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  1. #41
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    Quote Originally Posted by GregSY View Post

    What your RPC is doing is creating a 'fake' third phase that allows the motor to start. But it's not a real phase. I've not thought about RPC's in 15 years so I'm a bit foggy on the details. But from memory you are only getting about 2/3 of the motor's output...because the third leg is not really doing anything in terms of power production.
    If you do it right, the generated phase is as good as the others. If you have the same voltage and phase angle, the motor doesn't now the difference and it produces full power. You can do it with only capacitors, the idler just extends the useable range. I have had one going with a variac to reflect a variable capacitor and could exactly duplicate a "real" third phase. This is not a thought experiment but real experimental data. Jerry described on working on a similar setup with a bunch of switched capacitors. There is nothing fake about the voltage, only off the optimum setting.

    Bill

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  3. #42
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    Quote Originally Posted by cg285 View Post
    that's an idea. don't know what all is involved nor the reaction once engaged at speed
    If there is a belt in the power train, you my only need a longer one and an idler to control tension.

    Bill

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    Quote Originally Posted by cg285 View Post
    soft start has to sit by the motor?
    I'm getting the feeling we're not all on the same page.
    A Wye/Delta soft starter starts the motor with the leads tapped for Wye. After the motor starts it switches the taps to delta. So you would need access to the motor to run all six (or 12) leads out to the starting mechanism.

    This may help
    WYEStart-DeltaRun

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    My remembrance of an RPC is it operates on a fixed capacitance and will be optimized at one HP output only. If you optimize it for 10HP, it won't be optimized at 5HP Using variacs and switched caps to alleviate that gets you closer but are a lot of mess in terms of added pieces, wiring, controls, etc.

    The starting problem, from the sounds of it, don't lie in the fact that the RPC of 20HP is not big enough to run a 10HP motor. My guess is the entire system upstream of the motor is not adequate to support the current demand during start. Keep in mind the motor probably pulls 6X the current during start that it pulls at full load (10HP). so in basic terms, for the starting cycle it is pulling 60HP worth of current. The RPC could be a limiting factor but so could a breaker or your power source.

    The bottom line is the same....a VFD, in this application, solves all of these issues handily. No inrush. A small, compact, near silent wall mounted box.


    If you want to add some numbers, you can use an abacus or a 55lbs. old school mechanical adding machine. But it sure is more pleasant to go buy a calculator at Target.

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  7. #45
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    Quote Originally Posted by Larry Dickman View Post
    I'm getting the feeling we're not all on the same page.
    A Wye/Delta soft starter starts the motor with the leads tapped for Wye. After the motor starts it switches the taps to delta. So you would need access to the motor to run all six (or 12) leads out to the starting mechanism.

    This may help
    WYEStart-DeltaRun
    oh, you are referring to what i believe is a star delta.
    this is what i had come up with before - maybe it's not correct

    The two starter types we are comparing are the star-delta (wye-delta) and a soft starter, and they both limit the voltage on start. The short answer is a soft-starter is more efficient, so use it where you have a large motor that is often starting and stopping. The goal of this method is to reduce the starting voltage.

    i have a relatively large motor that requires frequent starts (for part inspection) while operating

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    Quote Originally Posted by Cole2534 View Post
    Where can a guy find a reasonably priced scope? . . .
    This is a great little scope. Very capable and not expensive: Hantek DSO5102P Digital Storage Oscilloscope USB 100MHz 1GSa/s 40K, 2 Channel, 2CH: Amazon.com: Industrial & Scientific

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    Quote Originally Posted by cg285 View Post
    oh, you are referring to what i believe is a star delta.
    this is what i had come up with before - maybe it's not correct

    The two starter types we are comparing are the star-delta (wye-delta) and a soft starter, and they both limit the voltage on start. The short answer is a soft-starter is more efficient, so use it where you have a large motor that is often starting and stopping. The goal of this method is to reduce the starting voltage.

    i have a relatively large motor that requires frequent starts (for part inspection) while operating
    having said that further research show soft start - low torque, vfd - high torque (related to voltage)

    if one was to size a vfd would he need to have a larger vfd than largest motor? ie: a 15hp vfd for a 10hp usage or 10 for a 10

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    i think i found the sizing answer. running off of single phase i would need 20hp vfd
    no sense running it off of a rpc

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    Quote Originally Posted by Larry Dickman View Post
    Larry, I realize I'm sticking out my neck:
    I think the diagram shown is simplified.
    There is a correct way of connecting a star/delta starter and there are several wrong ways.
    Normally, the purpose is to reduce the start current but care should be taken. Take a look on the diagrams suggested by a reputable manufacturer: One has to consider the actual labels on the windings, otherwise the start current will not be reduced.
    The correct diagrams will (according to norms) be drawn for motors turning clockwise. If turning anticlockwise, the diagram is a bit different, so if you want this you should not just swap two phases.
    There is a Danfoss(TM) note on this somewhere which I read back in the sixties. No doubt similar notes can be found in american papers.

    Don't ask me how I came to know about this. Suffice to say, many electricians over here don't know either. Nowadays our power supply don't much care anyway, but I suspect the OP could use this pointer.

    Just my .02 €,
    fusker

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    If you can see your motor nameplate, read what it says for full load amps. Whatever is says, you would need a VFD that can output that many amps. Generically, you need around 30 amps at 230v. But each motor will be a bit different.

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    Quote Originally Posted by GregSY View Post
    If you can see your motor nameplate, read what it says for full load amps. Whatever is says, you would need a VFD that can output that many amps. Generically, you need around 30 amps at 230v. But each motor will be a bit different.
    will be a task to get to the name plate but info that came with it says [email protected] full load. not starting @ i found tho

    what i read was if the vfd was supplied with 3 phase then the amps could be matched but if supplied with 1 phase it would have to be much higher. no sense having a rpc powering the vfd - in my mind

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    You can't power the VFDS with an RPC, if that's what you're saying. You would just connect the VFD directly to the 240 outlet.

    You're getting too worried about the VFD single vs. three pahse issue. Just use a VFD that can output 30 amps @ 240v 3 phase based on a single phase 240v input. Yes, it will be a larger VFD than a 30 amp 3 phase In/Out VFD but so what?

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    Before all this, why don't you measure the voltages between each leg you have now, running unloaded and when trying to start. You could produce more wisdom than has been expressed here with a Wiggy.

    Bill

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    Quote Originally Posted by Larry Dickman View Post
    why don't you just retap the motor for Wye and see if it works? It'll take 10 min and won't cost you a nickel. Then you'll know if a Y/Delta soft starter will work.
    after spending most of the day accessing the motor i find it's a 9 wire which i understand is already wye and can't be changed? i ohm'd out the windings and they appear ok. to ground i have approx 0.7xx meg which i think should be ok - probably should have had the meter on a smaller scale. i did find one wire nut i had to cut off (overheated/corroded) MAYBE i could get lucky with that.

    healing from my second elbow surgery in the last 5 months may the pricks that use a roll of black tape on every wire nut break both elbows.

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    Quote Originally Posted by GregSY View Post
    My remembrance of an RPC is it operates on a fixed capacitance and will be optimized at one HP output only. If you optimize it for 10HP, it won't be optimized at 5HP Using variacs and switched caps to alleviate that gets you closer but are a lot of mess in terms of added pieces, wiring, controls, etc.

    .....
    Your remembrance is a tad off.

    Just to set the record straight here, an "RPC" is a 3 phase motor that is driven with 2 wires. It will run like that, and yes, it can only produce 2/3 power. But the good news is, that it only has to generate 1/3 of the power.... simplified story is that there are three wires in 3 phase, and each carries 1/3 of the total power. So the RPC has only to provide 1/3 of the total.

    The other half is that no capacitors are actually "required". A perfectly good RPC can be made without any capacitors, relays, sensors, etc. The old school type is a big-ass 3 phase motor, started mechanically by spinning it with a "pony motor", or rope starting it like a lawn mower, or other simple means.

    Once started, it generates voltage in opposition to the incoming voltage, on all wires. two of them have power on them, but on the third, there is no power connected, the generated voltage is what you want. It is in-phase and close to (always slightly less than) the input voltage, and that is where the power to the third wire comes from. The windings of the motor force it to be in correct phase angle, and it will power load motors just like any 3 phase source, just a little unbalanced (unless the designer fixed that, which can be done). The various capacitors often used are either for starting it electrically (on single phase), or to adjust the voltage output.

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    Quote Originally Posted by GregSY View Post
    You can't power the VFDS with an RPC, if that's what you're saying.
    Assuredly one CAN do. it can add variable-speed and soft-start ("ramp-up").

    One can even power a VFD from Dee Cee - into or even substituting for its onboard post-rectification capacitor bank. See "DC bus" and it's value, special, but nonetheless "real world" edge cases.

    It just doesn't "usually" make much sense either technically OR economically to "stack" them that way when so many VFD can run directly off single-phase.

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    OK....I stand corrected. You CAN do lots of things...but why on Earth would you?

    A big-ass 3 phase motor started by a pony motor or a rope start? A VFD powered by an RPC? Guys, that's a lot of bits and pieces that have to be paid for and take up real estate in the shop. Why have any of it when you can have a small box, all by itself? And who wants to go yank a rope every time they want to use the lathe? And..don't forget the idler motor itself will pull inrush, even uncoupled.

    And this sort of all assumes the big-ass 3 phase motor is gonna come from someplace 'free' or 'really cheap'. Because if you have to pay for it new, you're already in the red.

    A VFD is as soft and ramped up as you can get on a motor start. An RPC feeding a VFD would offer nothing. And if you mean the VFD itself, they start with very little 'inrush' so there is no need for any sort of ramp-up on the VFD alone.

    It's not 1985 anymore. By now, VFD's are very affordable, reliable, and well thought out. A single VFD can power many shop tools. Much like an RPC, it simply needs to be sized as such. In some cases, an RPC can stand head-to-head with a VFD, still. But for the OP's situation, it's clearly VFD territory.

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    Quote Originally Posted by GregSY View Post
    OK....I stand corrected. You CAN do lots of things...but why on Earth would you?

    A big-ass 3 phase motor started by a pony motor or a rope start? A VFD powered by an RPC? Guys, that's a lot of bits and pieces that have to be paid for and take up real estate in the shop. Why have any of it when you can have a small box, all by itself? And who wants to go yank a rope every time they want to use the lathe? And..don't forget the idler motor itself will pull inrush, even uncoupled.

    And this sort of all assumes the big-ass 3 phase motor is gonna come from someplace 'free' or 'really cheap'. Because if you have to pay for it new, you're already in the red.

    A VFD is as soft and ramped up as you can get on a motor start. An RPC feeding a VFD would offer nothing. And if you mean the VFD itself, they start with very little 'inrush' so there is no need for any sort of ramp-up on the VFD alone.

    It's not 1985 anymore. By now, VFD's are very affordable, reliable, and well thought out. A single VFD can power many shop tools. Much like an RPC, it simply needs to be sized as such. In some cases, an RPC can stand head-to-head with a VFD, still. But for the OP's situation, it's clearly VFD territory.
    Threw away three VFD's, two brand-new, never wired up, the third a decent-grade Schneider Altivar 71 ten hoss.

    Spent about $1600 on a used P-P, then $4,000 on a NEW one. As with VFD wanting new capacitors every 7 to 12 years? P-P want new caps at THREE year intervals! RTFM.

    The RPC, all new parts? 10 HP Weg motor, one of Jim's Phase-Craft controllers?

    About $700. Caps will probably outlive me if not also impeachment as a national sport.

    Cheapest VFD I'd allow under-roof starts at $1,600 or so. It would operate exactly ONE machine-tool.

    The RPC and Phase-Perfect go to a load center and whole buncha CB's and Hubbell twist-lock wall outlets. By way of a "pure Sine wave" filter in the case of the P-P, then a Delta-Wye transformer for both. No corner-grounding or high-leg Delta for me!

    All my motors are OLD. They weren't BUILT for "inverter duty" any more than I was!

    Always liked GIRLS, y'see!


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    A VFD will be current limited and may not supply enough to overcome starting friction. A soft start ramping up is exactly what you don't want. You want to hit the motor with enough current to overcome friction as quickly as possible to minimize heating the windings. I will defer to Jerry for a final opinion, but you may need a much larger VFD than is required for running to supply the starting current.

    Measure the incoming voltage under locked rotor conditions and see if there is enough to do the job. If the voltage stays reasonable, measure the voltage between the generated phase and each of the lines. You will probably see a voltage like 200 between the generated leg and one line and a much smaller one on the other. Connect starting capacitors across the larger voltage points and see if it improves. Assuming that there is enough current available, as you add capacity eventually you will have equal voltages on both sides and if the motor is capable of pulling the load, it will then.

    In any case, read the voltages and currents and suspend this meaningless babbel.

    Bill

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    Quote Originally Posted by 9100 View Post
    In any case, read the voltages and currents and ...
    Take note that one of the older and larger commerial makers of RPC publish a table that takes hard starting under load into account as to RPC sizing.

    Add idler. Upsize the feed. Go make parts. No meter required.

    Not even CLOSE to a "new challenge". His rig WAS WORKING. It doesn't need much.


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