used.

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    I have a 30 hp Phase-a-matic RPC, and the current inrush pegs my meters at 100A. It's probably in the 150A range. Under heavy use it will trip a 100A breaker, but so far has not tripped a 125A breaker.

    This same breaker feeds the downstream 3 phase load center too.

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    Quote Originally Posted by rlsmith58 View Post
    Hi all, I'm a relative newbie to this forum and looking for some help.

    I recently purchased an older Fadal VMC 3016, wired 3 Phase. I've looked all over for the transformer to convert it to single phase, but have come up empty or looking at $3000+ pricetag.

    I then turned my attention to an RPC to run this and any other machine I may find. I found a reasonably priced 30HP motor in great condition and have studied different solutions, but few go into details I can understand or to match the size of this motor.

    One of the latest concerns I've had is the inrush of current to start this beast and whether my load center will support it. I have been toying with using a VFD to start up the idler, then switch to my single phase power with the created wild leg.

    If anyone can help with this idea or other options, it would be greatly appreciated.
    Starting load and its serious challenges aside for a moment..

    "scale" the Fitch Williams RPC design at 3 times the capacitor values.
    He publjshed his circuit right AT 10 HP to make that part easy.

    Now.. slow-ramping the UNLOADED 30 HP idler up with a MUCH less-powerful VFD works fine ..for that PART...

    What would be missing is any sychronization better than a dice-roll to the split-phase upstream feed's phase at the time you seek to cut it over.

    That could get.."dramatic"

    And then . VFD have some clever features, not all of which are always put to work. Taking input from a local shaft resolver or a remote data link is on that menu.

    It "should be" possible, in a like manner, to "inform" the VFD that there is an external (to itself) phase you wish it to match its own phasing and load motor TO?

    Ergo "somebody" has already done this. "Grid Tie" inverters do it, too.


    "Your mission...." etc. or that of we chik'ns? .. is to find that info .. and see if the method can be applied.

    Worth the research, IMNSHO.

    You are not even CLOSE to the only Seeker who would LOVE to have a better way to spin-up large idlers on barely adequate power.. that CAN easily haul the actual loads.. if but gotten up and running without a borderline major war, each attempt.

    Getting this to work could be a LOT more "user friendly" than trying to "pony" an idler up ... and using a "synchroscope", manually. and "each go" . .ELSE risking the "drama."

    The OTHER way to manage the inrush to a 30 HP idler is not to have it as so bad to begin with.

    Start a 10 HP or 15 HP idler. Pause. Add another idler. Then again.

    By four, if not three, or maybe just TWO?... the array of idlers is at your target 30 HP.. but it never hit the line for more than a portion of the inrush at any given point in time.

    Works for me .. and even AT the 30 HP target.. if I'd but replace my 3 HP with a 5 HP.

    10+3+7.5+7.5 = 28

    Would become:

    10+5+7.5+7.5 = 30

    Downside is that is a lot of motors as idlers. And it was a lot of FREIGHT cost to get them HERE.

    Upside .. and my primary reason to BOTHER -- is that I can tailor the idler HP actually on line to a wide range of loads. And put more online to START heavy loads than they will need to RUN in real-world loading... thus drop OFF some, if not MOST.. idler power once they ARE past THEIR starting inrush.

    A modest VFD would have been CHEAPER, and not need as much SPACE, contactors, circuit breakers, capacitors, Weigemann boxes, wire, transformers and control switches.. so.

    "Go for it!"

    Manually . .with a synchroscope for openers. Those CAN be really SIMPLE. Blinkinwinkinlight simple.

    It was how we paralleled-up multi-megawatt Diesel Gen sets "back in the....."

    "Theatre of operations, NO commercial power.."... Vet-Nam War.

    So the synchroscope part JF WORKS.

    Now teaching a VFD... to do that sync... or a pig to whistle...


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    Hmm. What about just doing an extreme form of reduced-voltage starting? Slap enough resistance/autotransformer in line that starting current is manageable, then short them out once up to speed. Should be fairly minimal spike as it's already most of the way up to speed - the slip should be quite minimal even at say 25% voltage, because all it's fighting is the fan and bearings.

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    Quote Originally Posted by SomeoneSomewhere View Post
    Hmm. What about just doing an extreme form of reduced-voltage starting? Slap enough resistance/autotransformer in line that starting current is manageable, then short them out once up to speed. Should be fairly minimal spike as it's already most of the way up to speed - the slip should be quite minimal even at say 25% voltage, because all it's fighting is the fan and bearings.
    Lumped-inductance in-series / NOT.. implies possible phase shift? Resembles an Old Skewl O'Scope's trigger delay line?

    Big-ass resistive might be better?

    I've used various cooktop elements. One is a 9kW "canning" element... for those big boiler/sterilizers.

    Because:

    A) They are CHEAP as "power resistors" go, as little as eight bucks each.

    and more importantly..

    B) They are MEANT to get hot!

    So they do not need a heat-sink nor a fan.

    Only a keep your FINGERS off.. perf, grille, or screen.

    And the heating time is short enough the heat isn't actually horrible, anyway. Especially when paralleled to drop the resistance...BEFORE they are bypassed, outright.

    To the good, "perfect" phase match is there from the outset.

    Meanwhile..

    2 idlers @ 15 HP, each, stagger-started, would still be pushing it?

    3 idlers @ 10 HP, ONE 10 HP-capable starter, two extra contactors.. not so bad?

    In "dial-an-idler" use, my case, the run/balance caps needed to live in a BIG Weigemann box ON each idler, wired motor side of the contactor in the same box.. so the overall capacitance better matches wotever was selected and "active".

    In the OP's case, one load, they could live in ONE box, (through) or "bussed" side?

    2CW

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    Yeah, I was considering that phase shift might be an issue. Experimentation might be needed. Ideal would probably be a really fat variac and wind it up, but that's probably cost-impractical unless you find a very cheap surplus one.

    The nice thing about a transformer is that it multiplies your current. Start at 50% voltage, motor pulls 50% of starting amps, line draw is 25% of starting amps (ignoring TX losses). You get 25% (0.5^2) starting torque but that's probably fine.

    You'd have to run a resistive starter at 25% voltage, 1/16th starting torque to get the same line current, and that might be too far.

    Wiring it for 480 might work but you still end up having to switch from low to high voltage (or vice versa, depending on how you're counting).

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    Quote Originally Posted by SomeoneSomewhere View Post
    Yeah, I was considering that phase shift might be an issue. Experimentation might be needed. Ideal would probably be a really fat variac and wind it up, but that's probably cost-impractical unless you find a very cheap surplus one.

    The nice thing about a transformer is that it multiplies your current. Start at 50% voltage, motor pulls 50% of starting amps, line draw is 25% of starting amps (ignoring TX losses). You get 25% (0.5^2) starting torque but that's probably fine.

    You'd have to run a resistive starter at 25% voltage, 1/16th starting torque to get the same line current, and that might be too far.

    Wiring it for 480 might work but you still end up having to switch from low to high voltage (or vice versa, depending on how you're counting).
    MOST of these bentlegged "solutions" would SUCK for a "service motor" of any kind.

    All MANNER of minor delay and annoyance can be tolerated if it gets a stout RPC up off its knees to run a shop not otherwise "run able".

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    I toyed with the pony motor idea to ramp up the speed, but I'm not sold on it mainly because of the cost of sheaves to fit this motor. I found a cheap hub at $25, but when looking for an "E" type sheave they started around $500.

    My next thought was to build a 10HP RPC, then feed the 30HP off of that. My 2 concerns with that was how to keep from back-feeding the smaller motor after it's cut out? Or, something I hadn't thought of was, syncing up the phases when I switch over.

    Thinking about the design with the 2 idlers, It would be nice to power up the 10HP when running smaller motors and adding the 30HP when I want to run my Fadal. How that would work again I'm not sure.

    Last thought was to start the bigger motor at 440v, then step down to 220v. This would require stepping up beforehand too, so now I'm buying 2 transformers which aren't cheap. That makes the sheave for the big motor looks more promising now, but still more than I'd like to spend

    Any thoughts?

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    What is the shaft diameter?

    secondly, when talking about starting the motor on 440.
    what that means is using 3 contactors aka 3, 3 pole relays to connect the motor for 440 and start it on 220.

    then change it back to 220 after some amount of time (sometimes they don't always come up to full speed on 1/2 voltage)

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    Quote Originally Posted by rlsmith58 View Post
    I toyed with the pony motor idea to ramp up the speed, but I'm not sold on it mainly because of the cost of sheaves to fit this motor. I found a cheap hub at $25, but when looking for an "E" type sheave they started around $500.

    My next thought was to build a 10HP RPC, then feed the 30HP off of that. My 2 concerns with that was how to keep from back-feeding the smaller motor after it's cut out? Or, something I hadn't thought of was, syncing up the phases when I switch over.

    Thinking about the design with the 2 idlers, It would be nice to power up the 10HP when running smaller motors and adding the 30HP when I want to run my Fadal. How that would work again I'm not sure.

    Last thought was to start the bigger motor at 440v, then step down to 220v. This would require stepping up beforehand too, so now I'm buying 2 transformers which aren't cheap. That makes the sheave for the big motor looks more promising now, but still more than I'd like to spend

    Any thoughts?
    Make a sheave. Even WOOD will work. Steel hub. They used a LOT of it.. and glued PAPER.. "back in the flat-belt day".

    Serpentine belt, not classical Vee. You CAN groove for it, but need not.

    As to sync? Keep your leads the same way whilst coming off the split-phase and thereafter, the string of players all automagically stay on-phase and IN phase together. Nature of the beast.

    As to spinning up a 30 with a TEN? If had to doo that, my "pony motor" would be an automotive starter motor off a lead-acid marine/RV 12V battery on a trickle/float maintainer charger.

    Otherwise.. look again at the sequence I posted.

    I start the TEN off the Phase-Craft with its capacitor.
    All the other supplementary idlers after it are SMALLER, not larger.

    10 is cap started
    10 contactor-starts 3.
    13 contactor starts 7.5
    20.5 contactor starts second 7.5

    ..and THEN I'm at 28 "nominal". And can drop OFF what is not needed.

    IF, repeat IF, I even GO that far. Read: "not usually".

    Two of my 3-P machines - a 1.5 HP and a 1 3/4 HP .. only need the THREE as idler.


    No meat on that 220/440 bone, BTW. You ain't the first Virgin as ever had this itch to scratch. If is was THAT easy, everybody would already have it.. built-in!

    What we DO get, (now and then, not often..) instead, is Wye start, Delta run. But that's no significant help HERE.

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    WRT starting at 440V, that means connecting the motor (using contactors) for 440V but supplying it 220V - half voltage. This is similar to star/delta starting but at 50% not 58%.

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    So the VFD is out of the picture, seems like there may be issues I don't know how to resolve.

    I do have a question about the whole pony motor idea though. If I were to use a single phase motor to ramp up the idler motor to speed i wouldn't have the inrush of power. What if I also had a small 3 phase motor attached to the idler. as the idler has begun to make 3 phase power I engage contactors to turn off the single phase motor and turn on the smaller 3 phase motor, powered by the idler? If I am thinking correctly, I have just begun producing my own power offline. I would still have to balance the power coming from the idler, but in theory it seems like it would work as long as the small motors have enough power to spin the idler.

    Please let me know if you think its feasible or what the stumbling blocks would be. Thanks.

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    Quote Originally Posted by SomeoneSomewhere View Post
    WRT starting at 440V, that means connecting the motor (using contactors) for 440V but supplying it 220V - half voltage. This is similar to star/delta starting but at 50% not 58%.
    I hear yah. But what happens next?

    Are the contactors meant to be arranged in a "transfer" contact form?
    If so, how?

    Or as break-before make?

    And if break-before-make, are we still aligned with the supply phase when the power comes back and hits the re-arranged coils?

    Wye/Delta is a tad different than series/parallel, yah?

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    I wrote the last post before catching up on the new postings. I still hope to hear your ideas, but will catch up on my reading.

    I didn't think of making my own sheave. I like it!

    Also, I have a lot to learn about electricity. Reading in a different posting, I learned that a motor is not a generator as I was thinking it would work unless there is current or some residual magnetism.

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    Quote Originally Posted by rlsmith58 View Post
    Reading in a different posting, I learned that a motor is not a generator as I was thinking it would work unless there is current or some residual magnetism.
    Actually ..it IS. The mechanism is called "back EMF" or "CEMF" Counter Electromotive Force".

    It is how a motor draws less power when lightly loaded, and part of how an RPC re-distributes the power from its single input phase to two other parasitical phases to cause them to have enough energy to "act as if" they were directly powered.

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    There are a LOT of people doing pony start RPCs, and the system pretty much just works.

    For ordinary motor loads, most do not bother with capacitors or balancing. For use with a rectified load like a CNC, you will probably want to have a balance method.

    The use of a transformer voltage boost for the generated leg is probably best for loads like a CNC, where the load is variable. Start the RPC with the transformer and loads disconnected, then connect when up to speed.

    Note that the "back EMF" is ALWAYS lower than the line voltage, which is the reason WHY there are capacitor schemes etc for boosting the generated leg.... it is automatically lower in voltage than the line, because the motor has to draw current.

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