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Torque in motor

  1. Jun 17, 2015 #1

    cnh1995

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    When current flows through the armature winding of a dc motor, what exactly happens in there that produces the torque? Is there a force due to the interaction between two magnetic fields or Lorentz force acts on the conductor electrons? Basically, why does motoring effect take place? Is it due to the magnetic field interaction or Lorentz force? Or are they one and the same?
     
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  3. Jun 17, 2015 #2

    Hesch

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    When current flows, a magnetic field in the airgap between rotor and stator is created. This magnetic field has an energy density = ½*B*H [ J/m3 ].

    Commutating the motor currents so that a pole pair is kept apart from each other, the magnetic field will cross the airgap skew, thus taking up a lot of volume. Nature ( included the motor ) wants to get rid of magnetic energy, which could be done by rotating the motor, thereby closing up the pole pair and minimizing the volume of the magnetic field. To turn the motor, torque is needed ( nature knows all about that ). When the pole pair is closing up, the motor currents are commutated so that the distance between the poles is kept constant.

    So torque is produced because the nature attempts to get rid of magnetic energy by reducing the volume of the magnetic field, containing energy density.
     
  4. Jun 18, 2015 #3

    cnh1995

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    That means interaction of two fields leads to the torque?In regards to this interaction, I saw a diagram in which it was shown that flux at one end of the pole is enhanced and that at the other end is reduced so that a net force is developed. Is that correct?
     
  5. Jun 18, 2015 #4

    Hesch

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    How do you count two fields? Which field is the other field?
    I don't know. I've not seen such a diagram.
     
  6. Jun 18, 2015 #5

    cnh1995

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    The two fields ard due to field current and armature current.. The torque is proportional to the product of these two currents.. My question is basically about the force experienced by a current carrying conductor placed in magnetic field.. Does that force act on individual electron(s) or is it the result of the interaction of the two fields?
     
  7. Jun 18, 2015 #6

    Hesch

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    Two magnetic fields can never overlap each other ( cannot interact ). Illustrating some magnetic field, lines/curves are often used. Two lines/curves can never cross each other. This is stated in Amperes law:

    circulation H⋅ds = N * I

    This means that if you have a 3D compass and from some place follows a curve, you will always end up exactly the same place. If two curves were crossing each other, you could "jump" from one curve to the other and thus not ending up the same place.

    Magnetic fields are circulation fields.
    You must read the post #2 again. The wires are often "hidden" in grooves, and thus is not exposed to a magnetic field of significance: The motor will yield a torque anyway.
     
  8. Jun 18, 2015 #7

    cnh1995

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    If the wires are hidden in the slots, what causes back emf?
     
  9. Jun 18, 2015 #8

    cnh1995

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    Forget about motor..If a wire is held in between two poles and current is passed through it, it will have its own magnetic field which will be in circles around the wire. This will distort the original flux(N to S) in such a way that it will increase above the conductor and decrease below the conductor (assuming proper polarity and current direction). Won't this lead to a reasultant force upside down? If no such interaction happens, where's the force coming from?? Also, is its msgnitude BILsinθ??
     
  10. Jun 18, 2015 #9
    In Quantum Physics, electric forces are sometimes modeled as virtual photons. But following that answer leads down a rabbit hole. (What is "virtual"? How do photons interact on a quantum level? Etc.) If you wish to know more, get a good textbook on Quantum Electrodynamics.

    Otherwise just accept that fields exist and the EM fields follow Maxwell's equations.
     
  11. Jun 18, 2015 #10

    jim hardy

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  12. Jun 19, 2015 #11

    cnh1995

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    Thanks jim..That helped a lot.. Especially that cross product mechanism which takes place twice..:smile:
    Now as I'm told that the magnetic fields do not interact, does this diagram make any sense?
    force - Copy.jpg
    Is this the effect of that cross product or what?? This is what I was talking about in #3..
     
    Last edited: Jun 19, 2015
  13. Jun 19, 2015 #12

    jim hardy

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    Yes. That helped me. Glad you liked it.

    If you can contort your right hand thusly

    You'll have to rotate your hand from the way this sketch is...
    CO2.JPG
    index finger in direction of current
    middle finger align with magnetic field , toward blue S pole
    you'll find thumb pointing down and that's the force.


    force-copy-jpg.84979.jpg

    In freshman physics we were taught the memory aid

    cross product: result has direction of a right handed screw rotated from first term into second.

    yes.
    Now let the wire move down, repeat the hand gyration and you'll see counter-emf oppose current.

    Magnetic fields dont interact ? Sounds cunterintuitive at first but think about it...
    If i hold two magnets close to one another, the force i feel is exerted on the magnets .
    Probably semantics - our mind tends to leap past the necessary baby steps of thinking .
    Likewise in the motor the force is exerted on the wire, even if only because the charges are constrained to stay in it. .
    This hyperphysics link is a bit more academic than my explanation, it introduces the idea of magnetic moment .
    (unrelated to my 'senior moments')
    http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magmom.html

    old jim
     
  14. Jun 19, 2015 #13

    cnh1995

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    Well, if the field due to the current in the above diagram is somehow nullified, will it still push the wire down? Because there will still be the charges flowing in a magnetic field, leading to the cross product. What really pushes the wire? Is it the distorted magnetic field or the motion of charges in the magnetic field?
     
  15. Jun 19, 2015 #14

    jim hardy

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    Nullified ? No field no force.
    Realize that magnetic field in the figure will be affected noticeably when current in the wire produces its own field that's noticeably intense in comparison..

    In rotating machines that's called "Armature Reaction" and searching on that will open a new world.


    I'm old school and think of it as the Lorenz force pushing the charges across the wire as in Hall effect.
    They cant get past the insulation so that force is transferred to the wire.
    Indeed the conductors in huge machines have to be securely wedged against magnetic forces.

    Astute folks here on PF have corrected me, and rightly so for there do exist field equations more modern than my simplistic 19th century mechanical analogy.. Jeff hinted at that in post 9.
    I leave Poynting vectors et al to people who are better versed than i in higher math.

    old jim
     
  16. Jun 20, 2015 #15

    cnh1995

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    Here is why my original question came up in the first place..I was studying indirect loading test on induction motor coupled to a dc generator. When the electrical load (lamp bank) was switched on, the motor on the other end slowed down. How does the motor know that it's been loaded?
    The motor (rotor) feels an opposing mechanical torque in order to slow down. How's it generated? Then I thought, it might be due to the armature reaction in the generator but it is nullified by the interpoles. That leaves only the Lorentz force mechanism.. That's what I was talking about in #13. Field due to the current is nullified but still there are charges flowing in a magnetic field. Also, is it technically correct to say that armature reaction is desired in a motor but not in a generator?
     
  17. Jun 20, 2015 #16

    jim hardy

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    Armature reaction in a machine seems at first glance a necessary evil.
    It complicates an analysis that we would prefer remained simple.
    But the clever giants on whose shoulders we stand figured out how to use it to advantage.

    Realize that a motor and a generator are identical, the only difference being direction of power flow.
    They added a third brush to generators in early automobiles to power the field. Armature reaction distorted the flux hence the current to the field which made them self regulating. Model A Ford(1932) is the last one of those i've personally encountered.
    https://en.wikipedia.org/wiki/Third-brush_dynamo
    300px-Three-brush_dynamo%2C_circuit_%28Autocar_Handbook%2C_13th_ed%2C_1935%29.jpg
    Should you encounter one here's a practical page to know about
    http://www.yesterdaystractors.com/articles/artint4.htm

    of course the same applies to motor operation of the three brush machine.
    Ford windshield wiper motors as late as the 90's had three brushes for better speed control.
    If you peruse junkyards grab one and take it apart .

    Automobile alternators are built with a lot of armature reaction which makes them current limiting.
    I guess i was lucky to grow up before things got sophisticated -
    my first alternator equipped automobile puzzled me because the voltage regulator had no current control coil.
    Only when i took AC machinery course did i understand why. In fact it was Prof Gross, mentioned in another thread, who helped me understand the Lundell automotive alternator. It's just a three phase synchronous machine with a rectifier added.

    Synchronous machines are inherently current limited by armature reaction.
    The terms to search would be "Synchronous Impedance" and "Short Circuit Ratio".

    But you're studying DC machines, right ?

    The field isn't nullified.
    Draw your vectors and you'll see the armature's MMF is in direction perpendicular to that of field.
    At low current there's hardly any effect, observe that in your right triangle a small opposite side doesn;t affect the hypotenuse very much.
    As current increases the hypotenuse and adjacent side separate changing the direction of the field.
    So the field is shifted and it's the job of interpoles to put it back where it belongs.
    Three brush machines omit the interpoles and use that shift for current control. It affects voltage at commutator over where the third brush is located , changing excitation to the field coil..
    It's beautifully simple. The old principle of designing so Mother Nature helps you out.

    I hope this helps you with the concept.
    I apologize for bringing a 'junkyard wars' meme to PF . But one learns an awful lot by working on machinery, and that's why lab courses are so vital to a university curriculum.
    Glad you were curious.

    old jim
     
    Last edited: Jun 20, 2015
  18. Jun 20, 2015 #17

    cnh1995

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    Ok..I guess I've got a hang of it..And what's your idea about that induction motor indirect loading? How does that work?
     
  19. Jun 20, 2015 #18

    cnh1995

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    Indirect loading means not applying direct mechanical load. Induction motor is coupled to a dc generator via a shaft. Now if the motor is run, it will generate emf in the generator. When the electrical load at the generator end is switched on, it will draw current from the generator armature. That's how the motor has been loaded indirectly. This will slow down the motor as it feels a load torque. My question was how does it know that its been loaded??
     
  20. Jun 20, 2015 #19

    jim hardy

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    Force on the rotor bars is in proportion to the current induced in them
    by relative motion of rotor bar to rotating field (slip).
    To get more torque requires more slip so the rotor slows down.

    Since you have a lab setup
    shine a florescent light on the shaft and watch very carefully. (old fashioned fluorescent tube with magnetic balast , not a CFL)

    You will see , albeit faintly, stroboscope effect. Specks of dirt and rust on the shaft will appear to stand almost still.
    If you have a strobe light(Strobotac) that you can synchronize to the line so much the better.

    When you get the hang of it, you can watch slip. A lightly loaded motor might take almost a minute to lose a whole turn with respect to the power line.

    That's slip.
    This curve shows that torque reverses when rotor field overtakes stator field and it becomes a generator.
    ns is line frequency rpm - for 60 hz 1800 or 3600 or 1200 or 720 or whatever.
    http://www.expertsmind.com/topic/induction-motors/torque-slip-characteristics-917822.aspx
    531_Torque-Slip%20Characteristics3.jpe

    old jim
     
  21. Jun 20, 2015 #20

    jim hardy

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    Last edited by a moderator: May 7, 2017
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