How to get 41amps from RPC...

Discussion in 'General Electronics Chat' started by catman81056, May 3, 2012.

  1. catman81056

    catman81056 Thread Starter New Member

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    I bought a transformer to run my 440v lathe. The transformer is an Acme T-3-53311-1s, 15kva. Thats what the Acme tech suggested. Now I need to come up with 41 amps from my RPC to run the transformer. My current RPC is a 3hp, 3ph motor powered by 220 1ph. It only puts out about 8amps. I'm sure I can come up with a bigger motor for the RPC but I don't know what size to get. Is there a way to up the amps whitout changing RPC motor size.
    Any help here would be appreciated.
  2. #12

    #12 AAC Fanatic!

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    What's an RPC? and why is your 3hp, 3 ph motor powered by single phase 220V?

    Whatever the answers, you need to input enough power to your 15KVA transformer to get out enough power for the 440V lathe. Start with figuring out the power or KVA required and work backwards.

    Edit: It's your lucky day. There are obviously several people here that know about this sort of thing...and I don't. Hence my silence for the rest of this thread.
    Last edited: May 7, 2012
  3. strantor

    strantor Well-Known Member

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    If I remember from your previous post a few weeks ago, your RPC is 220 single phase in, 220 3 phase out, correct? Well if you need 41A on the 3 phase side, then you are going to need many more amps on the single phase side. your RPC is going to have lots of losses. I full a number out of my rear: 50A. lets say you need 50 on you single phase side; that's a 15hp single phase motor, do you have one? can you get one? do you have 50A service & breaker?
  4. crutschow

    crutschow AAC Fanatic!

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    How much power does the lathe require? You don't necessarily have to supply the whole 15kVa to the transformer.

    You can't up the amps with changing the RPC size.
  5. catman81056

    catman81056 Thread Starter New Member

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    The lathe is powered by a 440v 3ph, 5hp motor that pulls 8.8amps. I'm just going by what the tech said about having to have 41 amps to power the transformer.
  6. gerty

    gerty Well-Known Member

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    This is your load, 8.8amps@440v, that's what your transformer will have to supply. When you double your voltage (step up) with a transformer, you have to double your current on the primary, about 18 amps.
    The start up current should be about 150% of the run current, but that only lasts for a second or two.
    I've only worked on a couple of your type RPC and the rule I learned was the pony motor (RPC) must be larger than the load motor(lathe).
    So it looks like you're going to need a 10hp 220v single phase motor to make this RPC work. You might want to look at Static Phase Convertors, that might be more economical.
    catman81056 likes this.
  7. takao21203

    takao21203 Well-Known Member

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    well do you have a schematics for your setup??

    Just the fact the transformer has 14kW rating, does not mean it will always draw 14kW.

    You use a 5hp motor, thats about 700W/hp or something the like.
    Minus some conversion losses, you don't put in more kW than you get out for the motor, basically.

    Do you only have single phase AC available? US system?

    Draw up everything in a schematic (the local supply, the transformer, the motor, etc.)
  8. crutschow

    crutschow AAC Fanatic!

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    Do you think he really needs a RPC motor with double the lathe motor power? I would think perhaps 7-8 Hp would be sufficient to allow for efficiency loses. But that's just my thoughts on the matter, not having any experience with this.
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  9. gerty

    gerty Well-Known Member

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    Good catch, :eek: I had 8.8 in my head and I transposed that to horsepower!!
    It's 5 hp and 8.8 amps :rolleyes:

    And yes I believe 7-8 hp would suffice
    Last edited: May 5, 2012
  10. Brownout

    Brownout Well-Known Member

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    I'm not experienced with pahse conversion and such, but I'm gonna jump in and validate gerty/crutshchow on this. It's clear that you can't power a 5hp motor with a generator that makes only 3hp. Your convertor needs to be>5hp, and so I agree that 7-8HP shold be about right.
  11. shortbus

    shortbus Senior Member

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    I thought your lathe motor was 440 - 220 3PH? Why not skip the transformer and connect for 220 3PH ?
  12. catman81056

    catman81056 Thread Starter New Member

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    If I supply the 440 3ph I don't have to change anything, nothing else to convert on the machine. I picked up a 20 hp 3ph idler for a RPC. I'll just rig a pony motor to start the 20hp so the inital current draw will be lower.
  13. gerty

    gerty Well-Known Member

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    I'd really like to see a schematic of what you have in mind.
  14. shortbus

    shortbus Senior Member

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    The change is just a few wires in the "pecker head", versus the cost of the transformer and the power loses in it.:confused:
  15. gerty

    gerty Well-Known Member

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  16. shortbus

    shortbus Senior Member

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    gerty likes this.
  17. takao21203

    takao21203 Well-Known Member

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    OP - The lathe is powered by a 440v 3ph, 5hp motor that pulls 8.8amps

    So he wants to power this motor from 200 volts (using 15kW transformer, 200 --> 440 volts)? Is this correct?

    (440/200) * 8.8 = 20 Amps input (w/o losses).

    Also it's not clear how much the transformer will draw w/o load (since it is used in reverse).
  18. crutschow

    crutschow AAC Fanatic!

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    A normal power transformer draws only a small magnetizing current, either forward or reverse, as long as its voltage ratings are not exceeded.
  19. catman81056

    catman81056 Thread Starter New Member

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    I think it's alittle more than switching a few wires, I have all the stuff needed, 3ph 20hp idler, 3 hp 220 1ph pony motor(I'll use this to bring the idler up to speed before applying power), the 6ga wire, and the 60 amp breaker box. All thats left to do is run the wire and fab up the frame work to mount the idler and pony motors. Thats some thing I enjoy doing. Here's what the electric panel looks like. It's alittle over my head.
    [​IMG]
  20. takao21203

    takao21203 Well-Known Member

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    Well here we can see the control box.

    More information is required or advice can only be guesswork:

    -Type of motor, 3-phase, or eventually additional starter coil?
    -Any capacitors used?
    -How many phases on the grid? Single phase 200 volts? 3 phases?

    -Any switches or buttons on the control box?
    -Are the relays used to change between star/delta configuration?
    -Or are they used to switch an external motor/generator? Is this done manually eventually for startup?

    -Have you connected the transformer already?

    Here a page I found containing information about "idler motor" circuits (to convert single phase to tri-phase).
    http://www.metalwebnews.com/howto/ph-conv/ph-conv.html
    Last edited: May 6, 2012

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