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PM motor with field winding

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  • PM motor with field winding

    Anybody ever heard of this? I woke up this morning with this idea on my mind. You have a permanent magnet motor, but you augment the magnetic field with a winding. Not sure if there would be any benefit to this- offhand it would seem that the magnets would alleviate the need for field current, but you could apply field current as required to boost torque when needed. At the same time the wiring could be done in such a way that it creates magnetic field pinning, which could increase torque as well.

    I have been wondering about neodymium magnets- particularly in regards to how much magnetic 'strength' the actual material in the magnet can handle before it saturates. Obviously steel will saturate at some point, and 'modern' electrical steels are better than they used to be in this regard. Can a magnet handle higher field strengths if you induce it with current through a winding? Or will it saturate and effectively remove itself from the magnetic circuit?

    So far as I'm aware, regular steel has to be about the same thickness as a neodymium magnet in order to handle the field strength of the magnet. Of course it will be affected by the grade of the magnet- N52 might be able to saturate steel, with the result that some magnetic field will exist outside of the steel and be 'wasted' unless the steel is thick enough- or of a more suitable alloy. Boosting magnetic strength on demand might just waste the energy applied- but potentially you could get a short-term increase in power while still maintaining a reasonable efficiency.

    I'm aware that some starter motors use permanent magnets- I have one or two that do. And they are just regular ferrite magnets- probably what they call 'wet' magnets, which are probably the strongest ferrite magnets- but still not in the realm of neodymium or some other types. These motors use a planetary gearing stage to get the required torque. A ferrite magnet would seem to be able to benefit greatly from having a winding to increase the magnetic field strength- but of course if the material saturates, the torque limits out and any extra energy pumped in is wasted as heat.

    I've tried to envision the neodymium material as just a material and not as a magnet, just to try and understand how it compares to electrical steel in terms of maximum field density- not a lot of info available on that as far as I've been able to find. Of course, as soon as you apply magnetizing current it becomes permanently magnetized- unlike steel- but that doesn't prevent it from having a maximum flux density figure. How does it compare to say amorphous steel?

    So many questions, so few answers. What a thing to have in my head as I wake up-
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    Waay outside my area of expertise, but I suspect that whatever benefit there might be would not be justified by the additional cost and complexity, or we would have seen this long ago.
    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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    • #3
      "Magnets" are actually non-magnetic..... They are already saturated, they cannot "carry" any further field strength, and in general behave much like air, or some non-ferrous metal, as far as any external magnetic fields..... they act as a "gap" as far as the the external field.

      That said, there is potential room for series wound coils on extra core bits, acting as "interpoles" to assist commutation, etc.

      I am not sure if you would want to attempt to do "field weakening" with any sort of extra coil, that might depend on the magnet type.

      You could potentially use the PM as the weakened field, and add to it, presumably with the windings on a solid part of the core, not the magnet..
      1601

      Keep eye on ball.
      Hashim Khan

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      • #4
        Darryl, what are you doing??? You don't have time to be wasting on all this magnetic foolishness!!

        You're supposed to be busy putting the finishing touches on that perpetual gyroscope that we're all on the edge of our seats waiting for. We're counting on you man!
        Lynn (Huntsville, AL)

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        • #5
          Yes- I'm a man of many projects, and master of none Tell you what I'm doing right now- cleaning out my garden shed so I can put my fiberglass storage unit in there. I discovered this rotary table I built some years ago that I have to have a home for as well. I found a table saw- and tires. Lots of tires. None of them fit any vehicle I own-? The rotary table is almost 4 ft across, you could almost walk around it if it had 40 square feet of floor space to sit in-

          I'd rather be working on the gyro- and I don't need this magnetic foolishness in my head-
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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          • #6
            Look at Minkota electric trolling motors., they do something similar for variable speed.

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            • #7
              Originally posted by lakeside53 View Post
              Look at Minkota electric trolling motors., they do something similar for variable speed.
              They have a cross-section of their motor on their site, and they do not seem to have anything like that showing, nor do they comment on it, which I would have expected.

              Perhaps they have their secrets, but if it is present in the motor, it is not a secret. No mention of motor patents, etc. I'm a little disappointed, it seemed like an interesting idea.
              Last edited by J Tiers; 07-09-2020, 10:02 PM.
              1601

              Keep eye on ball.
              Hashim Khan

              Comment


              • #8
                JT your explanation makes sense- if a magnet was capable of carrying any more flux it would probably already be magnetized to that level. That's what the N rating is all about, and why there's nib magnets in the first place.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  Originally posted by J Tiers View Post

                  They have a cross-section of their motor on their site, and they do not seem to have anything like that showing, nor do they comment on it, which I would have expected.

                  Perhaps they have their secrets, but if it is present in the motor, it is not a secret. No mention of motor patents, etc. I'm a little disappointed, it seemed like an interesting idea.
                  On those I disassembled there is a mass of "fields" coils embedded in a potting compound on the rear end of the armature. They switch speeds with small wire combinations and no electronics. Other versions are PWM. I'll dig though my junk pile.. I might still have some.

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                  • #10
                    I was unfamiliar with N-ratings for neodymium magnets. Here is helpful info:

                    https://www.apexmagnets.com/news-how...ymium-magnets/

                    There are also ferrite transformer and choke cores with material types like N27, N67, N87, and N97, for popular EPCOS cores. They vary in terms of, mostly, permeabiity, but also cost, usable frequency, saturation, hysteresis, power density, temperature, and other properties.

                    https://cosmoferrites.com/Downloads/...art%202011.pdf

                    https://www.tdk-electronics.tdk.com/...rite-materials


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

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                    • #11
                      You can google field weakening permanent magnet motor for some creative methods. Inser the words "d axis" into your search.

                      basically you can strengthen or weaken a motors permanent magnetic field but you need a steel pathway through the field winding.

                      Magnets (with the exception of alnico which has a permeability of 10 iirc) are airgaps and it takes too many amp turns and copper losses to apreciably strengthen or weaken a neodymium magnet. Ceramic magnets yeah, but no one uses then anymore.

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                      • #12
                        Long time ago, I used a 20 mm diameter neodymium magnet in e lock latch and wound a coil around the magnet to release the latch. I had to use quite a big coil powered by 12 vdc to counteract the magnet's power. It was used in a kind of a safety gate that would stay closed even if the power failed. It would open by the electric pulse or in an emergency, if you forced it with push.

                        I suppose that you could wind a few turns around the magnets and connect in series with the armature to react to the load much like a old compound DC motor,
                        Helder Ferreira
                        Setubal, Portugal

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                        • #13
                          A PM motor is inherently more efficient because no energy is lost in field coils. But strong magnets are expensive and possibly fragile and may lose strength at high temperature. A field coil only dissipates power due to resistance, so with enough copper (and perhaps cryogenic cooling) it can be made very efficient. Probably cheaper than magnets, and field weakening (or strengthening) can be used for speed and torque control along with adjusting rotor current. Brushes and commutator can be inefficient and require maintenance. Best design is probably BLDC or synchronous with slip rings and wound rotor. And once you commit to electronic commutation, you might as well go with three phase induction or switched reluctance designs.
                          http://pauleschoen.com/pix/PM08_P76_P54.png
                          Paul , P S Technology, Inc. and MrTibbs
                          USA Maryland 21030

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                          • #14
                            Many times I've wondered about how strong a magnetic field can be in a motor. First we had Alnico, then ferrite, then Samarium Cobalt, then Neodymium. There were others, and I'm not sure now what their makeup was- Magnaquench is one name I remember, and I think there was one other type- but in any event with the stronger fields it becomes important that the magnetic pathway can handle all the flux. Cogging could become a serious issue as well in conventionally designed PM motors. Moving coil, or iron-less armatures came into use- I have a few motors like that, one being a good sized servo motor with near instant acceleration and zero cogging. Both are brush motors.

                            Moving along we have brushless and ironless motors. Two things here- one is that the gap has to be larger in order that the coil has room and clearance, and this affects the strength of the magnetic field in the gap. The other thing is that the more powerful and compact the motor is, the hotter the coil will get- it could have a meltdown before the potential power output could be realized- making the whole design about as ridiculous as augmenting a permanent magnetic field with wire coils. If we had a magnet that was say 3 times as strong as neodymium, it could actually be quite useless, except perhaps in a radical design.

                            At any rate, this was not on my mind when I woke up today. I was not even thinking about how to remove that last free molecule from the vacuum chamber for the gyro
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                            • #15
                              A couple days ago I awoke from a dream in which I was apparently working as a writer for Vladimir Putin, and the piece I was writing was titled, "Never kiss a girl". So, add that bon mot to, "Never trust a dream", and "Never trust a fart". Especially an old one...
                              http://pauleschoen.com/pix/PM08_P76_P54.png
                              Paul , P S Technology, Inc. and MrTibbs
                              USA Maryland 21030

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