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Need help understanding strange idea (electric motor rotor and stator)

  1. Oct 25, 2015 #1
    A dc brushed motor with a commutator, the stator isnt fixed to anything and is left freely on a desk for example, now apllying power the rotor starts to turn, if I take the rotor and hold it with hand the stator now starts to turn around , I assume this happens because the stator poles and rotor poles push against each other and if one is stationary the other is left to move the one with less resisatnce to movement will move, but it can be either one since the magnetic field doesnt care which one it is correct?

    Now further what I want to know is what would happen if i took the rotor and fixed it permanently as it is against the stator , say for example welded the rotor shaft to stator at some point , now the stator makes up the magnetic field N-S or S-N as usual and the rotor poles tend to repel against that field also as usual but what would the result be ? say I put the whole assembly of stator and rotor into some extended bearings , would the whole motor (stator and rotor together ) now rotate around some external axis like one piece ?


    This is were I get lost because as long as the current runs through the rotor wires the rotor poles being locked with respect to the stator poles would try to constantly repel but since the stator is fixed with the rotor as one piece the only frame of reference to repel against not is the rotor shaft which is attached into some external bearings , the question being would it spin or what else would it do if not that?

    thanks.
     
  2. jcsd
  3. Oct 25, 2015 #2

    andrewkirk

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    It would not rotate, or move in any other way, unless there were other strong magnets in the vicinity. The magnetism is making the rotor exert a say clockwise torque on the stator and the stator exert an anticlockwise torque on the rotor. The welded joint exerts an equal and opposite torque between the stator and the rotor, so the torques cancel out.

    The system will be like a compressed spring that is locked in place. It cannot generate any motion unless the spring locking mechanism is released. In this case the locking mechanism is the welding.
     
  4. Oct 25, 2015 #3
    Hmm but why would it not.
    Ok lets simplify this whole thing forget about a separate stator with coil windings welded in place with a rotor with windings.
    Take just the rotor with windings , for simplicity the rotor has only two poles. now either there are two permanent magnets attached or the rotor itself is magnetized N-S or S-N doesn't matter. now the magnets are fixed with the rotor and their center is at the rotor pole center.
    Now make a dc power connection to the rotor poles, the rotor pole should try to push itself away to align with the opposite pole all the time but it couldnt because the alike pole is fixed at the pole pushing itself away from it.

    note slip rings are used instead of a split contact commutator since as the magnetic poles the current is fixed in one direction.
    this kinda puzzles me because the field from the rotor coils is at the correct angles to the magnetic field to exert a pushing force as normally in every universal motor , the only difference that in a usual motor as the rotor pole pushes itself away from the stator pole the current is fed to the next wire/pole to push again away from that same exact pole but here the wire coil/pole is permanently pushing away from the same pole , I don;'t know maybe it doesnt work because there is no stationary frame of reference or something along those lines could you folks please go in depth about this , thanks.

    P.S. but heres the point andrewkirk , a homopolar motor for example can work when both the coil carrying current and the permanent magnet creating the field against which the current can " push" both rotate together, technically there is no stator , the sort of magnet which would serve as stator in oridnary motors is fixed with the rotor coil/disc whatever and they both coorotate.
     
  5. Oct 25, 2015 #4

    andrewkirk

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    There's nothing to go into any depth about. Two things that are stuck together pushing against one another will produce no motion. When you are sitting in the driver's seat can you make the car go by pushing on the steering wheel? It's Newton's Third Law of Motion.
     
  6. Oct 25, 2015 #5

    Nidum

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    It does occasionally happen that real motors lock up .

    Large motors generally make loud noises for a short time and then burn out if no protection systems are installed .

    DC ones usually growl and AC ones usually buzz .

    Vibration levels can be very high .
     
  7. Oct 25, 2015 #6

    CWatters

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    Can you provide a reference for that?
     
  8. Oct 26, 2015 #7
    I'm not sure I can find a reference for this, but I could try to apply a bit of what I know and ask others to join in , for the classical faraday disc one has a circular permanent magnet with it, with this magnet the field cutting the disc is uniform all around the disc , so what difference would it make if the magnet rotated or not with the disc? if the magnet stays stationary and the disc rotates the conducting disc cuts a static uniform flux which exerts a force on the charges and if brushes are attached create a current or is voltage is applied and a current runs through the disc creates a force and starts motion.
    now if the magnet rotates with the disc , the disc still cuts a static uniform magnetic field the same as before....

    from a special relativity point of view if you are an observer and you corotate with the disc , say you are a part of the disc , now in both cases what you see is a static uniform magnetic field that you are being dragged through, and if you were charged you would be pushed sideways either one way or the other.

    one could also model this is a charged particle traveling along a straight line , in one case a magnet travels along the particle in other case the whole line is stationary and magnetized like a long magnet strip and the particle travels along that strip, i think the particle should get deflected in both cases but please comment on this if it's not the case.

    also you could have a disc between two magnets , one attached to the disc and corotating the other being stationary, now would the current induced be proportional to the strength of the stationary one or both... again the same puzzle.

    there is one interesting probability i would like to discuss but i think its better to clear these questions first out of the way.
     
  9. Oct 26, 2015 #8

    andrewkirk

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    You'll have to draw a diagram, because it's not possible to visualise what you are writing about.

    I am confident there is an error in your reasoning, but unless you can make your reasoning clear enough to understand, including a detailed diagram, we can't help you find it.
     
  10. Oct 26, 2015 #9

    russ_watters

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    Force on what charges? The magnetic field is not some stationary thing existing in space that a magnet can push on: It is generated by the magnets in the motor. So if you lock the rotor in place, the magnets are just pushing on each other.
     
  11. Oct 26, 2015 #10
    I'm not sure what much can I visualize and diagram about a flat copper disc corotating or not corotating with a uniformly round magnet like a speaker magnet. I'm just not that good at diagrams.
    I mean at this point I'm sure you know just as much about my post as I do myself.
    the very simple question also hides the deeper explanation within itself, do magnetic field lines rotate with a uniform axially rotated magnet or no?


    I have thought about this back and forth from every possible angle I could find to the point i have exausted myself and am unable to bring my **** together, I mean I do understand how ordinary generators and motors work , if the answer to my previous question would be yes I had in mind a interesting device that could be built but I guess being just " another brick in the wall" it would be unreal for me to come up with something that hasn't been already thought of before and either already made or discarded as impossible.
    God damn physics is killin me :D
     
    Last edited: Oct 26, 2015
  12. Oct 26, 2015 #11

    andrewkirk

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    Yes, if you rotate a magnet the magnetic field lines rotate with it.
     
  13. Oct 27, 2015 #12
    well please take no offence but i can't agree with that. well they may rotate with the magnet if you mean rotating a bar magnet fron N to S sideways , but take the same bar magnet and rotate it around it's imagined axis going from the center of it's N pole to the center of it's S pole., place another magnet nearby, now would that magnet feel anything at all , in terms of any change I doubt so.

    Also in the homopolar disc example, nothing happens whne you simply spin the magnet undearneath , it's only when you spin the disc itself that current is generated , because the current forms from charges and they are in the conducting disc and they need to be moving in a uniform field to experience a sideways force , but even if the uniform field was rotating what difference would it make? every next so called field line that the conductor would cut is the same strength as the previous one , so for you as an observer on the disc all of them would seem the same.?

    heres a simple thing i just did , i took an old spear magnet and a bar magnet , i placed the bar magnet on my desk and put the speaker magnet in hand and rotated it beneath the desk right were the bar magnet is , as i rotated the round speaker magnet underneath the bar magnet showed no signs of rotation at all , it only reacted if i moved the speaker magnet sideways up or down (closer or further away) and when i flipped the poles , just like i expected it to do.



    here is an interesting video i found on youtube, as one can see here the very conducting surface of a round neodymium magnet is used as the current path to form a homopolar motor, but in regard about the question whether the magnetic field lines spin with the magnet one has to think about it because the neodymium magnet spins together with its coating which due to being conductive is used as a homopolar motor , so technically this is a copy of a typical magnet being fixed with a copper disc spinning together.
    any comments welcomed please.

    thanks.
     
    Last edited: Oct 27, 2015
  14. Oct 27, 2015 #13

    Nidum

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  15. Oct 27, 2015 #14
    as usual in wikipedia , the article is a bit confusing , even though it kinda approves on what i said earlier that the motor/generator works with the magnet corotating with the conducting path which would lead us to think that a uniform field isnt affected by the physical rotation or nonrotation of it's source.

    in one part the article says that no matter whether the magnet rotates or not the field being uniform is there and the electrons in the disc spinning feel the lorentz force which produces the current in this generator.
    then further it says quote,
    which leads to think that if the magnet is stationary the current generated comes from the disc but if the magnet rotates with the disc the current generating part is now the stationary wire with the attached brushes part of the circuit.


    Oh and also there was a thread some time ago here on PF which was rather confusing , both the OP question and the replies he got.
    here is the thread i am reffering to
    https://www.physicsforums.com/threads/magnetic-field-rotation-or-not.805357/

    and the Op asked one question to which there was no answer maybe i can quote it here and see if someone comes up with one as it interests me just aswell.


     
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