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Rotary phase converter

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  • Rotary phase converter

    I have just come into ownership of a 3 phase 3HP electric motor. It weighs at least 75 lbs. Grunt.

    I am currently running my 3 phase 1.5HP Strands drill press using a static phase converter which uses a start capacitor and a run capacitor. It works ok but I now have the opportunity to easily make it into a rotary phase converter. It should run the drill press smoother.

    The question is: Is their any advantage to making a flywheel for the shaft of the 3hp idler motor?
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  • #2
    I would say the roter is heavy enough by itself. If another machine is running on the 3ph during the start of the second motor I don't feel much, if anything, is to be gained by having more inertial energy.


    • #3

      Unless the drill press has some very high speeds that create a huge inertia load to overcome, I would think that the double hp idler motor would work just fine. I’d try it first without a flywheel and see how it goes. If you make your idler a “self starterâ€‌ the flywheel is only going to make it harder to start and in any case, more expensive to run.

      To be honest with you, I really don’t see how a flywheel would help that much. So if you decide to go ahead with it, I’d sure be interested in the results.


      • #4
        At one time I ran a similar setup but started the generator with a pull rope.
        Give it a tug and turn on the power.

        A flywheel won't help start under that setup.

        I have been told the generating motor should be three times the capacity the machine.

        I don't know that as a fact.
        It is easy to test.

        The electritian who built mine mounted a washing machine motor to start the generator and keep speed up under load.

        It has a five second delay Grainger timer to that hits the 240 volt relay after the starter motor.


        [This message has been edited by kap pullen (edited 01-04-2005).]


        • #5
          I have done it on a 3hp converter myself.I found the only difference it made was in starting 3600 rpm motors like those on radial arm saws and grinders when the hp of the idler is closely matched to the hp of the motor being started.Other than that no real advantage.
          My own personal converters I use a centifugal switch and a start cap to start the idler.This way if the idler ever does see a heavy starting load and loses some shaft speed the centifugal switch catches it and drops the start cap back in momentarily to get the converter over the hump.Using this setup I can start a 5hp motor off a 3hp idler.
          One other note,I don't use any run caps in the system,they are not needed in most applications and only cause heating in the idler motor.

          BTW,Grainger has the centifugal switches and contact boards for cheap,they are very simple and reliable.Using them the start cap is in circuit the EXACT amount of time needed to start the motor.I have used the potential relays,but it sucks when one blows on the weekend when the hardware store isn't open.I have one converter I built a friend that has been running 40+hours a week for years and has never had a hiccup using the centi.switch.

          [This message has been edited by wierdscience (edited 01-04-2005).]
          I just need one more tool,just one!


          • #6
            No reason for a flywheel as it won't be needed and will cost more to operate. I did a ton of research prior to biulding my rotary and taught myself a bunch about them.
            Mine is 5 hp with heavy windings and balanced with 9 caps in total. It starts instantly with the flip of a toggle switch with no flywheel. Once its up to speed I can start either of the two machines and then bring the remaining machine up to speed. Thats a total of 12.5 hp runnning from it. I see no need to double the hp of the converter.
            Make sure your wiring can support the loads.
            Stay Safe


            • #7
              My static converter is my own design. It uses a fixed time delay to switch in the start cap for about 1 second after which the start relay drops out and leaves only the much smaller run cap in circuit. The drill press is a gearhead unit with a maximum rpm of 1200 or so. I do on occasion use it under heavy load. It is the only machine that will be running from the RPC at this time and I don't see any possibility of another in the near future.

              I don't understand why using a flywheel would increase operating cost. Once the idler is up to speed there shouldn't be any difference except when a load is thrown on the system.

              I suspect that a flywheel probably wouldn't give much advantage in this case. The motor is a super heavy unit designed for continous duty industrial use. I think it weighs closer to 100 lbs than 75.

              Wiring is no problem in my garage. I have 240 volt 50 amp service on a short 20 foot run of 8x3 guage wire. The only other unit on that circuit is my welder and I can't weld and drill at the same time.
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              • #8

                I see no advantage to installing a flywheel, but I wouldn't hesitate installing a nice buffing wheel



                • #9
                  Not a bad idea. But then I will have to make a stand for it instead of putting it on the floor. Just what I need, another project...
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                  • #10

                    Here is a nice one on Ebay (#3864162874):

                    10"od x 1.25"id x 3/4" face cotton buffing wheel

                    Untreated ventilated bias buff is Excellant for polishing aluminun brass and stainless steel.



                    • #11
                      I don't do e-bay. Besides, I can probably get one cheap from my wife. Good idea though.
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                      • #12

                        That's a $5 wheel on Ebay.

                        Don't waste your time making a table... Buy one for $19:

                        This one is perfect: