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  • phase converter???

    Neighbor gave me a large 3 phase motor to use as a rotary phase converter...my worry is that it is large .....like 30 HP large....I only intend on starting a 7.5 hp max motor...maybe 20 amps worth...but the 30 hp motor requires 80 amps...But since I am not really loading the 30 hp , just using it as an idler, will it work with a 20-30 amp breaker????any input would be appreciated...Shawn

  • #2
    I think you are bound to get a startup surge of much more than 30 amps.

    If you had a small starter motor for the 30-hp monster to get it up to speed before switching it on, you might stand a chance. You would need some sort of clutch so you could disengage the starter motor.

    I think your best bet is to get about a 10hp motor to use for the RPC.
    ----------
    Try to make a living, not a killing. -- Utah Phillips
    Don't believe everything you know. -- Bumper sticker
    Everybody is ignorant, only on different subjects. -- Will Rogers
    There are lots of people who mistake their imagination for their memory. - Josh Billings
    Law of Logical Argument - Anything is possible if you don't know what you are talking about.
    Don't own anything you have to feed or paint. - Hood River Blackie

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    • #3
      I agree plus the caps & other parts are going to cost more for the 30HP. I have a new 100HP motor I wanted built into a RPC & my guy that does it said the parts were about $1300.
      "Let me recommend the best medicine in the
      world: a long journey, at a mild season, through a pleasant
      country, in easy stages."
      ~ James Madison

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      • #4
        When I got a 15HP 3-phase motor for free for the same purpose, I just got simple and called American Rotary Power. From them I paid about $350 for a box that had all the starting capacitors needed, and it works fine on a 50AMP/220VAC/60hz circuit breaker. We've run all 3 of our 3-phase machines, under load, all at once just to check for any problems. Never even got the circuit breaker warm, and that was a 5HP lathe, 2HP grinder and 1HP Bridgeport.

        You really should have a minimum 15HP idler for a 7.5HP working motor, more if running multiple machines. You may not be all that far off with the 30 if you run more machines, or have any intent to get more. If not, see if someone might trade you for a 15HP motor.

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        • #5
          You will probably spend more to make that monster work than a VFD will cost.

          A 30 HP motor has a full load current of 80 A at 230V. Start surge without any special starters will be as high as 300A at 3 phase. A single phase start will likely be worse.

          In fact, most utilities will NOT want you to start a motor of that size by "dropping it on the line"..... you would be expected to at least do a Y-delta start on 3 phase, a choice you do not have. Reduced-voltage start could be done.

          I would expect that at idle, it could draw around 25A, but the power factor will be so low that there will be comparatively little power involved.

          Your breaker is not interested in power factor, so teh idle current is "real" to it. A 30A breaker might "hold" the idle current, but almost certainly will NOT allow you to start the motor without very special circuitry.
          1601

          Keep eye on ball.
          Hashim Khan

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          • #6
            I don't think you need anything more than a 10hp idler. That will be enough to start and run your 7.5hp motor, and I doubt you will come close to loading the motor to 7.5hp. 15hp would indeed be better, but don't think you need it, or realistically have the power to run it. (30hp would be better still, but unless you beef up you incoming power line a LOT I think that is impractical.)

            If you add more machines and run them at the same time, I think their motors will just add to the RPC effect. The idler doesn't need to get bigger. I doubt you could run multiple machines, anyway. You won't have the amps to run more than one motor at a time.

            J Tiers mentions VFDs. 7.5hp VFDs aren't cheap. Because you have single-phase power, and nobody seems to make VFDs designed for single-phase power larger than about 1hp, you would need to get a 15hp VFD designed for 3-phase input and de-rate it 50% for single-phase input. See for example this one: http://dealerselectric.com/item.asp?...ID=166&PID=647 Just what you need, but it's $836.00.
            Last edited by SGW; 03-19-2012, 10:16 AM.
            ----------
            Try to make a living, not a killing. -- Utah Phillips
            Don't believe everything you know. -- Bumper sticker
            Everybody is ignorant, only on different subjects. -- Will Rogers
            There are lots of people who mistake their imagination for their memory. - Josh Billings
            Law of Logical Argument - Anything is possible if you don't know what you are talking about.
            Don't own anything you have to feed or paint. - Hood River Blackie

            Comment


            • #7
              VFD is out because I intend to run several items off this one conveter....the lathe being the largest at 7.5 hp , followed by the air compressor at 5hp, and the BP at 1.5 hp, and possibly a radial alarm saw...other part that sucks right now is my service is a measly 60 A....just trying to get by until I can up grade my service....Thanks, Shawn

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              • #8
                Sounds to me that you need a donkey motor to bring your rpc motor up to speed before throwing the switch on it.
                "Patriotism is the last refuge of a scoundrel"

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                • #9
                  pony motor was in the plan...just wasn't sure about how many amps to feed it...shawn

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                  • #10
                    Originally posted by SGW
                    I don't think you need anything more than a 10hp idler. That will be enough to start and run your 7.5hp motor, and .
                    that's my set up, 10hp RPC. biggest motor is 7.5, next is 5 then down from there. No way you need bigger than 10. Its not a problem either running more than 10hp worth of motors on the 10rpc, each additional one you start balances it more.

                    My entire shop has a 60 amp service and I run all kinds of machines at once, rpc is on a 30 amp breaker and works flawlessly.

                    10 hp 3 p motors aren't that hard or expensive to come by used
                    .

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                    • #11
                      Originally posted by Mcgyver
                      Its not a problem either running more than 10hp worth of motors on the 10rpc, each additional one you start balances it more.
                      It's not a matter of balancing, but rather max current your idler's winding (and power circuit) can withstand.

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                      • #12
                        Can you run the 30 hp motor on half the windings and use it as a 15hp RPC? Then if you want, after spools it up on half the windings, then engage the other windings at full speed for full amps capacity if needed. Just seems like an easy way to limit inrush and idle current.
                        Not 100% sure on this, hope others will comment.
                        --Doozer
                        DZER

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                        • #13
                          Originally posted by Doozer
                          Can you run the 30 hp motor on half the windings and use it as a 15hp RPC? Then if you want, after spools it up on half the windings, then engage the other windings at full speed for full amps capacity if needed. Just seems like an easy way to limit inrush and idle current.
                          Not 100% sure on this, hope others will comment.
                          --Doozer
                          You might run it wired 460 on 230..... that will lower the current.

                          But that also drastically increases the impedance of the generated leg.... ALL the problems with respect to the generated leg, balance, etc, etc arise from the series impedance of the idler in series with that leg, which is impedance NOT present in the other legs.

                          So going HIGHER on impedance is not a good way to go.
                          1601

                          Keep eye on ball.
                          Hashim Khan

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