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Large Radius High Speed Stepper Motor

Discussion in 'Electronic Design' started by [email protected], Dec 8, 2004.

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  1. Guest


    I am trying to make a fan using a stepper motor where the radius of the
    rotor is very large, say, 1 meter, and the drive is applied at the
    periphery of the fan (the fan will have a rim at the outer

    I want super high rotational speed. The faster the better. Natuarlly
    the atmospheric drag will increase with the rotational velocity, but I
    was hoping that I could simply increase the speed by increasing the
    power delivered.

    I can drive the stators in any sequence at any rate with any amount of
    current under software control (USB).

    Is there a speed beyond which there is a fundamental limitation on how
    fast I can pulse my stators? Can I expect to be able to get the fan to
    turn faster simply by increasing power? What type of efficiency can I
    expect (electrical power to frictional atmospheric forces?) I am
    assuming that efficency losses will be in joule heating and radiation,
    so it seems that I should be able to get high efficiency. And finally,
    a big one, what is the optimum geometry of the magnetic assembly at the
    perimeter? How many magnets should I use? Is more really better?
    Should I use some sort of super cap array to deliver the pulses? What
    types of (electro) magnets should I use? Will I have to worry about
    vibration? Lubrication? Metal bearings for direct contact? d-phi-b-dt

    I know I am asking a lot, and I don't need all the answers now, just
    some hints to keep me from trying all the silly things.
    -Chaud Lapin-
  2. Paul Burke

    Paul Burke Guest

    You increase the speed of a stepper by increasing the step frequency. It
    will have a huge angular momentum, so don't try to accelerate quickly.
    But why a stepper motor? Surely you don't need to control the
    instantaneous position of a fan? I would have thought a linear induction
    motor operating on an aluminium band round the rim would have been a lot

    Paul Burke
  3. Anders F

    Anders F Guest

    You are indeed looking for trouble!
    Of course it takes power to do work (move air)... So you're right on that
    one. Fast rotation of a 2m diameter rotor: kWs - and a lot of them....
    You'll burn a lot in driver/coil losses.
    What's the application? Sounds like a no go in my ears... I'd go for more
    smaller (traditional) fans.

  4. Guest

    You increase the speed of a stepper by increasing the step frequency.
    Hi Paul,

    No, I don't need to be able to control the position accurately, but I
    would like to be able to reverse the direction. It's ok that the
    blades would have high angular momentum. As a matter of fact, I was
    thinking that it might actually be better that there is some rotational
    kinetic engergy for stability.

    My overall concern, I guess, is to get maximum efficieny in transfer of
    engery from an electrical source to the blades, either in the
    rotational kinetic enery or through forced movement of air by the
    I am going to check out the linear induction motor.

    -Chaud Lapin-
  5. Guest

    Hallo Anders,

    The application (among other things) is a wind tunnel where the input
    is free-atmosphere. Again, the radius of the fan needs to be 1 meter
    at least.

    I am mostly concerned with the efficiency of transfer of energy from an
    electrical source. Though I have near-zero experience with stepper
    motors (college work is over a decade old), it seemed intuitive that
    placing the magnetic drive at the perimeter of a large radius and
    pulsing the stators under software control at just the right moments
    could yield some very interesting scenarios. I also get the feeling
    that brief pulses delivered through low-reistance coils will minimize
    core losses. I would use standard techniques like quadrature to
    determine angular position and velocity. The key in all of this would
    be the software, not so much to force the rotor to assume a certain
    rotational pattern, but to make the rotor turns as fast as possible
    using strategically activated pulses, more or lest opportunistically.
    -Chaud Lapin-
  6. Boris Mohar

    Boris Mohar Guest

    Do some math on the angular momentum. Since you want the rotating part of
    the motor to be on the circumference, the largest concentration of mass will
    also be there which is the worst possible place. The requirement for the
    tensile strength of the blades will doom the project.


    Boris Mohar

    Got Knock? - see:
    Viatrack Printed Circuit Designs
  7. Pat Ford

    Pat Ford Guest

    Having some experience with a BIG fan wind tunnel Boris is right. I've
    worked on a large fan ( 6Mw motor, folker craft wing segments for the
    blades) You dont want the mass on the outside of something that big.

  8. Ken Smith

    Ken Smith Guest

    Remember that power goes as roughly the cube of speed. Doubling the speed
    takes about 8 times the power.
    The stator is inductive. The rotor shunts that inductance with some
    resistance. All energy you put into the inductance must be removed when
    you stop the current. You will be sloshing around large amounts of
    energy. The faster you go, the higher the losses.

    No, supercaps have too much internal resistance. You are better off
    trying to ensure that the current draw is near constant for the whole
    Very expensive LN2 cooled or LHe cooled ones.
    I wouldn't call it worry. Something closer to !!!!PANIC!!!! would be the
    right level.
    If you are going fast, air bearings are the way to go.
  9. Rich Grise

    Rich Grise Guest

    You're reinventing the "fan"?

    To heck with electronics - if you want lots and lots of air, use a RAT:
    Rotary Air Turbine.

    Good Luck!
  10. Rich Grise

    Rich Grise Guest

    Hasn't this been done? Look up "wind tunnel". Maybe even "turbine bleed
    air". I haven't checked these things, but it sounds like all you want is a
    controlled wind tunnel.

    Designing and building a stepper motor from scratch is just stupid.

    Good Luck!
  11. Guest

    You increase the speed of a stepper by increasing the step frequency.
    Well, I must say, I feel like a dummy. LIM is exactly what I had in

    Not like I thought the idea was new or anything, but all the various
    "extra" features that I thought I could add like rapidily discharching
    stored engergy using supercaps has already been done. That are several
    nice, big, fat juicy papers put out by U.S. Department of Energy on
    this subject.

    -Chaud Lapin-
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