Magnetic Flux in BLDC Rotor

Discussion in 'The Projects Forum' started by TheBlindGuy, Apr 22, 2018.

  1. TheBlindGuy

    Thread Starter New Member

    Apr 22, 2018
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    Hello, everyone Thanks for checking the thread. I've been trying to create a BLDC motor and I designed something random without any calculation or simulation. Unfortunately it looks like I've made a mistake and now I can't get any flux to flow from rotor to stator laminations. My intention was to create four poles with permanent magnets in the N-S-N-S formation but the core seems to attract everything with a weak magnetic force. Even when I try to get the same poles of two magnets, it still attracts the magnetic core. I used electrical steel laminations and neodymium magnets. You can find the design attached to the thread. Can someone enlighten me as to why this is happening? I suspect I created a short magnetic circuit inside the rotor and all the flux is just going from one magnet to another. I would expect at least a low amount of repelling movement but it just attracts everything, regardless of the pole's position. Oh, and one more question: If this is the case, I don't have the chance to design the laminations again so what happens if I just create a stator made out of an aluminum block? I know the eddy losses will be magnificent but can I at least get it to work? Thanks for your answers. Have a great day!
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to AAC!
    If the rotor is made of ferromagnetic material then yes, the magnets' flux is shorted out by it, so little will couple to the stator. Without any coils energised the force between rotor and stator will always be attractive.
    The magnetic coupling would be improved if the magnets' poles were closer to the stator. Can you annotate the pic to show which way the magnets are polarised and where the poles are in relation to the magnet faces?
     
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Not to be rude but your second sentence says it all,
    Did you even look at how a BLDC motor is built? Or how one works? There are tons of pictures on line of the inter workings of a BLDC. They use the principle of "salient poles", not what you have made. You may be able to save the stator, but a total remake of the rotor will have to be done.
     
  4. TheBlindGuy

    Thread Starter New Member

    Apr 22, 2018
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    Thanks for the responses, guys!

    Of course. I just tried to draw the working principle for you. And as for the explanation, the motor was designed to have 6 stator poles and 4 rotor poles. Unfortunately I could only find laminations with 18 poles for the stator so I decided to connect them in parallel so 3 stator poles are polarized in the same way by a single source and the other three facing them is polarized in reverse. I tried to create two of the same poles (N-N) in the stator so that the rotor (S-S) can follow it. I also connected stator poles in star formation so that I can polarize one pole as N while the other one next to it is polarized as S. This way I thought while one coil is attracting the rotor poles, the other one is going to repel, doubling the torque.


    Yeah, I'm aware that designing something without any calculations was a mistake but can you please elaborate as to why this is becoming a problem? I've seen a few examples on the internet that looks exactly like this design (Circular core with rectangular magnets embedded in the rotor) and they seem to work fine for inrunner types. I thought the core was supposed to be made of silicon steel to reduce the reluctance and I thought I could lead the magnetic flux from magnet surface to stator poles if I created another magnet pole closer to it than the next rotor magnet. How can I calculate the position of the magnet? Right now I can only say that the closer they are to the surface (or stator) the better. Is there a specific way to determine the magnet volume / core diameter ratio?
     
  5. TheBlindGuy

    Thread Starter New Member

    Apr 22, 2018
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    Do you guys think replacing the current rotor with one of these designs could help? Can you please explain Why it wouldn't work if you think so. Thanks!
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    I too have a design for a different motor, one called a SRM, switched reluctance motor. Similar to a BLDC but different in that they use no permanent magnets just the salient electromagnet poles.

    That said I'm also going to use an existing stator, one from an induction motor like you're using. The thing that needs to be done to those stator cores is removing the unused poles. Especially in a BLDC. To get the magnetic pole pieces out of the way, so that the only reaction to the PM's(permanent magnets) is with the EM's(electromagnets). Without removing the unused poles the PM's are bound by the unused poles. Cutting the unused pole pieces can be done with a band saw, if you weld the blade joint with the blade on the inside of the opening. Machine shops do this all the time. Though it may take a couple of blades to get them all removed, the core steel is very hard on teeth. Until I started on building a motor I didn't realize how expensive the raw core stampings were.

    There was a few online thesis papers about how to calculate the magnet to pole measurements. I haven't done anything with my motor for awhile but a Google for "BLDC design" should get you some hits.

    Another source of cores and even end bells for experimenting is car alternators. Junkyards around here sell bad ones pretty cheap. They used to give them away but they get around $10 now from the cores, rebuilders buy them.
     
  7. TheBlindGuy

    Thread Starter New Member

    Apr 22, 2018
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    I'm afraid I passed the point of sawing poles off the stator part but I don't understand what's the downside of switching poles in parallel. Wouldn't it work if I just connected 3 poles in parallel to end up with 6 poles? (3x6=18 poles in total)
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Again not wanting to be rude, but this quote -
    Makes no sense if you want to end up with a working motor. Do you mean that you have already wound the stator, that you would waste all of the wire? If that is the only reason, you have to also look at it as, you have already wasted it, since it doesn't work. If money and time was the only problem you should have done some research on how a BLDC works before even starting the project. Information on how a system works or should work is the cheapest thing in any project, the planning at the start.

    I can't walk you through all of this online if you have no knowledge of the basics. You have to have some idea on how and why one of these work, and just looking at a few pictures online or a few Youtube videos won't do it for you. I'll try to help you but you need to show some understanding of the theory of how they work. Again I'm not trying to be rude or condescending to you, just truthful.

    You need to get a book or two on the theory of the motor. One I like is called, "Brushless Permanent Magnet Motor Design, by Dr. Duane Hanselman". Another good one is, "Design of Brushless Permanent-Magnet Motors (Monographs in Electrical and Electronic Engineering) by J. R. Hendershot" I bought mine used a years ago, but they may be available online by now.
    If you can't find them cheap enough you could try your local college library, they may have them.
     
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    @ TheBlindGuy, How are you planning on detecting the poles for the commutation of this motor?
     
  10. TheBlindGuy

    Thread Starter New Member

    Apr 22, 2018
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    I'm thinking hall effect sensors. I plan to insert 3 of them in between stator poles and try to detect the alignment of the permanent poles. Either that or I can try to use the back EMF of the stator coils but it's way more complicated than that so that's a plan B. I'm sorry I couldn't answer sooner. Don't worry about being rude for it's not rude to inform someone of his mistakes. I'm a student after all. How else am I supposed to learn from my mistakes if they are not stated clearly, right? Thanks for the book suggestion, I will try to acquire it right after my exams and start reading. The reason why I can't change the design now is because I wanted to make this as a school project with a few friends because I was interested in the subject but I need to present something soon, regardless of the efficiency. Right now I'm just trying to make the best of my failure of a project ^_^ I'm going to read the book when I'm through with the exams and try to design one that's actually usable.

    For now, I just made a new rotor from scrap from a diamagnetic material with poles attached on the sides so there shouldn't be any magnetic field interruptions but I'm expecting a very unstable rotation which means I won't be able to switch it at a high frequency. The stator is completely wounded and the rotor is ready so as soon as I find a couple bearings, I'm set (hopefully).
     
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