magnetic stirrer

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.
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device
motion
stirrer
that
this.
You are right, these things have a small shaded pole motor in the base with a ceramic magnet glued to a circular steel disk. The magnet is just below the hot plate under-surface. The disk is attached to the motor shaft with a set screw to adjust the height. The motor is an inch or two below the hot plate heating element which is attached to the aluminum sub plate. The upper part of the hot plate is usually ceramic or pyroceram material (Corning).
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.
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| 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.
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wrote:

Liquid cooled / lubricated bearings comes to mind. What sort of liquid? Canned-rotor with water-lubricated bearings have been used where there is zero tolerance for leakage.
daestrom
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snipped-for-privacy@ipal.net wrote:

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.
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wrote: | snipped-for-privacy@ipal.net 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.
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----------------------------
wrote:

------------------
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).
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| ----------------------------
| 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. |> |> -- |> |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.
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snipped-for-privacy@ipal.net wrote:

Maybe shaping the rotor so that a layer of fluid keeps the rotor from touching the sides, like the heads on a hard drive? Eric
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|> 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.
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----------------------------
wrote:

--------------- 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.
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| ----------------------------
| wrote: |> |> |> 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.
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Don Kelly wrote:

Unless a solenoid effect will keep the rotor in the field? All unclear.. Eric
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wrote:

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
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daestrom wrote:

That's clever, I wonder if it would work with saltwater?
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Old news:
http://query.nytimes.com/gst/fullpage.html?res=9C0CE7DB143EF936A25756C0A966958260
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Greg Neill wrote:

http://query.nytimes.com/gst/fullpage.html?res 0CE7DB143EF936A25756C0A966958260
Which begs the question, did it work, or was it one of those great ideas that just never went anywhere due to technical hurdles?
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Well the idea works for liquid *metals*. But saltwater? It has a lower conductivity so I suspect there would be some issues there.
The liquid metal pumps have to distribute the current through the metal liquid in an even cross section. You don't just put a 'button' electrode on each side of the pipe. That results in some liquid moving but the majority of the cross section won't have a force applied so the liquid metal just 'churns' in place. You have to spread the current across the pipe's cross section so that much of the metal has a uniform current density.
To apply this on a grand enough scale for ship propulsion, well I'm sure someone is probably working on it, but getting the current distribution right would be tricky.
As far as stealth, the no-moving-part propulsion would certainly be *quiet*. But the large magnetic field that needs to be applied around the tube would make detection with a magnetometer pretty easy.
daestrom
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