Sizing start & run capacitors for 100hp 3 Phase motor
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    Default Sizing start & run capacitors for 100hp 3 Phase motor

    Hello, I've been reading this post: Please help with capacitor sizing for 7.5 HP RPC

    as well as some others about setting up a rotary phase converter.

    I have a 3PH 50HP 4 Pole load that I need to run so I got a 3 Phase 100 HP motor.

    The Idler motor;
    100HP 75kw
    220/440 volts
    236/118 amps

    The load motor;
    50HP 37KW
    220/440 volts
    124/62 amps

    There is a document that I am also following: https://www.practicalmachinist.com/FitchWConverter.pdf

    Along with this youtube video: YouTube

    In the above video and PM post both were using 7ish hp motors.

    I am using 100HP which is a bit more.

    Some articles that I've read says it takes 100-200 microfarads per hp and it's been said that this scales linearly. Is that true?

    I understand that balancing a RPC uses different capacitors for start and run purpose.

    I plan to run with 220V since that's the closest thing that I have access to. Can someone assist me with properly sizing start capacitors?

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    Look into using a pony start, I think it is your only chance of making it work.

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    So, what is your mains supply like, as far as current capacity? You will have a huge current surge on startup if you try to start that with a capacitor setup as if it were a 1 HP, the lights will dim all around you, and the powerco will hate that.

    It's exactly the sort of situation that a pony start is good for.

    I'd suggest not using balance capacitors, or at least not connecting them until it is running..... they can cause a load on the motor when starting, if there is any residual magnetism in the iron, as there may well be.

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    Beware that in the video he is interchanging the words 'pony' and 'idler' for the same thing.

    The video also confirms that a potential relay is not the way to go.

    The wood sheet for a base is fine for prototyping. But not for the final product.

    His idler is 7.5Hp and his shear motor is 5Hp. The factor is 1.5. Your factor is 2 which has bought you a lot more to deal with.

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    I watched all of of the videos, and noted some errors.

    In the first video he calls and labels the idler motor as a "pony motor". They are not the same thing. A pony motor is a smaller motor used to spin the idler motor prior to power being provided to the idler. Using the pony motor means you do not need to use start capacitors to start the motor spinning. Using the pony motor to spin up the idler also reduces the inrush current.

    In the second video he used the ground line as a neutral line for both the contactor relay and the pilot light. This is a significant safety issue

    In the third video he uses a plywood inner panel. You do not want flammable materials inside your enclosure.

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    My non RPC 15 starts snappily on 1000 mfd. 100 would - I suppose - take 6700

    Now you can imagine the inrush to suddenly fill those up

    That 15 by the way gives the pair of slow blow fifties no heart burn

    Now you can imagine you need 350 amp slow blows

    By the way does such seriousness also need huge size copper feeds? Like way beyond 4/0?

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    I would also suggest that the idler be connected through a resistor during startup to reset the magnetic field of the stator. There will be an amount of residual magnetism at startup that needs to be synchronized to the power line. Connecting the two out of phase will generate a pulse of current. The resistor provides a small amount of ac to demagnetize the core and allow smooth coupling. The resistor is switched out or bypassed after the transition to running. Well known problem for large transformers.

    Tom
    Last edited by TDegenhart; 11-29-2019 at 02:42 PM.

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    I am still waiting to hear what the service panel is for running this as questioned by JST. At full load the single phase current will be running north of 200 amps. In a 200 amp panel, most bus bars are only rated for 100 amps per stab. I do not know what the stab rating is in a 400 amp panel or larger but breakers over 100 amps might be expensive or non existent for your panel. Your largest challenge may be getting adequate power to the RPC.

    And as stated by others, it is not practical to capacitor start a 100 hp idler. The fact that you have asked about start capacitors indicates to me that you have only begun to explore the construction of this RPC and have possibly got ahead of yourself looking at the details instead of the big picture.

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    What are you trying to run that requires 50Hp?

    I'm going to play psychic and say I see a large generator in your future

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    Quote Originally Posted by Bill Simmons View Post
    I am still waiting to hear what the service panel is for running this as questioned by JST. At full load the single phase current will be running north of 200 amps. In a 200 amp panel, most bus bars are only rated for 100 amps per stab. I do not know what the stab rating is in a 400 amp panel or larger but breakers over 100 amps might be expensive or non existent for your panel. Your largest challenge may be getting adequate power to the RPC.

    And as stated by others, it is not practical to capacitor start a 100 hp idler. The fact that you have asked about start capacitors indicates to me that you have only begun to explore the construction of this RPC and have possibly got ahead of yourself looking at the details instead of the big picture.
    There are published papers on line that discuss large converters for stuff like farm equipment. The circuits don't look anything like that little Fitch.
    If you just are beginning like Bill S. mentions and you have already looked at the big picture and this is what you want, then I think a pony motor is
    what you need. And that won't be too bad.

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    Quote Originally Posted by newbea View Post
    Hello, I've been reading this post: Please help with capacitor sizing for 7.5 HP RPC

    as well as some others about setting up a rotary phase converter.

    I have a 3PH 50HP 4 Pole load that I need to run so I got a 3 Phase 100 HP motor.

    The Idler motor;
    100HP 75kw
    220/440 volts
    236/118 amps

    The load motor;
    50HP 37KW
    220/440 volts
    124/62 amps

    There is a document that I am also following: https://www.practicalmachinist.com/FitchWConverter.pdf

    Along with this youtube video: YouTube

    In the above video and PM post both were using 7ish hp motors.

    I am using 100HP which is a bit more.

    Some articles that I've read says it takes 100-200 microfarads per hp and it's been said that this scales linearly. Is that true?

    I understand that balancing a RPC uses different capacitors for start and run purpose.

    I plan to run with 220V since that's the closest thing that I have access to. Can someone assist me with properly sizing start capacitors?
    You need to re-think this. Starting with the 50 HP load.

    Have a look at commercial ready-made units and note their published specs.

    Grainger list a Phase-a-Matic 100 HP implemented as PAIRED 50 HP.

    "Stagger start" come to mind? 325 Amp load on 460 VAC service sez it might do.

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    Lots of good advice, imho.

    To OP:
    Since You seem to have sufficient power to run 100 HP, most of the inrush current problems and scares mentioned are probably non-issues, if You just have any kind of resistor/limiter on the power lines in the rpc.
    To limit the sub-second peak power in and out.

    One good option is old-fashioned light bulbs for smaller loads, or perhaps water heater resistors for larger loads.
    A say 2 kW nominal resistor for a water heater can typically take 10x for less than a second, as that is what they do 24x7, in use.
    Maybe 20x, or 40 kW peak, 500 ms or so.
    And you can parallel them, and they are very cheap.
    Gang 3, done.

    Resistors for heating, and light bulbs, take huge power for tiny periods, until they heat up, and start to work near their rated range.
    They act as large shunts reducing shock loads.

    As most-all said, the pony motor is probably the right way to go for 3-phase startup at 100 hp output.

    One point:
    Does the 100 Hp load come up at startup, at 100% ?
    Mining conveyors, water lifts, might do this.

    But maybe 4/5 or 80% of normal use cases might not need 100% capacity at startup.
    A tiny 2 kW motor can start a 75 kW motor, at no-load.

    --
    Highly recommend double-insulated cables for the setup.
    Highly recommend a couple of dumb analog thermometers glued here and there.
    A measuring/peaking USB volt/amp tester left in place for a few weeks is a good idea.

    Power company demand charges / peak reserve pricing / whatever might easily be reduced by 200$ per month, once You understand your needs, use, and actual peaks.
    And a pony motor, or clever electronics, might also drop your demand charges by 200$/month, maybe more.
    Depending, details.

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    Quote Originally Posted by Bob-J-H View Post
    Look into using a pony start, I think it is your only chance of making it work.
    Looking below a lot of people recommend the pony motor but I am currently testing at a factory that builds and tests motors. They have very high voltage and amp rated electricity; well above 600v and 300amp.

    I could use a pony motor since there are a few lying around.

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    Quote Originally Posted by JST View Post
    So, what is your mains supply like, as far as current capacity? You will have a huge current surge on startup if you try to start that with a capacitor setup as if it were a 1 HP, the lights will dim all around you, and the powerco will hate that.

    It's exactly the sort of situation that a pony start is good for.

    I'd suggest not using balance capacitors, or at least not connecting them until it is running..... they can cause a load on the motor when starting, if there is any residual magnetism in the iron, as there may well be.
    The power at this location is 600v 300amp.

    I didn't actually try balancing it yet, I wired up the connections as referenced in the pdf and wanted to give it a start.

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    Quote Originally Posted by rons View Post
    Beware that in the video he is interchanging the words 'pony' and 'idler' for the same thing.

    The video also confirms that a potential relay is not the way to go.

    The wood sheet for a base is fine for prototyping. But not for the final product.

    His idler is 7.5Hp and his shear motor is 5Hp. The factor is 1.5. Your factor is 2 which has bought you a lot more to deal with.
    I did notice his errors and used it more for the practicalmachinist pdf that he linked as well as looking at the overall schematic. Can you expand on why a factor of 2 is more to deal with?

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    Quote Originally Posted by J_R_Thiele View Post
    I watched all of of the videos, and noted some errors.

    In the first video he calls and labels the idler motor as a "pony motor". They are not the same thing. A pony motor is a smaller motor used to spin the idler motor prior to power being provided to the idler. Using the pony motor means you do not need to use start capacitors to start the motor spinning. Using the pony motor to spin up the idler also reduces the inrush current.

    In the second video he used the ground line as a neutral line for both the contactor relay and the pilot light. This is a significant safety issue

    In the third video he uses a plywood inner panel. You do not want flammable materials inside your enclosure.
    I did notice those errors, I mainly use the first one as a reference, along with the pdf he links from this website.

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    Quote Originally Posted by newbea View Post
    Looking below a lot of people recommend the pony motor but I am currently testing at a factory that builds and tests motors. They have very high voltage and amp rated electricity; well above 600v and 300amp.

    I could use a pony motor since there are a few lying around.
    They - pony start motors - start making sense at as little as 10 and 15 HP idlers where source power is more constrained than what you seem to have.

    They make progressively MORE sense the larger you go. Not just for reduced shock load on the mains, either.

    Because they have very enduring service-stress and calendar LONGEVITY advantages over many and large capacitors.

    Capacitors age, by the calendar, and degrade, by the stress of use.

    The count of "starts" ages a motor more than straight-running hours do, but even so - decent pony might last fifty years or more?

    START caps? Not so long as "run" caps. And even that's closer to ten or twenty years for good "run" caps.

    This rig seems to be about conservative "worst case" Engineering, unattended reliability prioritized as has been said. Pony it. Or do something else altogether.
    Last edited by thermite; 11-30-2019 at 08:06 AM.

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    Quote Originally Posted by johnoder View Post
    My non RPC 15 starts snappily on 1000 mfd. 100 would - I suppose - take 6700

    Now you can imagine the inrush to suddenly fill those up

    That 15 by the way gives the pair of slow blow fifties no heart burn

    Now you can imagine you need 350 amp slow blows

    By the way does such seriousness also need huge size copper feeds? Like way beyond 4/0?
    The wire sizes inside the motor was about 18mm^2 so I went a few sizes thicker to what was readily available; 22mm^2
    It should be fine but I am not 100% sure.

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    Quote Originally Posted by TDegenhart View Post
    I would also suggest that the idler be connected through a resistor during startup to reset the magnetic field of the stator. There will be an amount of residual magnetism at startup that needs to be synchronized to the power line. Connecting the two out of phase will generate a pulse of current. The resistor provides a small amount of ac to demagnetize the core and allow smooth coupling. The resistor is switched out or bypassed after the transition to running. Well known problem for large transformers.

    Tom
    I will give this a shot and update the wiring once i do some calculation on sizing the resistor for this type of load.
    Thank you

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    Quote Originally Posted by Bill Simmons View Post
    I am still waiting to hear what the service panel is for running this as questioned by JST. At full load the single phase current will be running north of 200 amps. In a 200 amp panel, most bus bars are only rated for 100 amps per stab. I do not know what the stab rating is in a 400 amp panel or larger but breakers over 100 amps might be expensive or non existent for your panel. Your largest challenge may be getting adequate power to the RPC.

    And as stated by others, it is not practical to capacitor start a 100 hp idler. The fact that you have asked about start capacitors indicates to me that you have only begun to explore the construction of this RPC and have possibly got ahead of yourself looking at the details instead of the big picture.
    The power at this shop has 600v 300amp, they can start 1000hp motor with no problem. They build those them. My purpose is to test this size rotary converter to see would it be practical to build it here then start it without such huge power. In the end the idler might be the best bet since it should eliminate the inrush current. The 100HP idler motor will run a 50HP motor that starts unloaded.


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