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  • three phase question

    On one of my machines, if I run it long and hard, the reset on the machine will go. It is in a separate box and lookes like a thermal overload breaker, it will be warm when I reset it. I use a phase generator and notice that one leg (the one being generated) has little or no current threw it after starting. Could this be causing the other legs to pick up the slack and over heat the reset?

    Is there a fix?

    Thanks Bob

  • #2
    Take a current amp probe (clamp on) and check the legs to see if current is even.

    My 3ph lathe was kicking the breaker, not the overloads.

    David

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    • #3
      I used the clamp on, and there is only current on the two legs from the single phase. The leg that is being generated has no or little current. The current will spike when I start the machine only. It only seems to effect one machine.

      What can I do to get current going threw the generated leg?

      Bo

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      • #4
        what is the HP of the phase converter, and what values of run caps are you using? you may need to fine tune the run caps to smooth out your phases. either that or turn on another 3-phase machine to use as a second idler in the circuit.

        andy b.
        The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining

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        • #5
          I don't have any run caps. I use capacitors to start the generator, but that is all. The generator runs five different machines of diffeent HP, one at a time. I was told the run caps would not work due to the different hp of the other machines.

          What should I use for run caps and how do I figure out how much capacitance?

          Thanks Bob

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          • #6
            Are you using a static converter or a rotary phase converter? It almost sounds like you are using one of the cheap static converters that only supplies the third phase while the motor is starting. These units do not allow the equipment being powered to develop full HP rating (since it is really only running on 2 legs of electricity)

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            • #7
              Bob, you might check on the Practical Machinsts board. They have a section just for phase converters and elec. There are some pretty knowlegable guys that can probably help you out. James

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              • #8
                Actually,

                Single phase power is 180 degrees between power waves.

                3phase power is 120 degrees between waves.

                A phase convertor staggers the pulses with capacitors and inductors slowing the discharge. You still have two legs that are 180 apart. A sensitive machine can tell you all about the out of Rhythmm power.

                My milling machine was rattling to beat a snare drum before I added a inverter. Nothing was wrong with the mill head, it just rattled the speed changer.

                Adding in extra motors, or a rotary phase convertor will smooth the 3rd generated phase. But it will never be true 3 phase power. The phase convertor litature rates the hp output of a motor 100% when you install a idle motor. I do not firmly believe that thou. It is a claim.

                On your system:
                The 3rd generated leg should have been pulling power too, Thou I am not that familiar with the current draw during operation.

                My system:
                I did have to oversize the service going to my old 24" lathe, silly me calculated it for 3 phase power. It ran for about a minute then tripped breakers. I have not ran it since I put in the phase convertor-panel.(belt problems) I built it with a small single phase panel, insulated a terminal bar (was a ground bar) put in a extra ground, wired the 3rd leg from the convertor to the insulated bar. To engergize something you turn on the 2 pole breaker. I have to remember while the main is on, all equipment has ONE HOT LEG coming off the terminal bar. All my small 3 ph machines have a drum switch on them too.

                The more 3 phase motors running, the smoother they all are. They act like push-pull and help generate the 3rd leg.

                Sorry I am not more help, I do instrumentation and industrial electrical mainly thou and this was just recently aqquired knowledge. You never see anything like this in a industrial enviroment.

                The old way was a Reliance Add-a-phase panel as big as a truck hood for a small motor. I never heard about this homebuilt method of running motors till I joined here. None of the home-built methods I have saw are as good as real 3 phase power. Dupont has the strangest power arrangement I have ever saw, the center tap is grounded in all there 3 ph power. It is a transformer connection thou. You get in the middle or amp out the circuit it is powered and carrying amperage.

                David

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                • #9
                  The practacle machinist was very helpfull, thanks for the link. I do have a rotary converter, it is 15hp. I really only need 5hp but that was all I could find at the time. I have an old 3hp but it is too small. I think I'm going to try to find a 5hp and use the run caps as stated in the practicle machinist. I suppose I could always use the run caps on the 15hp, but it seems like a lot of overkill.

                  My lathe has the biggest hp motor at 5hp.

                  What do you guys think? 5hp or 15hp?

                  Bob

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                  • #10
                    You know it just occurred to me? I wonder if running another small machine at the same time might balance it out? of cource it might draw too much current with two machines on, and blow the braker. I'll have to give it a try.

                    Bob

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                    • #11
                      Well I just check to see if running another small motor would help but it did not. I'm going to have to go with run caps.

                      Bob

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                      • #12
                        YOu have of course opened the Peckerhead and checked motor taps?

                        Should be Wye connected with 456 together for 220 volts.. Normally.. paired off input T-leads. This is a normal 9 lead motor configuration.

                        Ohm motor through out while it is opened from the service. Make sure it is not a internal problem.

                        You have 3 phase overloads? Sized how?

                        Usually the Simple things.

                        David..

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                        • #13
                          Yup,
                          The motor is wired for three phase 220v. The overloads are actually the ones on the one machine, it is a 2 HP. It is a thermal overload and I beleave is poping due to an unbalanced load. When running without load, L1-6A,L2-6.5A,L3-2A. When loaded heavy L1 and L2 goes higher and heats up the overload.

                          I'm thinking a 5HP will be easier to get run caps for. I need 12 mf per HP for two of the legs.

                          Bob

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                          • #14
                            I don't want to sound like a jerk, but why would some one go thru all of this instead of just getting some VFD's?

                            Comment


                            • #15
                              bob,

                              i would definitely try the run caps i got a whole mess of different ones and the total was only about $60 (including shipping).

                              http://www.usamfg.net/capacitorsrunvolt.htm

                              andy b.
                              The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining

                              Comment

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