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I I have a 2-part question about electromagnetism and motors

  1. Aug 4, 2016 #1
    So, I'm working with a DC shunt wound motor and I want to calculate the actual voltage production on paper mathematically before building. I have been studying the biot-savart law and understand it to a fair degree. One of my questions is after you get your measurements in Teslas from said law do you convert that number to Webers to find the flux density? And if so what is the easiest unit conversion to understand?

    Next question, what formula do you use to find out how many volts are produced after cutting through these lines of flux?

    Thanks
     
  2. jcsd
  3. Aug 5, 2016 #2

    NascentOxygen

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    Hi James Birt. smiley_sign_welcome.gif

    You're constructing a motor from scratch?
     
  4. Aug 5, 2016 #3
    Trying to, I'm a journeyman electrician and my step-dad is a mechanic with 20+ years experience. We are both working on it and have different understandings of how it works... well, fundamentally the same I guess. We are both interested in attempting it though. It is not going to be a large scale project, the motor will be relatively small with a small supply.. I'm just having slight difficulties with the math and would like to have the ability of knowing it would work before investing the time it will take to accomplish the task.
     
  5. Aug 5, 2016 #4

    NascentOxygen

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    You're constructing an armature using laminated steel, with slots, and a commutator, carbon brushes, and all? I'm impressed!

    Something slightly larger than a slot car motor?

    I hope there is someone here who can advise you further.
     
  6. Aug 5, 2016 #5
    Yes, that size or slightly larger but you're in the ballpark. I'm just having issues determining if I'll get the right voltage output so I've been studying magnetomotive force and flux density but the math is complex.... hey, speaking of the commutator, you know the gap where the brushes jump? I'm sure you do, what is the best way to tell how much the flux is disrupted during the change in cycle? An oscilloscope maybe?? Thanks, and thanks to anyone else that can shed some light on the subject.
     
  7. Aug 5, 2016 #6

    David Lewis

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    The commutator is retarded with respect to the geometric neutral plane. You manually adjust the offset angle by trial and error until arcing is minimized.

    Back EMF = (1/2) * (angular speed) * (number of turns) * (rotor radius) * (average magnetic flux) * (conductor length)
     
  8. Aug 6, 2016 #7
    What formula do you use to find average magnetic flux?? I can get the other numbers, and thanks for the feedback I'm stuck on this.
     
    Last edited: Aug 6, 2016
  9. Aug 6, 2016 #8

    David Lewis

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    Do you have a flux meter?
     
  10. Aug 7, 2016 #9
    No, I do not. Did you see my first post about unit conversion? I would really like to know the math.
     
  11. Aug 7, 2016 #10

    David Lewis

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    Measurements are data obtained from experiments.
    When you apply a physical law, however, the quantity you seek drops out the bottom of an equation.

    Before you proceed, review which quantity corresponds to which unit
    (square brackets enclosing a physical quantity mean the unit with which said quantity is measured):

    [magnetic flux] = weber
    [magnetic flux density] = tesla

    The formula is:
    (flux density) * (flux area) = flux
     
  12. Aug 7, 2016 #11
    Ok, I have been using the biot-savart law and that law is for magnetic fields and comes out in Tesla units. So, if the flux density is measured in Teslas then that should be the number I need. Thank you very much for clarifying this for me! I can now move forward with my calculations and hopefully construction very soon. Thank you again Mr.Lewis, I can't express to you how helpful that information is.
     
  13. Aug 7, 2016 #12

    NascentOxygen

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    Do you know the relative permeability of the steel you'll be using, James?

    How many commutator segments are you planning?
     
  14. Aug 8, 2016 #13
    Using an iron core, (2.5x10^-1). As far as commutators, from what I've seen, 2 is a pretty common number on motors the size I'm talking about putting together with many windings.
     
  15. Aug 8, 2016 #14

    David Lewis

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    If the relative permeability of the iron is 0.25 then it has 4 times less permeability than free space.
     
  16. Aug 8, 2016 #15

    NascentOxygen

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    I think there are "issues" with the use of two commutator segments, including the shorting out of the supply unless the commutator gaps are wider than the brushes. And with wide gaps the motor will sometimes need a helping hand to start it rotating.

    A motor with two-segment commutator would make a great demonstration machine, though. Are you intending your machine to do useful work, or just serve as a functional demo?
     
  17. Aug 9, 2016 #16
    No, I need the motor to function... what would you advise? I'm sure you've worked with them before, I have worked with them enough to understand their function but not enough to easily understand their construction. I know the gap in the commutator can cause interference in the magnetic flux and smaller magnets can be used to offset that particular variable but if I could avoid having to do that all together I would rather. Is there a better way?
     
  18. Aug 9, 2016 #17

    NascentOxygen

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    I think you'll find what you need from further web searches---more details about practical motor design. It's outside my area, I'm afraid.

    Are you sure you want a shunt motor, as it seems to me that a permanent magnet motor would, in principle, be simpler. (Though home-building an efficient motor with permanent magnets may not be a realistic proposition.)
     
  19. Aug 9, 2016 #18

    jim hardy

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    that's good iron.
    0.25/μ0 = almost 200,000 relative permeability


    If you want a working, useable motor....
    Before making one from scratch
    i'd suggest you fellows take apart an old DC generator or motor and take careful physical measurements .
    Study how its armature is wound. , and the field coils.
    Figure out how they arrived at number of turns on field
    and trace the wires around the armature.


    Junkyards are full of old cars. Most are too new to have a generator
    but a DC motor and generator have no practical difference
    and there are fans motors, windshield wiper motors, electric seat and electric window motors

    and of course starter motors. Converting a starter from series to shunt field would be a good exercise for you guys
    and you'll learn nearly as much , maybe more because you won't get discouraged machining that tedious copper segmented commutator
    Old Chrysler starters (early 60's) had two fields - a series field to provide extreme torque for those big B block V8's and a shunt field to limit RPM in case the Bendix drive gear fails to engage.

    Here's a video of a guy replacing fields in a generator...

    http://www.bing.com/videos/search?q...033C9AECEAA50410C9D0033C9AECEAA50&FORM=VRDGAR

    Read up on magnetic circuit
    learn what are meaning and units of
    magnetomotive force MMF , Ampere-turns
    flux Φ , Webers
    Reluctance ℝ Amp-turns per Weber

    e = blvsinθ, voltage induced in a wire = b(Flux Density) X l(ength of wire) X v(elocity) X sin(angle between Velocity and Flux)
    http://ibphysicsstuff.wikidot.com/electromagnetic-induction

    old jim
     
    Last edited: Aug 9, 2016
  20. Aug 10, 2016 #19
    I misunderstood what you were asking, it will have only one commutator.. thought about self-excited but may be unnecisary. I'll take a look at some other motors and see what makes em tick.. that's an excellent idea. I have plans for the one I want to put together though, but getting a better understanding of existing ones does seem like the most logical starting point. I'll check out those videos in the morning and get back to you. Thank you all for your input, it's been very very helpful.
     
  21. Aug 10, 2016 #20
    And the iron I want to use is 95% pure, but it also depends on cost if I go with it or not. The magnetic permeability isn't hard to figure out though.
     
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