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  • #16
    Originally posted by Norman Bain View Post
    Am a total novice to this ... though do have 2 commercial RPCs.

    A question; can the discussion be extended to say that a static converter could be used to "spin up" a 3 phase motor; then (say using make-break-make switch or contactor) turn off the static converter and (while the idler is still spinning) switch the incoming mains source power directly into the idler motor.
    .....
    Yes. That is essentially what an RPC is, given a few more parts that are usually included to get the "generated leg" voltage up to match the incoming voltage.

    The static converter, and the automatic start circuit of an RPC, both automatically disconnect themselves when the motor has started and the generated leg voltage has come up to reasonably close to the input volts. So nothing needs to be added, in general, to disconnect the start circuit, that function is normally already included.
    Last edited by J Tiers; 11-20-2017, 04:01 PM.
    CNC machines only go through the motions

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    • #17
      Originally posted by lakeside53 View Post
      Take care with their badly worded technical description that it only provides 2/3 of the power. The static converter does not provide any power - most "power" is what YOU draw via the loading of the motor. The motor will be quite happy to draw increasing power until it overheats and/or stalls. It is up to the user to limit FLA current to less than that on the nameplate...
      Am I reading this right. JT says that the static converter switches out once the motor is up to voltage; hence (if the OPs suggested device does work that way) the input voltage is then directly powering 2 x legs of the 3-phase motor.

      Are you saying that because the load on the motor (the drill bit, milling cutter or whatever) has to be managed using just 2 x legs AND that, as a result, the use (expectations) of the motor can easily be exceeded then the user needs to be careful.

      Does adding another (I assume larger) idler motor solve this problem? That is, when do we get to "real" (or real enough) 3-phase supply that the motor on the mill/lathe etc is able to be loaded at close to nameplate useage?

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      • #18
        Originally posted by Norman Bain View Post
        Am I reading this right. JT says that the static converter switches out once the motor is up to voltage; hence (if the OPs suggested device does work that way) the input voltage is then directly powering 2 x legs of the 3-phase motor.

        Are you saying that because the load on the motor (the drill bit, milling cutter or whatever) has to be managed using just 2 x legs AND that, as a result, the use (expectations) of the motor can easily be exceeded then the user needs to be careful.

        Does adding another (I assume larger) idler motor solve this problem? That is, when do we get to "real" (or real enough) 3-phase supply that the motor on the mill/lathe etc is able to be loaded at close to nameplate useage?
        If you power a machine's 3 phase motor with single phase, the power you can get from that motor is limited by the current the motor is rated for. Motors are limited by two things.... breakdown torque, and heating. Breakdown torque is the maximum torque the motor can produce without stalling. Heating is caused by current flow.

        The breakdown torque is reduced by running single phase. But the motor current is way beyond the allowable continuous current by then.

        The power output of the motor comes from input current. With only two wires providing input current, the power is limited to 1/1.73 = 0.578 of rated power, unless the current increases above rated current. If the current DOES go higher, then the windings are in danger of overheating. Thus the limit.

        In an RPC idler, that is not an issue, since the idler is only required to produce an output equivalent to 1/3 of the power of the load, since it provides only the power for one wire of the three. That , plus losses, is going to require a current well under 0.578 of the idler rating.
        CNC machines only go through the motions

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        • #19
          The mill I have running(starting) on a static converter has NEVER over heated, failed to start or stalled due to lack of power, but neither has any machine I run off my RPC.
          The only reason I've never wired the one mill into the RPC is because it does fine on the static converter.
          Another thing about a RPC is that each motor powered up after the first adds to the HP rating.

          THNX RICH
          People say I'm getting crankier as I get older. That's not it. I just find I enjoy annoying people a lot more now. Especially younger people!!!

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          • #20
            Originally posted by v860rich View Post
            The mill I have running(starting) on a static converter has NEVER over heated, failed to start or stalled due to lack of power, but neither has any machine I run off my RPC.
            ......

            THNX RICH
            Yep, folks can work for years with just a static converter. That's why they are still sold. Few hobby people, (unless they run production) will ever run a machine to its real limits. Nothing wrong with that it it works for you.

            It IS still single phase, and with lathes in particular, that can be a problem with chatter etc. I know that when I switched over to an RPC and 3 phase motor, I had an instant improvement in a lot of problems that I had been having; chatter. overall power, flat belts slipping, etc. All much better on 3 phase.
            CNC machines only go through the motions

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            • #21
              Originally posted by Doozer View Post
              Why not? All mine are.

              -D
              Because its safer and the recommendation of commercial RPC makers. I have to ask why would you want to put more load on the generated leg?
              Mike
              Central Ohio, USA

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              • #22
                Because it is safer??? How?
                I connect my contactor coils and control transformers to the generated leg of the RPC
                because that way, if the RPC gets shut off, all my machines shut off also, and not
                just get only single phase. This seems safer to me. When I leave my shop, all I do
                is turn off my RPC and all my machine contactors drop out. No potential for them to
                start up. It saves me from pushing the "Stop" buttons on all my machines that are
                powered up, but not spinning. No issues ever.

                -Doozer
                DZER

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                • #23
                  I guess if you wire contactors to the generated leg,
                  when the machine starts and the generated leg drops
                  down from the current draw, the contactors could drop
                  out, but I think that would mean your RPC is too small
                  anyhow.

                  -D
                  DZER

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                  • #24
                    Originally posted by Doozer View Post
                    Because it is safer??? How?
                    I connect my contactor coils and control transformers to the generated leg of the RPC
                    because that way, if the RPC gets shut off, all my machines shut off also, and not
                    just get only single phase. This seems safer to me. When I leave my shop, all I do
                    is turn off my RPC and all my machine contactors drop out. No potential for them to
                    start up. It saves me from pushing the "Stop" buttons on all my machines that are
                    powered up, but not spinning. No issues ever.

                    -Doozer
                    Mostly if you have heavy loads, the 3rd leg may get pulled down and cause dropouts. I "get" the safety aspect of what you are saying, but that approach often causes "stuttering" relays, and other problems. So it is not recommended.

                    You just have the RPC wired wrong. Never consider the RPC as a separate source. Make it so that shutting it off is done by shutting off all the 3 phase.

                    That is how mine is set up. RPC is auto-start, when the input power is applied to the RPC by the contactor, it starts. That contactor also controls the 2 legs going to the 3 phase loads. To tirn off the RPC that contactor is opened, stopping the RPC, and that kills all 3 phase circuits as well.

                    If power goes down, the whole 3 phase drops out when the RPC opens. The individual machines therefore also drop out.
                    CNC machines only go through the motions

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                    • #25
                      No affiliation, but just a personal good experience...
                      I bought a phase converter Control Panel from
                      northamericaphaseconverters.com
                      and it is a great unit at a great price. I don't think
                      I could scrounge the components to home build one
                      for the price they charge.

                      Anyhow I bought their CP-7 unit and I use it with a
                      6hp Leeson motor, 1200 rpm and delta connected
                      at 230 volts. This is a massive motor at 220 pounds
                      and it is TEFC cast iron. I believe because it is connected
                      delta, that it makes a smoother running RPC motor,
                      without all the buzzing that most motors make when
                      pressed into RPC duty. Maybe I am wrong about the
                      delta being better for RPC use, someone smarter please
                      comment.

                      I did have to unhook one starting capacitor
                      from the Control Panel, as the starter panel is rated to
                      start a 7.5hp motor, and my motor is 6hp. The symptom
                      was the motor would start most of the time, but only
                      humm and cut out sometimes. Since unplugging one
                      cap, it has always started perfectly.

                      One nice thing about
                      the 1200 rpm motor, is the huge rotor mass. It stores
                      potential energy (flywheel) and allows the RPC to not
                      dog down so easily when large or long starting loads
                      are applied.

                      This setup will start and run up to a 5hp motor on a
                      machine tool in my shop, and it is more quiet than
                      any setup I have ever seen. I just lucked on to that
                      big Leeson motor. An ebay purchase, motor was brand
                      new and I won the bid at $10. When I went to pick
                      it up, the guy actually had 2 identical motors.
                      Said I could have both for $20! I think I may use the
                      other motor to convert my Hendey Toolmaker's lathe
                      over from DC to a VFD setup.

                      --Doozer
                      DZER

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                      • #26
                        Ah Jerry, I do see what you are saying.
                        Yes my wiring scheme is a bit unconventional.
                        Your way is safer.

                        -Doozer
                        DZER

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                        • #27
                          Originally posted by J Tiers View Post
                          It IS still single phase, and with lathes in particular, that can be a problem with chatter etc. I know that when I switched over to an RPC and 3 phase motor, I had an instant improvement in a lot of problems that I had been having; chatter. overall power, flat belts slipping, etc. All much better on 3 phase.
                          My original question arose from the realization that a static phase converter will produce less than 2/3 of the rated hp of the motor. This statement provides another reason to use a mill (or other 3 phase motor/device) as an idler RPC. In my case, the mill is 2 hp. The lathe is a 1963 SB Heavy 10 with a 1 hp motor. Should work.
                          It's all mind over matter.
                          If you don't mind, it don't matter.

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                          • #28
                            It can EITHER be an idler, OR produce power for a machine, not both at once. Since there is one of you in the shop, that works.
                            CNC machines only go through the motions

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