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magnetic stirrer

Discussion in 'Electrical Engineering' started by [email protected], Jun 4, 2008.

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  1. Guest

    Long ago when taking chemistry I saw these magnetic stirrers. There was a
    coated (nylon, glass, or ceramic, I presume) bar magnet placed over a device
    that drove it. At the time I didn't think much of it. I just assumed a
    motor would turn a corresponding magnet inside the base. Now I wonder how
    hard it would be to make such a device operate without any mechanical motion
    inside the stirrer base, using just a changing magnetic field. The stirrer
    had an adjustable speed over a very wide range. So it would seem to me that
    a wide range motor controller wired to a set of stator coils could do this.
    What I'm most curious about would be the shape of the core(s) these coils
    would be wound around to be most efficient in this role.
     
  2. James Sweet

    James Sweet Guest


    I'm sure it could be done. I've only ever opened up one of those things
    and it had a small shaded pole motor spinning a magnet under the platter.
     
  3. Guest

    | The motor and its single coil is well below the hot zone in this design.
    | What you describe could work but would be much more problematic keeping the
    | coils cool. And, you would need several coils to create the phase difference
    | to cause rotation. Even though it wouldn't have rotating mechanical parts it
    | would likely be more complex.
    |
    | Small shaded pole motors are very cheap and readily available. They're used
    | in fans, can openers and a thousand other devices needing little power or
    | torque at a low price. It would be hard to design a non mechanical
    | replacement and probably not worth the effort. Don't forget the KISS
    | principle.

    The idea I have for this involves a "rotor" inside a pipe, instead of a beaker.
    It would be for applying some pumping pressure instead of stirring. So the
    stator would wrap around the pipe instead of being on one end. No heating
    would be involved.

    One thing I'd like to do is make sure the "rotor" does not rub against the
    inside wall of the pipe while rotating. I don't know if this can be done
    magnetically, or if it would have to apply some outward force of the liquid
    to balance the rotor in the center.
     
  4. Don Kelly

    Don Kelly Guest

    ----------------------------
    ------------------

    Rotors/axial pumps (i.e. propeller blades) inside pipes with polyphase
    stators outside have been used for many years where it is important not to
    break the integrity of the pipe- pumps for liquid sodium come to mind as an
    example(the pipe in the region has to be non conductive). The rotor /pump
    would have a frame and bearings to hold it in the center of the pipe.

    As for stirrers, you can have a stirrer for conductive fluids- simply get a
    stator from a polyphase motor and put it around the vessel (a single phase
    stator with a resistance or capacitance supplying the start winding could do
    the job. A case where this was applied was in a high power CO2 laser where
    there was a plasma discharge which had to be kept uniform and not form "hot
    spots". For non conductive fluids, put some sort of wobbly gizmo in the
    fluid. Some sort of spindle to hold it is sufficient ( A beer can simply
    dropped on the end of a paper holder spike in a rotating field will rotate
    well without trying to fly off to one side - drink the beer first as you
    want to use the opening where the pull tab is).
     
  5. Guest

    | wrote:
    |>
    |>
    |> | The motor and its single coil is well below the hot zone in this design.
    |> | What you describe could work but would be much more problematic keeping the
    |> | coils cool. And, you would need several coils to create the phase difference
    |> | to cause rotation. Even though it wouldn't have rotating mechanical parts it
    |> | would likely be more complex.
    |> |
    |> | Small shaded pole motors are very cheap and readily available. They're used
    |> | in fans, can openers and a thousand other devices needing little power or
    |> | torque at a low price. It would be hard to design a non mechanical
    |> | replacement and probably not worth the effort. Don't forget the KISS
    |> | principle.
    |>
    |> The idea I have for this involves a "rotor" inside a pipe, instead of a beaker.
    |> It would be for applying some pumping pressure instead of stirring. So the
    |> stator would wrap around the pipe instead of being on one end. No heating
    |> would be involved.
    |>
    |> One thing I'd like to do is make sure the "rotor" does not rub against the
    |> inside wall of the pipe while rotating. I don't know if this can be done
    |> magnetically, or if it would have to apply some outward force of the liquid
    |> to balance the rotor in the center.
    |
    | There are magnetic bearings that can do the job. Also, you might be able
    | to use conventional bearings, lubricated by the working fluid. Depending
    | on what this fluid is, of course.

    My understanding is that magnetic bearings generally require some kind of
    sensor that tells the control circuit where the rotor is in the radial
    direction so the control circuit can adjust the fields to keep it in the
    correct position at the axis. With the rotor inside of a pipe, I won't
    have that option. So I would either need some kind of passive magnetic
    system, or something based on applying some liquid pressure to the outer
    edge in the radial direction, as it rotates.
     
  6. Guest

    | ----------------------------
    | |>
    |> | The motor and its single coil is well below the hot zone in this design.
    |> | What you describe could work but would be much more problematic keeping
    |> the
    |> | coils cool. And, you would need several coils to create the phase
    |> difference
    |> | to cause rotation. Even though it wouldn't have rotating mechanical
    |> parts it
    |> | would likely be more complex.
    |> |
    |> | Small shaded pole motors are very cheap and readily available. They're
    |> used
    |> | in fans, can openers and a thousand other devices needing little power
    |> or
    |> | torque at a low price. It would be hard to design a non mechanical
    |> | replacement and probably not worth the effort. Don't forget the KISS
    |> | principle.
    |>
    |> The idea I have for this involves a "rotor" inside a pipe, instead of a
    |> beaker.
    |> It would be for applying some pumping pressure instead of stirring. So
    |> the
    |> stator would wrap around the pipe instead of being on one end. No heating
    |> would be involved.
    |>
    |> One thing I'd like to do is make sure the "rotor" does not rub against the
    |> inside wall of the pipe while rotating. I don't know if this can be done
    |> magnetically, or if it would have to apply some outward force of the
    |> liquid
    |> to balance the rotor in the center.
    |>
    |> --
    |> |WARNING: Due to extreme spam, googlegroups.com is blocked. Due to
    |> ignorance |
    |> | by the abuse department, bellsouth.net is blocked. If you post
    |> to |
    |> | Usenet from these places, find another Usenet provider ASAP.
    |> |
    |> | Phil Howard KA9WGN (email for humans: first name in lower case at
    |> ipal.net) |
    | ------------------
    |
    | Rotors/axial pumps (i.e. propeller blades) inside pipes with polyphase
    | stators outside have been used for many years where it is important not to
    | break the integrity of the pipe- pumps for liquid sodium come to mind as an
    | example(the pipe in the region has to be non conductive). The rotor /pump
    | would have a frame and bearings to hold it in the center of the pipe.

    I'm looking at doing this without any frame/bearing inside. So I need to
    find some means to keep the rotor steady inside the pipe. It needs to stay
    away from the inside wall of the pipe, as well as avoid moving along the
    pipe.


    | As for stirrers, you can have a stirrer for conductive fluids- simply get a
    | stator from a polyphase motor and put it around the vessel (a single phase
    | stator with a resistance or capacitance supplying the start winding could do
    | the job. A case where this was applied was in a high power CO2 laser where
    | there was a plasma discharge which had to be kept uniform and not form "hot
    | spots". For non conductive fluids, put some sort of wobbly gizmo in the
    | fluid. Some sort of spindle to hold it is sufficient ( A beer can simply
    | dropped on the end of a paper holder spike in a rotating field will rotate
    | well without trying to fly off to one side - drink the beer first as you
    | want to use the opening where the pull tab is).

    So I think my problem will be holding the rotor steady when it cannot come
    into contact with the pipe wall while rotating.
     
  7. James Sweet

    James Sweet Guest


    That's clever, I wonder if it would work with saltwater?
     
  8. James Sweet

    James Sweet Guest

  9. Roy

    Roy Guest

    From: (James Sweet)
    daestrom wrote:
    "Don Kelly" <>
    ----------------------------
    | The motor and its single coil is well below the hot zone in this
    design.
    | What you describe could work but would be much more problematic
    keeping the
    | coils cool. And, you would need several coils to create the phase
    difference
    | to cause rotation. Even though it wouldn't have rotating mechanical
    parts it
    | would likely be more complex.
    |
    | Small shaded pole motors are very cheap and readily available. They're
    used
    | in fans, can openers and a thousand other devices needing little power
    or
    | torque at a low price. It would be hard to design a non mechanical |
    replacement and probably not worth the effort. Don't forget the KISS |
    principle.
    The idea I have for this involves a "rotor" inside a pipe, instead of a
    beaker.
    It would be for applying some pumping pressure instead of stirring. So
    the
    stator would wrap around the pipe instead of being on one end. No
    heating
    would be involved.
    One thing I'd like to do is make sure the "rotor" does not rub against
    the
    inside wall of the pipe while rotating. I don't know if this can be done
    magnetically, or if it would have to apply some outward force of the
    liquid
    to balance the rotor in the center.
    --
    |WARNING: Due to extreme spam, googlegroups.com is blocked. Due to
    ignorance |
    |         by the abuse department, bellsouth.net is blocked. If
    you post to |
    |         Usenet from these places, find another Usenet provider
    ASAP. | | Phil Howard KA9WGN (email for humans: first name in lower case
    at ipal.net) |
    ------------------
    Rotors/axial pumps (i.e. propeller blades) inside pipes with polyphase
    stators outside have been used for many years where it is important not
    to break the integrity of the pipe- pumps for liquid sodium come to mind
    as an example(the pipe in the region has to be non conductive). The
    rotor /pump would have a frame and bearings to hold it in the center of
    the pipe.
    ISTR liquid sodium pumps didn't have any moving parts inside. Some
    electrodes passed a current through the sodium while a magnetic field
    was established outside the non-conductive pipe. Sort of a linear motor
    with the sodium being the 'conductor'.
    daestrom
    That's clever, I wonder if it would work with saltwater?
    ----------------------- I'll Say :)
    Think it will work with liquor ? Perhaps., though in the case of
    liquor., fluids ability to ignite is made apparent., with enough current
    going through, it could ignite.
    [the process must be enclosed & properly vented for gas emissions or
    steam] Right?

    Roy Q.T. ~ US/NCU ~ E.E. Technician
    [have tools, will travel]
     
  10. Guest

    |> So I think my problem will be holding the rotor steady when it cannot come
    |> into contact with the pipe wall while rotating.
    |>
    | Maybe shaping the rotor so that a layer of fluid keeps the rotor from
    | touching the sides, like the heads on a hard drive?

    Something like that might work.
     
  11. Guest

    |> Rotors/axial pumps (i.e. propeller blades) inside pipes with polyphase
    |> stators outside have been used for many years where it is important not to
    |> break the integrity of the pipe- pumps for liquid sodium come to mind as
    |> an example(the pipe in the region has to be non conductive). The rotor
    |> /pump would have a frame and bearings to hold it in the center of the
    |> pipe.
    |>
    |
    | ISTR liquid sodium pumps didn't have any moving parts inside. Some
    | electrodes passed a current through the sodium while a magnetic field was
    | established outside the non-conductive pipe. Sort of a linear motor with
    | the sodium being the 'conductor'.

    OTOH, the fluids I might be using (water, antifreeze, or oil) will not conduct
    very well.
     
  12. Don Kelly

    Don Kelly Guest

    ----------------------------
    ---------------
    Why not a simple frame to hold the axle centered. It need not be any more
    than a few "spokes" which will have minimal interference with the flow.
    KISS.
    Note that anything that depends on the fluid flow or layers will not keep
    the rotor free from the pipe on starting (unless you repeal the law of
    gravity) so after a few starts, bits of the impellor will be floating
    downstream.
     
  13. Guest

    | ----------------------------
    | |>
    |> |> So I think my problem will be holding the rotor steady when it cannot
    |> come
    |> |> into contact with the pipe wall while rotating.
    |> |>
    |> | Maybe shaping the rotor so that a layer of fluid keeps the rotor from
    |> | touching the sides, like the heads on a hard drive?
    |>
    |> Something like that might work.
    |>
    |> --
    |> |WARNING: Due to extreme spam, googlegroups.com is blocked. Due to
    |> ignorance |
    |> | by the abuse department, bellsouth.net is blocked. If you post
    |> to |
    |> | Usenet from these places, find another Usenet provider ASAP.
    |> |
    |> | Phil Howard KA9WGN (email for humans: first name in lower case at
    |> ipal.net) |
    | ---------------
    | Why not a simple frame to hold the axle centered. It need not be any more
    | than a few "spokes" which will have minimal interference with the flow.

    If the frame is part of the whole mechanism, and not affixed to the inner
    wall of the pipe, maybe that can be done.


    | Note that anything that depends on the fluid flow or layers will not keep
    | the rotor free from the pipe on starting (unless you repeal the law of
    | gravity) so after a few starts, bits of the impellor will be floating
    | downstream.

    At startup it won't matter (much). At full speed I want it not contacting
    the wall. But even at stop when it is contacting the wall, it cannot be
    affixed to it. It has to be able to slide out and leave the wall intact.
     
  14. Don Kelly

    Don Kelly Guest

    ----------------------------
    -------------
    If it is free to slide, then it will. The result will be that the rotor
    slides "upstream" rather than pumping fluid "downstream" This is not
    desirable as not only will pumping efficiency drop but the rotor might move
    out of the driving field. You have to somehow anchor the rotor in the pipe
    and transfer reaction forces to the pipe.
     
  15. Don Kelly

    Don Kelly Guest

    ---------------------
    There is a solenoid effect (as in a conventional motor) but remember that
    the full pumping force equals the reaction force and this, I expect, will
    far exceed any expected solenoidal forces involved. Possibly a design could
    increase the solenoidal force but there are easier alternatives. --

    Don Kelly
    remove the X to answer
    ----------------------------
     
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