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not understand anemometer

Discussion in 'Electronic Basics' started by developer, Jan 20, 2005.

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  1. developer

    developer Guest

    http://www.otherpower.com/.html

    The internal circuitry of our brushless DC motor consists of 12 coils, and a
    permanent magnet ring that spins around them. These motors are NOT like a
    normal DC motor...they require a special driver circuit to make them spin.
    If you apply plain DC current to the motor, it will simply seek a point
    aligned with the coils, stop, and burn out. The motors have 3 leads...a
    common in the center and 2 outputs (well, actually inputs!). For connection
    to measuring equipment, you need only connect to the center common lead and
    one of the side ones. The output of the motor when spun as an anemometer can
    be measured with a multimeter set for AC volts, or by counting pulses with a
    frequency meter or BASIC stamp. We had the best results using a Fluke 87
    multimeter set for measuring Hz (cycles per second). There are 12 internal
    coils in the motor, but we only measured the output of half the coils (since
    we connected to only one power lead). Therefore, a meter reading of 6 Hz
    equals one revolution per second (60 rpm). Both frequency and voltage
    readings from the motor are quite linear, making for easy calibration. The
    other advantage of counting frequency over measuring voltage is that the
    length of the data cable would affect voltage readings; when counting
    frequency it can be any length and the calibration will stay the same.

    why you use the AC volts to measure a DC brushless motor?
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  2. John Larkin

    John Larkin Guest


    The rotor spins. AC is just DC in motion.

    John
     
  3. developer

    developer Guest

    still do not understand
     
  4. no_one

    no_one Guest

    that is how you drive a brushless motor; with an AC power that is
    synchronized to the shaft rotation.
     
  5. Tim Wescott

    Tim Wescott Guest

    Because they shouldn't call them "DC Brushless Motors", they should call
    them "permanent magnet synchronous motors". But nobody would understand
    that, so they stick to the misnomer.

    Inside of a "normal" DC motor you'll find a little gizmo called a
    commutator. It's job is to take the DC from the power wires and switch
    it around to the coils -- making it AC with just the right frequency to
    drive the synchronous AC motor that's inside of nearly every DC motor*.
    A brushless "DC" motor takes the commutator out of the motor and puts
    it into a circuit, eliminating one of the major wear points of a DC motor.

    * except for the homopolar motors, but they're weird.
     
  6. NSM

    NSM Guest

    | developer wrote:
    .....
    | A brushless "DC" motor takes the commutator out of the motor and puts
    | it into a circuit, eliminating one of the major wear points of a DC motor.
    |
    | * except for the homopolar motors, but they're weird.

    They're going to amend the Constitution to ban those.

    N
     
  7. John Larkin

    John Larkin Guest


    Coils create magnetic fields and attract the magnets. If you apply DC
    to a coil, the motor moves to some position and then stops; you noted
    this yourself. To get the motor to spin, you have to keep changing the
    current in the coils to keep the rotor moving; you have to keep
    changing the "place where it would stop"

    If you use the motor as a generator, by externally spinning the shaft,
    the magnets move past the coils and generate voltage. As a magnet
    approaches a coil, the voltage swings positive, and then as it moves
    away, it swings negative. This happens over and over as the shaft
    spins, so the average voltage induced into the coil is zero. So
    there's no DC to measure.

    Actually, as I said, AC is just moving DC. If you were to turn the
    shaft very slowly and look at it with a sensitive DC voltmeter, you'd
    see the slow positive and negative swings. But the voltage would be
    very small. As the speed goes up, a DC voltmeter can't follow the
    swings and reports the average value, 0. An AC voltmeter essentially
    rectifies the voltage before measuring it, so you get a number.

    John
     
  8. I do. Thanks John, that makes much more sense to me than Tim's answer :)
     
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