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Homework Help: DC Motor Problem - Magnetic Field Calculation

  1. Oct 22, 2003 #1
    I really need help in calculating the magnetic field generated by a horse-shoe shaped solenoid which is being used as a permanent magnet in a DC motor.

    The calculation is VITAL (well, I think so) as I need it to be able to predict the angular speed (in revs per minute) of my motor.

    If there is sa way to do the prediction without this, I would gratly appreciate it.

    FYI - We are being source a 10 volt, 2 amp current to make the motor spin. I need help ASAP as if my motor doesn't work, I fail this subject at University.
  2. jcsd
  3. Oct 22, 2003 #2
    The magnetic field in a long solenoid is
    H = IN/l, where
    I is current,
    N is number of turns,
    l is length of solenoid.
    I think you can use this, even though the solenoid is not straight.

    The B field is
    B = [mu]0[mu]rH,
    where [mu]0 = 1.2566 * 10-6 Vs/Am.

    [mu]r depends on your core material. Furthermore, it depends on how strongly the material is magnetized. OK, if you use a DC of 2 Amps, I should suspect that the core is always fully magnetized. At least in the stator. For iron, you can assume a maximum value of 5500. But be careful here, since other materials can have very different values of [mu]r.

    I think the least risky method is, to measure B directly with a Hall probe.
  4. Nov 17, 2003 #3

    thanks for the help on this problem. Your calculations made sense BUT (ironically and I think I got what I deserved 'coz I started researching so late) our prof gave us the exact formaula to use in our calculations!!!!

    w = (V - Ir) / k * magnetic flux

    w = angular speed
    V = voltage
    I = current
    r = resistance
    k = constant (taking account friction etc.)
    (v - Ir) = back EMF induced by motor

    Wen enquired about the magnetic field (B), he said it's not needed!!


    Anyway, FYI my motor spun at 900rpm, which was 200rpm slower than the winner.

    thanks anyway:smile:
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