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  • Phase Converter Question

    For you 3 Phase gurus....

    If I'm using a static converter, and have a motor (say a mill, for example), idling on the circuit, does the idling motor function as a rotary converter, generating full 3 phase for another motor (a lathe) on the same circuit?

    my mind takes some interesting detours...
    It's all mind over matter.
    If you don't mind, it don't matter.

  • #2
    The short answer is yes but the idler motor needs to be at least the same HP as the additional motor.

    Comment


    • #3
      If you have an idler motor making the 3rd leg you have a Rotary Phase Converter, not a Static.

      Comment


      • #4
        Originally posted by MrWhoopee View Post
        For you 3 Phase gurus....

        If I'm using a static converter, and have a motor (say a mill, for example), idling on the circuit, does the idling motor function as a rotary converter, generating full 3 phase for another motor (a lathe) on the same circuit?

        my mind takes some interesting detours...
        You are absolutely right on. Good thinking!

        Yes, it will generate a 3rd phase. All that a standard rotary converter really IS is a 3 phase motor with a start circuit that amounts to a static converter. Some of the companies that make static cinverters have application notes to do just that, showing how to wire it up, etc.

        it IS true that as a "rule of thumb", the "idler" should be about 1.5x the HP of the largest motor you want to run. For hard-to-start loads, like some import lathes on high speed range, you may need to have an idler of 2 or even 3x the size of the lathe motor. In some cases, when you have a load like that, turning on another 3 phase motor may allow it to be an auxiliary idler, making the start easier.

        And, when using an ordinary motor as a converter, you may want to add "balance" capacitors to raise the voltage to equal the input voltage (cancels out the inductance of the idler motor). A purpose-made RPC may have the third winding made so as to already have the higher voltage, but you do not get that choice when using a random motor.
        CNC machines only go through the motions.

        Ideas expressed may be mine, or from anyone else in the universe.
        Not responsible for clerical errors. Or those made by lay people either.
        Number formats and units may be chosen at random depending on what day it is.
        I reserve the right to use a number system with any integer base without prior notice.
        Generalizations are understood to be "often" true, but not true in every case.

        Comment


        • #5
          It's an interesting question but lacks any hint of what you might want to do with the circuit and doesn't tell us what your static converter is or it's configuration.
          You might as well ask us if it will rain tomorrow, as of course it will, somewhere! ;-)
          If you benefit from the Dunning-Kruger Effect you may not even know it ;-)

          Comment


          • #6
            To add to this... The static converter functions only as a "kick" using start capacitors to get the motor running, once running they cut out. Once you have a motor running yes it will generate the 3rd leg for anything else connected. However commercial rotary phase converters also employ run capacitors to try to balance the wild leg. When checking voltage L1-L2 you will be right at your local power co's feed voltage (say 240V) but if you check L1-L3 and L2-L3 it will be all over the place which is harder on equipment. In addition to the balance issues that may not concern you the wiring throughout this solution needs to sized correctly. Also be sure that all motor starters/contractors, control transformers etc are NOT connected to the 3rd leg.
            Mike
            Central Ohio, USA

            Comment


            • #7
              Originally posted by Ohio Mike View Post
              ,,,,, However commercial rotary phase converters also employ run capacitors to try to balance the wild leg. When checking voltage L1-L2 you will be right at your local power co's feed voltage (say 240V) but if you check L1-L3 and L2-L3 it will be all over the place which is harder on equipment. .....
              Yes, and no. Input voltage will be at powerco voltage, if the wiring is adequate. Generally, output voltage will be lower on the 3rd, or "generated" leg, due to how a motor actually works.

              A motor inherently will generate a correctly phased 3rd wire output. It cannot help that, it is set mechanically by how the coils are located. Balancing is not needed for correcting the motor phasing.

              The "balancing" is intended to bring up voltage to counteract the fact that a random motor will nearly always generate a lower voltage unless something else affects it. The generated voltage is just the back EMF of the motor, and that is always LESS THAN the line voltage, so that current can flow.

              A commercial RPC COULD have that winding changed to boost the output to the incoming line voltage, and I understand some do. And, a large motor used as an idler may work great with no capacitors at all. Many people actually rope start an oversized motor (or start with a smaller "pony motor") and use it alone as the RPC, with no extra parts.

              Most folks using random motors DO use a balance capacitor setup to raise the generated leg voltage up to a good level. Some even correct the power factor on the input with capacitors. But you do not actually HAVE TO do either one.

              Wiring, disconnects, and motor size are all related issues that have to do with what the loads are in specific cases. They don't affect the principles of operation, but they are important when you actually install a unit.
              Last edited by J Tiers; 11-19-2017, 09:30 PM.
              CNC machines only go through the motions.

              Ideas expressed may be mine, or from anyone else in the universe.
              Not responsible for clerical errors. Or those made by lay people either.
              Number formats and units may be chosen at random depending on what day it is.
              I reserve the right to use a number system with any integer base without prior notice.
              Generalizations are understood to be "often" true, but not true in every case.

              Comment


              • #8
                Originally posted by MrWhoopee View Post
                For you 3 Phase gurus....

                If I'm using a static converter, and have a motor (say a mill, for example), idling on the circuit, does the idling motor function as a rotary converter, generating full 3 phase for another motor (a lathe) on the same circuit?

                my mind takes some interesting detours...
                What you are describing is, almost, exactly what I use as an RPC the only difference is I use a 5 HP 3 PH motor on an old lathe instead of on a mill.
                I use mine to power a 16" SB lathe, a Bridgeport and a 12" disc sander.
                Mine has been in use for at least 20 years. I checked the outputs of each leg when it first went into use and they were all so close I left well enough alone.
                I use a Phase A Matic static converter to start the 5 horse motor.
                Along those same lines I also have a BP clone that uses a static converter only, as I have never wired it into the RPC, and it has never been a problem, either. This static converter has been in use on this mill since the 80's, with not so much as a hiccup!!!!

                THANX 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!!!

                Comment


                • #9
                  I actually saw in action what I'm describing. A machine shop with 18 three phase machine tools. No CNC, just older machines with no fancy motor starters that might sense a "wild" leg with low or high voltage.

                  They had a single 1/2hp three phase motor hooked to a single phase 230V circuit. First thing in the morning they kick started the pulley on that motor. Three phase motors run on single phase if you spin them up close to running rpm first. That small 1/2hp motor was generating the third leg. They may have had some motors, like a continuously running fan, that acted as idlers.

                  Anyway they were able to run any of their eighteen machines off that setup. Probably because of load they couldn't run all eighteen motors at once.

                  This was a backwoods shop obviously not wired to code. I was there on a weekend to look at a machine they were selling. The seller started the operation as we walked in the door to demonstrate the three phase machine.

                  Comment


                  • #10
                    Years ago a friend ran the several three phase machines in his shop using single phase powering a three phase idler motor. In the morning he would wrap a rope around a starting pulley on the idler motor and rope start it. With the idler motor runnning, he could then run any one or all of his three phase equipment.

                    Comment


                    • #11
                      Originally posted by Ohio Mike View Post
                      .... Also be sure that all motor starters/contractors, control transformers etc are NOT connected to the 3rd leg.

                      Why not? All mine are.

                      -D
                      DZER

                      Comment


                      • #12
                        Lots of good info here, exactly what I wanted to know. I have several friends running home businesses who use RPCs that a good friend makes from 3 phase motors (one actually rope starts his). I just ordered one of these units:

                        https://www.ebay.com/itm/3-4-5-Hp-HD...72.m2749.l2649
                        It's all mind over matter.
                        If you don't mind, it don't matter.

                        Comment


                        • #13
                          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. If you have a machine with motor overloads you can take care of this, but small motors (and even many mills) do not have these. A larger ( and not commonly well understood problem) is phase imbalance. By definition that type of static converter has huge imbalance. NEMA recommends a large derating for even 15% imbalance.

                          The only saving grace is that non-cnc HSM usage has relatively low duty cycles, but keep an eye on motor temperatures, and current if you can..

                          Comment


                          • #14
                            Yep.

                            The real number for a "single phased" 3 phase motor, which is what you get with a "static converter", is about 58% of nominal power It is based on current rating, not power. About the same as with an open-delta transformer setup, and for much the same reasons.

                            A static converter, so called, is a "motor starting device". A few of them DO have a run capacitor to the 3rd wire, that provides a little 3 phase effect, and will help, but even then you should not expect more than the 2/3 figure that is normally suggested.
                            CNC machines only go through the motions.

                            Ideas expressed may be mine, or from anyone else in the universe.
                            Not responsible for clerical errors. Or those made by lay people either.
                            Number formats and units may be chosen at random depending on what day it is.
                            I reserve the right to use a number system with any integer base without prior notice.
                            Generalizations are understood to be "often" true, but not true in every case.

                            Comment


                            • #15
                              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.

                              [EDIT] I only suggest the switching out of the static converter to "avoid" running larger load than necessary thru it as there seems to be some suggestion that that device has a load limit in itself. Feel free to correct me if my understanding is wrong on this.
                              Last edited by Norman Bain; 11-20-2017, 04:49 PM.

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