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Electric brushless DC motors

  1. Oct 28, 2011 #1

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    Hi,

    I use brushless DC motors for my hobby things (planes, cars, helicopters). These motors have a special ESC which outputs a 3-phase AC current to the motor. These motors have stationary coils, and a rotary magnet array. The motors have an incredible torque for the size. I have, however, though of a brushless motor which runs on solely DC current, no electronics, no AC anywhere. A motor that can be ran solely by a battery with no fancy ESC to run the motor. Is it possible to make such motors - in theory, perhaps in practice?

    PS! Brushless, and no ordinary DC motor with rotating coils.

    Vidar
     
  2. jcsd
  3. Oct 28, 2011 #2
    There are two types of brushless dc (BLDC) electric motors. Larger ones have internal Hall Effect sensors to sense the permanent magnet rotor orientation and switch the dc polarity to the stator coils. Smaller sensorless BLDCs use external sensing of the Faraday-induced voltages to switch DC polarity.
     
  4. Oct 28, 2011 #3
    It can also be done by winding two sets of coil with the current flowing in opposing directions alternately.
     
  5. Oct 28, 2011 #4

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    I was thinking of a motor which do not change polarity at all, not externally or internally. It is fed with DC, it works with DC, it is DC internally, no cogging, no polarity change, no sensors or fancy electronics - just pure DC all the way. I think more like a brushless homopolar motor. Maybe I think too much outside the box :uhh:
     
  6. Oct 28, 2011 #5

    cmb

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    You could take a look at a homopolar motor. It works off very low voltage very high current. You may be pleased to know it doesn't even have coils!! However, in a conventional configuration you need some contact at the outer edge, a brush is usually used but you could use a roller or geared arrangement.

    In theory, however (and it may well have been done already) you could run two of them in a way that geared them together at the edge. This would avoid the need for a brush.

    The same principle was, in fact, the very first 'motor' ever built. It was Faraday's first demonstration of electricity turned into mechanical motion. He used a dish of mercury as the conductor of the electricity and the outer conductor, whereas a homopolar motor uses a rotating conductor.

    I do not know of any other means to achieve a motor with an unswitching DC.

    It might be amusing for you to take a look at all the various electrostatic generators, and try to figure out if you could make any of those work in reverse, as a high voltage DC motor.
     
  7. Oct 28, 2011 #6

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    Is it possible to build a homopolar motor with several windings in order to increase voltage and decrease current?
     
  8. Oct 28, 2011 #7

    cmb

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    A hompolar motor has no windings. It works simply by a big fat radial current discharged acorss a metal disc, the current then interacting with a magnetic field normal to the disc.

    To make it operate on a bigger voltage, you make the disc bigger. But then you end up with big magnets to find.
     
  9. Oct 29, 2011 #8

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    I guess you're right CMB. I have seen and tested homopolar motors myself with small nickel plated disc neodymium magnets. I have understood that the conducting coating on the magnet is a wire too. As long the magnetism in this part of the "wire" can escape from the magnetism in the magnet, it must be brushed - or using a gear ofcourse - but that will not make it possible to wind several windings in the same motor. The motor works more or less like a rotary "rail gun".

    What about a looped speaker driver motor system? The coil is aligned in a magnetic field. The longer the coil is the longer the stroke, but I can't imagine how this coil can be looped like a toroid with a pole piece in the middle and a magnet around it - AND make it work. The magnetic flux must return to the other pole, and by that crossing the windings again, which in turn will stop the motor from running....

    I think I will continue to use my regular brushless motors.

    Vidar
     
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