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Home built rotary phase convertor

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  • #31
    Originally posted by dvo
    Which potential relay should I look for or what spec do I need to look at? Will universal potential relay work?

    Steveco 90-66 is good for 240 VAC usage. They pick up at 208-239 V and drop out at 90, normally closed. I used one and it's proven to be completely reliable.

    They are now built by White-Rodgers. They are pricey, however.

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    • #32
      For what it is worth, the ARCO brand RPC which I have does not use a relay of any sort.

      It has a double-conductor rotor, and a single starting/compensating capacitor.

      There are thinner (higher resistance) copper conductors in shallow slots on the rotor, which respond to the high slip frequency at start, and whip it up to speed fast.

      And there are thicker regular aluminum conductors buried deeper, that come into play when the slip frequency is low (rotor is near synchronous speed).

      This is a standard technique in the induction motor world for getting good starting as a somewhat separate design consideration from the run characteristics.
      1601

      Keep eye on ball.
      Hashim Khan

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      • #33
        I just bought (locally) what they called a 'contactor' to replace the aging one in our RPC. Call it a relay, call it a potential relay, a contactor, whatever- it's got a coil that activates a pair of contacts. That, plus 12 ft of high temperature wire and a dozen spade connectors, plus taxes- less than $20.

        Replaced burnt wires and connectors, replaced the 'device', all is well. This is a 20 hp RPC.

        The burnt wires- the guy who built it used 14 ga wire to wire up all the caps, but he daisy-chained them. The result is that the first wire in the chain carries all the current shuttled through the capacitors. Each bank was wired like that. I think we were lucky that it didn't go on fire. Better would have been to make a local connector block which would fan out to all capacitors in each bank, and run a heavy wire from each block to the appropriate winding.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #34
          Originally posted by gnm109
          VFD's are an alternate method. I used an RPC because I have a variable speed mill and I can fix any part of the RPC.


          I know that VFD's are popiular, though.
          What does having a variable speed mill have to do with the type of converter?? RPC or VFD??

          JL.............

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          • #35
            Originally posted by JoeLee
            What does having a variable speed mill have to do with the type of converter?? RPC or VFD??

            JL.............

            The VFD will control speed through variable frequency fed into the 3 phase induction motor.

            The RPC provides 3 phase power to my variable speed mill which has a mechanical range of 60 to 4,000 rpm that is built-in.

            So as long as I can change speeds im the machine itself, I can use my RPC to provide power.

            Some people will like the convenience of a VFD even though they have a variable speed mill since changing speeds is very easy - just turn a potentiometer.

            Others, like myself, like to be able to tinker and build their own phase converter, although changing speeds on my mill is not all that difficult, just turn a knob. Sometimes you have to change ranges to the back gear but that's not difficult.

            I like the fact that with an RPC my motor is running at it's ordinary (near synchronous) speed all the time and is getting full cooling through its fan at all times. It must work since the mill is a 1987 model and when I had the motor off two years ago to clean and inspect it, it looked like new.

            That's good because a new one would cost $900.

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            • #36
              gnm109, reason I asked is I have always run my BP mill off a static converter that I built...... it was one of those little kit deals. It's run the mill fine for years but the motor is noisy. I replaced the bearings and put new bushings on the sheaves etc. and it still just doesn't run as smooth and quiet as others I've seen, it's not the quill or anything in between. I've been thinking it's an unbalanced voltage related issue. I have a VFD but never tried running the mill off of it. I might just try it today.

              JL......................

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              • #37
                Originally posted by JoeLee
                gnm109, reason I asked is I have always run my BP mill off a static converter that I built...... it was one of those little kit deals. It's run the mill fine for years but the motor is noisy. I replaced the bearings and put new bushings on the sheaves etc. and it still just doesn't run as smooth and quiet as others I've seen, it's not the quill or anything in between. I've been thinking it's an unbalanced voltage related issue. I have a VFD but never tried running the mill off of it. I might just try it today.

                JL......................

                If you already have a VFD then it would be an excellent idea to install it. I understand that they are an excellent concept. I'd like to try one myself but I don't want to spend the money on one right now. I've been spending my extra cash on tooling and so on.

                If you have an oscilloscope or access to one, you could look at the output of the static converter. It's possible that the power is not all that clean.

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                • #38
                  Static converters are missing one phase completely. The rotor/stator are designed for all three. That makes the motor vibrate or be noisy in exactly the same way as an rpc, which is why rpc idler motors are generally mounted on rubber feet - not bolted down hard. Bearings in motors on static converters have a lower life.

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                  • #39
                    An alternative design...

                    I read through this thread and did not see where anyone mentioned the alternative design offered by Eurotech - they use SCRs to switch capacitors in and out on the fly depending on load conditions. The less expensive model uses three SCRs and offers three levels of 'balancing', while a more expensive model has four SCRs and offers 7 levels - in either design one SCR is used for starting.

                    Without explanation of why, they specify the motor should be delta wound, rather than Y wound. There is a seller on ebay, "single2threephase", that sells the special controller and/or plans for building converters using their contoller.

                    I built one of their designs using a 15HP delta wound motor and their four SCR controller with very good results.

                    They claim the balance is good enough to run three phase VFDs, but I did not test that claim.

                    Below is a pic of the converter I built - the motor came on the wooden structure shown and I used that to hold all of the parts. The large coils at the top are specified by Eurotech and are there to limit the surge currects when the SCRs switch under load.



                    This pic shows their controller:



                    I sized the unit to fit under the workbench shown.
                    TexasTurnado

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                    • #40
                      You have done some beautiful work there. I can see the benefit of switching capacitors in and out to compensate for varying loads. The other side of that is the increased complexity.

                      In my case, I'm only running one 3 hp motor and that's probably the extent of my 3 phase work. I've got a very nice setup that's fairly simple and has been running nicely for two years now.

                      I do think that a good quality motor makes a difference in how much balancing is needed but I have no scientific proof of that. I was lucky and scored a NOS 5 hp Boston gear motor on eBay. I guess no one else wanted it because it was a flange mount. I got it cheap and built my own mount.

                      Nice job!
                      Last edited by gnm109; 10-25-2011, 03:52 PM.

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