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Make a Brushless DC Motor this way?

  1. Sep 5, 2010 #1
    I know the magnetic field makes a circle around a wire with current flowing through it. So I wondered if a motor could be made in the following way. Take a round plastic sleeve with an inner diameter slightly more than the wire. Glue 4 magnets on the sleeve, such that they form a circular magnetic field (for example, the north pole of each magnet could always face to the right when looking down at the vertical sleeve from above). Now run the wire through the sleeve and allow current to flow. It seems to me that there will always be a constant net force on each magnet, resulting in a constant torque to spin the motor.

    But some things don't seem right about this. As the motor spins, it appears to me that the magnetic flux experienced by the wire is not changing. Thus, there will be no back EMF generated to limit the speed of the motor. So the motor could reach infinite speed (neglecting friction). For the same reason, the device could not be used as a generator, which seems strange from a symmetry perspective.

    What is wrong with my idea? I know from a practical standpoint the current in the wire would have to be high, since there is only a single wire involved vs. the multiple turns in a conventional motor. But in theory, would this device work?
  2. jcsd
  3. Sep 5, 2010 #2
    BLDC motors require a sequenced (basically AC) drive current, so I think you have designed a motor that will work with AC current. With DC there will be no change in magnetic field, so nothing to push the magnets after they line up with the first ON-pulse. Coiling the wire to concentrate the magnetic field will make a more efficient use of the current flow. Spinning the magnets will produce an AC current, not DC...
  4. Sep 5, 2010 #3
    Won't there be a constant force on each of the magnets (force on a magnetic dipole in a magnetic field)?
  5. Sep 6, 2010 #4
    I believe the field from a DC current in the wire is "polarized" in a fixed direction ...the right-hand twist rule or something like that... The magnets will line up with that direction to remain in balance. If you change the field strength, or better reverse it, then you get the magnets to move.
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