magnetic stirrer

[[email protected]]

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.

 

[James Sweet]

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.

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.

 

[[email protected]]

| 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.

 

[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.

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).

 

[[email protected]]

| [email protected] 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.

 

[[email protected]]

| ----------------------------
| |>
|> | 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.

 

[James Sweet]

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?

 

[James Sweet]

Old
http://query.nytimes.com/gst/fullpage.html?res=9C0CE7DB143EF936A25756C0A966958260

Which begs the question, did it work, or was it one of those great ideas
that just never went anywhere due to technical hurdles?

 

[Roy]

From: [email protected] (James Sweet)
daestrom wrote:
"Don Kelly" <[email protected]>
----------------------------
| 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]

 

[[email protected]]

|> 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.

 

[[email protected]]

|> 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.

 

[Don Kelly]

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

|> 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.
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.

 

[[email protected]]

| ----------------------------
| |>
|> |> 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.

 

[Don Kelly]

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

| ----------------------------
| |>
|> |> 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.

-------------
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.

 

[Don Kelly]

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

---------------------
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 [email protected]
remove the X to answer
----------------------------