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PM motor with field winding

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  • #16
    I think the question is not unlike the control circuitry in conventional welders. By another name, they are known as "saturable core reactors" and "magnetic amplifiers" both of which are true. In the older shop welders, they have an extra small winding down near the core which passes DC from the control circuitry. This sets up magnetism in the transformer core which varies with the desired output. When the core is magnetically saturated, the inductance goes to zip and you get no current at the output. Turn the saturation down and the output current goes up. And of course it is possible to vary the saturation level at audio frequencies -- there are some high-end lab amps based on this idea.
    25 miles north of Buffalo NY, USA

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    • #17
      Alomg that line, while it would be, as mentioned, hard to do much with a magnet in the path, since the magnet is a huge air gap and makes establishing a strong field difficult, there may be anther way.

      A "magnetic shunt", that would take some of the magnetic flux away from the "business end"..... by saturating that , or not, or only partially saturating it, the shunt could be turned from a shunt to effectively an air gap. A few practical considerations would apply.......
      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.

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      • #18
        N-C-F, I think you have that backwards. The DC current does saturate the core and reduce the inductance, but that allows MORE current to flow.

        https://en.wikipedia.org/wiki/Saturable_reactor

        http://electriciantraining.tpub.com/...ircuit-136.htm

        http://rfcafe.com/references/radio-n...march-1952.htm

        https://trace.tennessee.edu/cgi/view...t=utk_graddiss

        https://patents.google.com/patent/US2798571A/en

        This is pretty much obsolete technology. And I don't think it is very useful for high quality audio lab amps. It is difficult to change the DC current quickly in the saturable core winding because of the inductance. I once tried to use a saturable reactor to adjust the current on an AC load which had a variable inductance in a time frame of 50 mSec to 2 seconds, and it was all but impossible. It would have required several thousand volts at 10 amps or more, which would have meant a 10-100 kW amplifier. This was a 1970s project and high current PWM and VFDs were uncommon. The problem was solved by adding resistance in the primary circuit which reduced the current variation to acceptable levels.
        http://pauleschoen.com/pix/PM08_P76_P54.png
        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030

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        • #19
          There is a patent on a variable gap transformer.

          This one references a variable gap welding transformer.

          http://pauleschoen.com/pix/PM08_P76_P54.png
          Paul , P S Technology, Inc. and MrTibbs
          USA Maryland 21030

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          • #20
            Originally posted by PStechPaul View Post
            N-C-F, I think you have that backwards. The DC current does saturate the core and reduce the inductance, but that allows MORE current to flow.

            https://en.wikipedia.org/wiki/Saturable_reactor

            http://electriciantraining.tpub.com/...ircuit-136.htm

            http://rfcafe.com/references/radio-n...march-1952.htm

            https://trace.tennessee.edu/cgi/view...t=utk_graddiss

            https://patents.google.com/patent/US2798571A/en

            This is pretty much obsolete technology. And I don't think it is very useful for high quality audio lab amps. It is difficult to change the DC current quickly in the saturable core winding because of the inductance. I once tried to use a saturable reactor to adjust the current on an AC load which had a variable inductance in a time frame of 50 mSec to 2 seconds, and it was all but impossible. It would have required several thousand volts at 10 amps or more, which would have meant a 10-100 kW amplifier. This was a 1970s project and high current PWM and VFDs were uncommon. The problem was solved by adding resistance in the primary circuit which reduced the current variation to acceptable levels.
            Its possible that I remember wrong, because it has been some years since I played with electronics. But I can't see how a transformer with zero inductance would work -- all the welders that I looked inside of were wound on a single core.

            You might enjoy this: https://www.analog.com/media/en/tech...17334AN211.pdf
            25 miles north of Buffalo NY, USA

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            • #21
              In a transformer, drastically lower inductance would bring the "mutual" inductance too low to get secondary current. So saturated = no output. (inductance drops to "air core" value.)
              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


              • #22
                Thanks for the link to the current feedback amplifier. I'll have to read it when I have extra time. I found some conflicting information on saturable core welding transformers and reactors.

                https://www.electricaledition.com/20...lications.html

                https://canteach.candu.org/Content%2...y/20053425.pdf

                http://www.tpub.com/neets/book8/32n.htm

                https://www.yourarticlelibrary.com/w...nd-types/96860

                http://www.freepatentsonline.com/3665150.pdf

                http://pauleschoen.com/pix/PM08_P76_P54.png
                Paul , P S Technology, Inc. and MrTibbs
                USA Maryland 21030

                Comment


                • #23
                  Most of them are actually saturable reactor types, even though there is a transformer involved. The actual saturable transformer would be a one piece system, but would still need to maintain a substantial inductance in the primary.

                  So, most of the systems that vary the transformer work directly on the primary-secondary coupling mechanically. That way the influence on the primary is less, although even there, the inductance of the primary must change if magnetically coupled iron is changed.

                  The moving coil type, if it does shift iron, would change the inductance to less as the coupling is reduced. Generally, the coil only is moved, and the iron stays the same, with a setup that has less mutual inductance as the coil is moved away.

                  The shunting type would maintain it decently, even increasing it in the shunted (low output) case. A properly set up control winding on that leg would affect the inductance the least.
                  Last edited by J Tiers; 07-10-2020, 10:37 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

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