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Author: Subject: DIY magnetic stirrer
Rosco Bodine
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[*] posted on 20-2-2006 at 16:15


Quote:
Originally posted by 12AX7
Quote:
Originally posted by Rosco Bodine
Anyway , if you got something better for the job ,
well I've showed you mine , ........

So now show me yours ,


In the other thread, someone already posted one as good as I would come up with. Simulation results were even provided.

Tim



Aside from showing a 1.5 watt control pot which by itself
costs more than my whole project , and having too low a voltage rated Mosfets , no surge or static protection , and no startup pulse logic , no level control for defining the operating parameters , as well as requiring complementary N/P Mosfets ......it's just wonderful .

If that's the best you could do .....

then why do you have a low opinion of the practical design shown here which is sorted out in all its specifics and ready for the soldering and testing ?

[Edited on 21-2-2006 by Rosco Bodine]
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[*] posted on 22-2-2006 at 03:17


Here is the final revision schematic ( I hope ) . The graphic is cleaned up a bit and I added a couple of resistors and tweaked the values for better noise immunity . An added resistor across the varistor is something of a safety device , since without it , and no load on the output , a meter test or probe might show no voltage across the output , if it was a high impedance probe .

The circuit which energizes the gate and turns on the output mosfet is completed through the load , so without some sort
of lower impedance " dummy load " there across the output continuously , the " pseudo " open circuit might appear to instruments not to be energized when it actually would be very quickly energized if it was touched by a finger for example :D So I figured it was safer to have the output unambiguously energized any time the circuit is powered up , and put a small load across the output to assure this .

Most of the parts for the project are on hand now and the few remaining parts I should have soon . I have some
other business to catch up while waiting on the remaining parts to arrive , and will probably take a break from this
work before beginning the construction of the prototype ,
since I have reached a " stopping point " with the completion
of the theoretical model . The apparent simplicity of the finished schematic for something like this is very deceptive about the amount of mental work which goes into sorting out and specifying the details of such a design , especially when the design is unconventional . An electrical schematic
is inherently a complex mathematical expression , and
it has been plenty of mental exercise developing this idea
for what seems so simple now in hindsight . I look at this
schematic and realize it took me three weeks to sort this
idea out and get the design down on paper in a buildable
specification . It may not be perfect , it is experimental .
But it is good enough of a theoretical model for me to proceed with the build using this as my " blueprint " .

Anyway , I deserve a break now and I am taking it :D

[Edited on 17-5-2007 by Rosco Bodine]

AC Power Handler for Magnetic Stirrer Motor experimental prototype schematic final resize.jpg - 68kB
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Rosco Bodine
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[*] posted on 23-2-2006 at 20:48


Today I found a way to get a quality color graphic file
of compact size to export in PDF format from my
ECAD software and am attaching it here because of
the usefulness of the red highlighting which shows
the interconnections between discrete components .
This attachment can be high resolution printed in
color or in black and white if you specify a black
and white only output in your printer properties .

Sorry for all the bulky JPEGS posted earlier . Since
there is nothing in the ECAD help files about doing
any sort of exporting of the ECAD output file as
JPEG or PDF file types , I had to improvise some
way to share the file content , which involved
printing , scanning , and image editing before .

But today I found that I could use a freeware
" virtual printer " from Adobe called Acrobat PDF Writer
to " print to file " ( a .pdf file ) from the print command
in my ECAD which by default only prints a hard copy
on paper , .....except to create a PDF , I just select
a different " printer " to send the " print job " ......
which is the " Acrobat PDF Writer " , and it makes
the PDF file that I am needing . A very handy little
freeware tool from Adobe ....as good for PDF filemaking
as is Acrobat Reader good for viewing those files .

This color graphic ECAD output file as PDF is higher resolution and only one quarter the size of some of the black and white scanned JPEG files , which were a headache to get
clear at any reasonable file size .

Attachment: AC Power Handler for Magnetic Stirrer Motor experimental prototype schematic final.PDF (26kB)
This file has been downloaded 1452 times

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[*] posted on 23-2-2006 at 22:15


Rosco,

An easy way to get any image into a file is:-
Get the image you want on screen in the application it was developed in at the size that you want it.
Press the key marked "Prt Sc"
Go into your favorite image editing package.
Start a new image.
Past the clipboard into the image.
Crop and resize as required.

Prt Sc copies what is on the screen to the clipboard.

Mike.
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Rosco Bodine
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[*] posted on 23-2-2006 at 22:58


I have a screen capture and printing utility called SnagIt
which does something like what you are talking about and it will output various file types . I tried it but the file size required for achieving comparable resolution just doesn't come near to the small file possible with a good direct output to Acrobat pdf . There is also a problem when the aspect ratio for the image is optimized for printing on an 8.5 X 11
sheet of paper , rather than for the screen . But the print
to PDF takes care of that sizing and optimizing automatically .

Open that file I just posted above and use the enlarge
button to zoom in and magnify the page to maximum ,
which is 16 X normal view , 16 diopters or 1600% magnification ......and the image quality holds
perfectly clear and precise , which is amazing for a
a 26KB page file . That is only possible because the
PDF file is largely composed of the actual PostScript
descriptor language which is the " raster file " for
screen or printer and the efficiency and precision
is unmatched by anything else ....precisely why
PDF and PostScript are the industry standards .

I may have spoken in error about the Acrobat PDF Writer
being freeware , I think it is the PostScript Virtual Printer that was the actual freeware and , the Acrobat PDF Writer is bundled as a feature of the Acrobat Pro Version , which I have .

[Edited on 24-2-2006 by Rosco Bodine]
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[*] posted on 19-4-2006 at 20:59
update on prototype stirrer


The experiments which I have been doing lately
involves evaluations of several PSC motors to see
what is their variable speed performance , when
supplied with a variable voltage and driving a
fan load , which is a quadratic type load similar
to the load which will be felt by the same motor
used in a stirrer . For these bench tests I have been
using an ordinary variac as the power supply ,
as I have not yet built the solid state power supply
described in the attached schematic , which is one
of a collection of several solid state controls which
will be evaluated . For these tests I have been
looking at the PSC motor performance , and it appears
good enough that a variety of different controls may
be useful and provide satisfactory performance , the
choice depending upon how critical is the application .
The power supply type is likely not so critical as is the matching
of the motor to the driven load so that a very smooth speed increase
is produced in response to the increasing power input .
The speed decrease should
likewise track nicely with decrease in power input .

I have tested more than a half dozen different PSC motors
of various power and voltage ratings from 1/20 to 1/3 hp .
Guess which one I like :D , that's right the big one :D
The motor selected as having characteristics which fit
my best guess as a candidate motor for the prototype
is a 1/3 hp 230 volt 1075 rpm motor which was originally
built for use as a condenser fan motor for an HVAC
application . But the motor will be run in highly derated
fashion , across the voltage range of perhaps 15 to 110 volts ,
so the actual plate rating of the motor is something
like 4 times the actual maximum loading which the motor will
ever be run in the " off label " use which I am performing .
It is a 48 frame motor with a 1/2 inch shaft .

One of the things I quickly discovered from testing motor
performance is that the " dynamic range " of the motor
performance is extremely important , that is the greater
the range of operating voltage possible for the motor ,
the better the motor will perform , and some motors are
better than others in that regard . A good candidate motor needs
to be able to start reliably and run at about 13% of its rated voltage
without any further modding to enhance its low end performance .
After certain changes are made , like increasing the capacitor value
by about 66% over the plate value used for conventional fixed speed high efficiency ....
the minimum starting voltage for the motor can be decreased
by about one third below the stock performance .
The precise capacitor value which is optimum
must be determined by carefully testing
and comparing and charting the results for a particular motor ,
recording the current draw and voltage incrementally
across the operating speed range for the motor both unloaded and loaded ,
on the stall slope speeds particularly , but also looking at what happens if the speed increases into the normal operating range where the counter-EMF or " regeneration " voltage becomes a factor and excessive capacitor current could develop .
It is a tedious process selecting the optimum capacitor value for
the particular application . PSC motors which seem to
fit the requirements best have larger than average capacitor values for their hp rating in usual stock configuration and
this is some indication of a more hefty auxilliary winding ,
a motor which will have a higher starting torque and stall torque by design than the average motor . These motors
will also have a bit lower ultimate efficiency than motors
designed to labor less and accellerate more slowly up to
their near synchronous operating speed where the main winding then does most of the work .

A factor which I have discovered has bearing on the minimum starting voltage for the motor is the static friction
of the bearings and the lubricant film on which the weight of the rotor is resting . It is something like the effect of a sled
sitting on the snow , it requires an extra force to breakaway
the runners sitting at rest , due to " adhesion " forces , but
once that initial " slippage " occurs ...away it goes in motion .
To eliminate the weight of the rotor on the thrust bearings
which is causing this adhesion , I am installing a magnetic thrust bearing consisting of ring magnets around the exit shaft which will levitate or nearly levitate almost all of the rotor weight , so that the only remaining resistance to rotation on starting is simply the viscosity of the oil film on the bearings . This should reduce the starting power for
the motor from its stock configuration minimum of about 15 Watts input , to less than 5 Watts being required for the motor to reliably start and run without stalling at very low speeds which would otherwise not be possible without
using a magnetic support bearing . I will provide more
information on what are the results of my tests of the
magnetic support bearing , when this part of the prototype
is completed . I have the magnets for the bearing and also
have the magnets for the driven rotor , but the mountings
have not yet arrived for these magnets , and I will update
the progress after these parts arrive .

The motor in derated operating mode appears to be
still operating in excess of 50% efficiency at the shaft while
driving a 16" four blade fan load , and observing the
ammeter dropoff as the rpms reach past 80% of synchronous
speed .....so the top end penalty on efficiency is under thirty watts from optimizing the low end and stall slope speeds performance . The input power and speed response
is very linear across the entire stall slope rpm range ,
and the practical range is roughly from 6 Watts to 175 Watts
or taking 50% of that at the shaft as actual mechanical
output ....the rest being dumped easily as heat .
A large blower wheel will be used as a fan canopy to
provide cooling for the motor . And the drive magnet
armature is a 5" length of 1" wide and 5/8" thick barstock
on which are mounted each end N40 block magnets which
are 2" long by 1" wide by 1/2" thick , doubling this thickness
by pairs if required . The planned configuration will allow
an actual delivered horsepower at the drive magnet
across the range from .004 to .117 hp , a 29.3 to 1 control range ,
or from 3.4% to 100% output as real power . If this
motor to load match is accomplished as predicted , then no
elaborate power control schemes will be required for open loop operation which would be adequate for non-critical
applications . The motor has enough torque to pull any
hill without needing a cruise control and without laboring too much under any but extreme viscosity change conditions ,
ordinary speed variations should be minimal even without
any active feedback speed control . The starting torque of
the motor is also sufficient that it appears that nothing
beyond an ordinary variac having its minimum voltage pinned
to a fixed limit for starting , or an adjustable minimum starting voltage relay added to the motor ....is all that would be needed in the way of a power supply :D

What could you do with this stirrer ? Stir anything from
a 500 ml beaker ......up to a 200 liter barrel :D ,
even coupling through the bottom of a hardshell mantle
sitting on top . I am intending to establish a new standard
for " heavy duty " :D

The thing may not have to go on casters , but it is
moving that direction weightwise .....it looks like
maybe 35 pounds for the whole shebang . Ever noticed
how it seems like any equipment that is worth a damn
seems to have a bit of heft to it ?
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Rosco Bodine
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biggrin.gif posted on 29-4-2006 at 12:42
The magnetic support bearing works !


The mounting hub for the magnetic support bearing
arrived the other day and has now been installed on the
motor shaft . I am doing the first testing today ,
and it is already confirmed that my theory was correct
about the weight of the rotor itself and its static friction
on its thrust washer bearing , being a large determinant
for the minimum starting voltage , the breakaway voltage
for the motor . Evidently half or more of the static friction
which must be overcome by torque from the stator field magnets to cause the rotor to breakaway into rotation , is indeed friction from thrust load present simply from the dead weight of the rotor which weighs several pounds .

Without the magnetic support bearing , the minimum
positive reliable starting voltage was 28 Volts @ .70 Amperes
which is 19.6 Watts of minimum reliable starting power .

With the magnetic support bearing installed and adjusted to
levitate the rotor , and driving the same test load , the
minimum positive reliable starting voltage reduced to
16 Volts @ .41 Amperes which is 6.6 Watts of input power ,
only one third of the previous input power required as
a minimum reliable starting power :D

Quite a difference , and this is important for the slow speed performance . The dynamic range from minimum power
input to maximum power is tripled (2.96) , simply by the addition of the magnetic support bearing .

It appears certain that indeed a variac alone would be
sufficient as a power supply for an open loop speed control
which would have adequate speed stability for any but
the most exacting sorts of applications . I mentioned this earlier and it appears that simply pinning the lower limit of
the variac or using a set voltage tripped relay would be
sufficient for setting a minimum powered on voltage ,
but this is not really essential as protection since the input
power is so low , it doesn't matter if the motor is stalled
or not ....it likely wouldn't overheat and has an auto reset
thermal breaker in the motor for protection anyway .
At higher power settings , some sort of " stall detection "
switch could be used to detect any abnormal condition like
could occur in stirring a thickening slush of heavy crystals
which might cause a problem . Perhaps some sort of
motion detector or airflow sensor input combined with
a voltage comparator , and a settable delayed response
relay could be configured as an automatic breaker for
the stirrer when it is left operating unattended overnight
for example . This could automatically shut down the unit
if a stall or abnormal condition occurred . It is not
certain to me that this sort of added circuitry is essential ,
given that the thermal protection breaker in the motor itself
would also function in such circumstances , although more
slowly in its response , it would still get the job done .

Anyway , the motor performance on the bench looks
good at this point with the addition of the magnetic support bearing , and the AC power simply being supplied by an ordinary variac .

The motor is an A.O. Smith #175 1/3 hp 1075 rpm 230 Volt
motor , being run at 16 to 125 Volts . The endshields were
removed and supported on phenolic blocks , and a 1/8" pin punch and hammer were used to punch out the 13 vent slugs
on each endshield , to allow for cooling airflow . The capacitor value for reduced voltage operation and improved
performance for this niche application is changed to 12.5 uF
at this designers specification , after much testing has shown
that value optimum , as opposed to the 7.5 uF value which
is the manufacturers recommendation for rated voltage
operation at rated output and rpm ....which is of course
irrelevant to this application . I have found this capacitor
value change to be about the ballpark proportional change desirable for operation of a motor in a derated scheme of
approximately half the rated voltage . It may vary a little
either way depending upon the amount of asymmetry present about the auxilliary winding versus the main winding , for optimizing lower voltage and stall slope performance , and should be tested by trial and error
carefully charting the performance , and watching for
the power efficiency " sweet spot " for a given motor on
the stall slope , without undue circulating current losses
at the higher speeds and unloaded speeds .

The ring magnets are .51" ID X 1" OD X 1/4" thick N40 .
A sheet of .005 brass shim stock was wrapped around
the .4998" motor shaft to align the ring magnet concentrically
around the shaft , a very small amount of silicone grease
applied as a bore release agent and the ring magnet epoxied
to the bearing housing facing where the shaft exits the motor . When the epoxy was set very firmly , but not yet
completely cured , the shim was pulled out leaving the ring
magnet positioned with a .005" clearance fit between
the sides of the shaft passing through the opening
in the ring magnet .

A second identical ring magnet was mounted against the
face of a machined steel hub having a 1/2" bore and 2 set screws spaced 90 degrees . A 1/2" drill bit shank was
inserted through the hub whose bore was lightly greased
and the brass shim was again used to assure concentric
alignment of the ring magnet , epoxied to the hub , being
certain to position the like pole of the exposed face of the ring , to correspond with the exposed like pole of the
first magnet mounted on the motor . Again , when the epoxy had set firmly enough that the magnet would not move , but was not yet fully cured , the shim was pulled
out leaving the magnet aligned concentrically with the
bore of the hub , in a way that would also assure alignment
of the faces of the two ring magnets in operation later .

After the epoxy set completely , the hub mounted magnet
was placed on the motor shaft and moved downward until
there was about 1/4" air gap between the faces of the
repelling rings , the gap adjusted until the repulsion force
levitated the weight of the rotor , and the set screws tightened to fix the position for the rotating magnet which
turns with the shaft above the stationary magnet on the face of the motor housing . The rotor can be seen to be
floating in the slight shaft endplay distance when the shaft is
pushed down by fingertip pressure on the end of the shaft ,
or lifted up slightly , and the gap can be adjusted very easily to levitate the weight of the rotor , so it has a little endplay
both up and down as it is levitated by the magnetic bearing .
It is a very slick setup :D The motor manufacturers should just build 'em this way to begin with , and save me the trouble of putting them right :D But the magnets are sort
of pricey at about ten bucks apiece which makes for an expensive bearing , but one that will never ever wear out :D

To use this kind of magnetic bearing requires about 3/4" of unflatted clear round shaft emerging from the motor , before any milling cuts for the flats on the shaft , some motors will
have almost no clear round shaft length , but a few will
meet the requirement .

There have been concerns expressed by the motor manufacturer about the potential for sleeve bearing damage
in a motor running less than 500 rpm due to the failure of the oil to circulate from the slinger washer . I am unsure
about how much or little problem this will prove to be for
a motor having babitt sleeve bearings with oiler wicks ,
so long as it is kept well oiled and not used continuously
at only low speed .....my opinion is it won't be any problem
for an intermittent duty application , but maybe after a month of continuous windmilling yes it could be a bearing would go dry . The cure is replacement with sintered bearings if the dry bearing concern should ever become an issue for the solid sleeves . A ball bearing motor can be used as is , and depending on the quality of the bearings ,
may not even need the magnetic support bearings to
start at an acceptably low voltage . The minimum starting voltage is not a figure that is published by the manufacturers
which would simplify things greatly . But the fact is that
the manufacturers don't really want to admit that PSC motors driving a quadratic or " fan load " can be speed controlled using a variac . Why tell people who could be their customers for a six times more expensive ECM motor ?
So it is worth testing any ball bearing motors which might seem likely candidates , you might find the one that does
exactly what the manufacturer knows it will do but will never tell you about . In fact they will lie to you about it before they will level with you about it .

Cooling could be supplied by a narrow blower wheel inverted to enclose the end of the motor which will just slip inside it , so that the wheel draws its air through the motor .
I am unsure that this rather extreme measure will even
be needed . The rotor has some small vanes on its ends
which create a bit of airflow , and at low speeds there is very
little power dissipation which the motor housing just seems to heatsink without even feeling warm to the touch .
It may be that the main magnet rotor can simply have some
aluminum blade appendages fastened to it and this will
provide ample air circulation for cooling , as a sort of
paddle fan assembly .....and I will probably try this first
since indications are it will be sufficient and simplify things .

A tripod body band mount will probably be used to mount the motor to a heavy baseplate with rubber feet . The rotating parts will be dynamically balanced so the stirrer runs smoothly and quietly .

Everything seems to be right on track at this point , with no
surprises really except that the motor seems to have better performance than was expected ....which greatly simplifies
the speed control scheme . This thing could end up requiring
absolutely nothing in the way of solid state electronic control , simply for the good balance of mechanical components ......being a nuts and bolts project versus any control circuitry problem at all . I won't know for sure on that score until I actually have a drive magnet armature on the end of the motor shaft spinning closely underneath an aluminum plate , and driving some stirbars through their test paces . But so far my guesses have been on track and
my guess is that the eddy current and stirrer loads will
behave about the same as the 16" four blade fan load which
has been my test load on the bench ......in which case
this thing should work beautifully . The geometry of the
drive magnet is one thing I hope I get right first try ,
because I really don't want to get into six months of experiments with field shaping and coupling fine points
for magnet and armature combinations to find what works right .
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[*] posted on 29-4-2006 at 20:31


You know what we want Rosco -- PHOTOS!



\"One of the surest signs of Conrad\'s genius is that women dislike his books.\" --George Orwell
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Rosco Bodine
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[*] posted on 29-4-2006 at 23:23


Yeah okay , I'll take some digital pictures of the
setup later . I had planned on doing the pics when the
assembly is near to the end stages and I have the
final components . It has been a spread of parts
and wires to this point , nothing really much to show
that is worth a photo presentation and I wanted to make sure it all works before posting the bragging pictures .
It's pretty straightforward , but I know
folks want the pics if nothing else to prove I'm not
just making all this stuff up :D Hey trust me it's for real .
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[*] posted on 10-5-2006 at 16:08
PWM and triacs


Hi,Quince. I tryed the PWM + triac solution some time ago and it didint work. The reason is that triacs needs a pulse of at least 0,2 ms to go on and will shut off on the next AC zero cross. I´m working now on a zero cross detector circuit (usin PIC microcontrolers) to phase control triacs. With PWM you going to end up with erratic triac firing (just put a lamp bulb and watch the control ou get!)
As soon as I find a solution I´ll post it here

armo
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[*] posted on 10-5-2006 at 16:17


I'll tell you that there is for some reason more
heating loss and less mechanical efficiency ,
as well as a low power hum , and less smoothness
and linearity to the mechanical power when
the same PSC motor is controlled by an ordinary
triac " light dimmer " type of speed control .......
compared to the smooth and efficient
pure sine wave power from a variac which
works beautifully .

[Edited on 11-5-2006 by Rosco Bodine]
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[*] posted on 10-5-2006 at 16:34


Hi Tacho,
Some time ago I found on the net a site about using stepper motors. I think it was Jones on step motors. In this site he teaches how to use damaged hds. May be you can just use the spindle as a small centrifugue.
armo

Quote:
Originally posted by Tacho
I tried to make a driver circuit for the spindle motor, but it didn't work. I used variations on stepper motor circuits, but it seems that this motor works with positioning sensing, probably written on the magnetic disk.

I posted a circuit to drive a stepper motor sometime ago. It's a bit complex because I wanted do use TTL logic and have reverse spinning. One could make a much simpler driver circuit for steppers using a 555, one CMOS chip and a ULN2003 chip.
I may do it one of these days.

I like the centrifugue idea. For the tubes I would use 2 x 5ml syringe barrels. Maybe just two wire loops passed through holes drilled on the disk would work as the swivel (sp?)joints. Humm...

[Edited on 1-6-2005 by Tacho]
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[*] posted on 10-5-2006 at 16:49


armo, that's because I didn't use triacs. I used a floating gate bidirectional MOSFET switch, and tcouple of kHz). That gives me fully linear control; no worries about phase or anything. The only thing here is that the gate supply and driver has to be floating, so I used a second small transformer in order to do that (it's referenced to the AC line instead of ground). The PWM circuitry feeds the floating gate driver through a standard optoisolator.

I no longer have schematics since my HD crash.

[Edited on 11-5-2006 by Quince]




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[*] posted on 16-5-2006 at 22:19


Update : The X269 Marathon PSC motor

A specific motor recently tested appears to have speed control characteristics which make it an excellent choice
right out of the box , with one minor modification which is
easily done . It is a ball bearing fan motor , which can be easily modified for stirrer duty by reducing the thrust preload force on the ball bearings to about one third
to one fourth of the preload used in the motor as it ships from the factory . There is inside the bearing housing a
wave washer which places a thrust preload on the ball bearings to limit shaft endplay and takeup any clearance
in the ball bearings themselves . The wave washer spring is stiff enough to also oppose the thrust load of a fan blade which would in the intended use be pulling
on the shaft . The spring is pretty stiff and causes too much static friction on the bearings for good low voltage starting and low power operation . The factory assembled preload
tension is quite high , possibly 20 kilos , and this really causes sluggish starting at reduced voltage . For example in
the stock configuration the motor requires 46 volts @ .3 amp which is 13.8 Watts of input power to start , and
following breakaway into rotation it gradually spins up
accelerating to its rated slip from synchronous speed
at 1625 rpm . Removing the wave washer completely
and eliminating any thrust preload on the ball bearings
and retesting , the mimimum starting voltage was
reduced to 15 volts @ .060 amp which is 0.90 Watts ,
Yes less than 1 Watt of input , and the motor runs
stable at about 50 rpm , speed controlling beautifully
from a variac when driving a fan load . However it
was found that eliminating all of the preload was
unacceptable because of bearing vibration at certain speeds . A much lighter force of preload on the ball bearings
has the effect of quieting them in operation without
adversely increasing the starting voltage . After
flattening the stock wave washer and rebending it
to a lower curvature and lighter tension , it was reinstalled . The adjusted thrust force required for
quieting bearing vibration , raised the starting voltage
to about 19 volts @ 0.085 amps or which is 1.6 Watts
and again after starting the motor was well behaved
in speed response , which seems to hold true at anything
below 5 watts input unloaded . This area of low power
starting and operation seems to be a region where the
motor is actually working most simply to stir against the viscosity of the lubricants in the motor bearings which
prevents the motor from simply climbing the torque
slope and accellerating on up to synchronous speed
even with no other load whatsoever . The bearings are high enough precision that the presence of the added weight of a magnet rotor or fan seems to only increase the minimum starting voltage by less than a half volt above the unloaded figure , essentially no difference . Ball bearings are not dead silent like sleeve bearings . There is a slight hum or whizzing sound especially at high speed operation , a slight zishing sound almost like
escaping air from a slightly cracked open valve . Maybe
there are ball bearings that are dead quiet but I haven't found them yet . But these are quiet enough I think
for instrument use , with the reduced preload . No really obnoxious rattle or whine was noted , but more of a soft
mechanical low pitched hum with a harmonic shift at
a couple of speeds as is typical even for sleeve bearings .

One thing I also did was put about five drops of ashless
two-stroke oil at the gap in the bearing shields to slightly
thin the gluelike grease which the bearings are fully packed with from the factory . The stock grease is an extremely sticky mineral-polyurea that is as thick and
sticky and viscous as almost cured gasket cement or
epoxy putty ....more like a glue than a lubricant . This
is the sort of grease you could fling a golf ball sized
lump against a window and come back a few years later
and it would still be hanging there , maybe sagging
slightly from where it was . It was so thick I knew a bit of oil would help loosen it a bit which was needed for the tests I was doing and for the few hours break-in running .

Different capacitor values were tried and charted
and I was pleasantly surprised to find that the motor
designer at Marathon had indeed already worked out
the most efficient combination of capacitor value and
assymetry ratio for the way the motor was wound ,
and the best compromise value of capacitor for both
speed control and power efficiency is clamped to the
case of the motor from the factory . The stator coils
have been laced in tight bundles and the stator has
been varnish dipped and baked . The rotor has been
dynamically balanced . And even though the output
shaft is a half inch single flatted shaft , it is a turned
down exposed section of the actual rotor shaft which is ~17mm journaled in shielded ball bearings which appear
to be 40mm OD units , supported in machined
aluminum bearing housings cut directly into the cast aluminum endshields . The case and endshields are
themselves a heatsink , and there is an automatic reset
thermal breaker . The motor frame is 48Y , 5 5/8" dia.
The motor appears to be a jewel of quality hand
built craftsmanship , carrying a little blue label ,
" built with pride , made in USA "
and it looks like it :D

This is a motor where you can adjust the bearing preload to a reduced tension to clean up the low end low power performance and quieten it for instrument duty use as described , put a balanced rotor magnet assembly on it and power it from a simple variac .....
and it will probably run for ten years continuously , or intermittently for a hundred years , re-bearing it and keep right on going like it was brand new all over again
for the ones who inherit it long after you are gone .
Built like a bank vault as the old saying goes .

So it looks like this particular motor may be one of those
elusive matches which I was saying I believed was
possible early on in the process of considering a PSC
motor in open loop speed control operation for a
magnetic stirrer . I am glad to have found this one ,
which does seem to be a good match of an existing motor to the intended task , without any exotic
accomodations or modifications . The motor is
rated 1/6 hp 60Hz 230 volts 1.2 amps 1625 rpm
class B insulation , 40C ambient , continuous duty ,
air over , open enclosure , permanently lubricated .

I will probably use this motor and run it derated
from 20-125 volts . It was my original idea to use
a four pole motor anyway because of the higher rpm
capability when using smaller stirbars . The torque
is significantly lower than for the 1050 rpm six pole motor .....but there is ample power and the power
band covers a wider speed range , so I believe this is a better all purpose choice . And having discovered the
" secret " of reducing the bearing preload , the same
method could be used to tune the low end performance
of a ball bearing 1050 rpm motor , without having to
worry about any sleeve bearing lubrication issues at low speeds and not having to use magnetic support bearings either . Adjusting the bearing preload is
a general method which could likely be used with many different ball bearing motors to adapt them from fan duty to stirrer duty applications . It has cost me a lot of time and money to discover this simple fact . And it
sort of gripes me because it seems so simple that the
bearing and lubrication and motor experts I have consulted , likely knew this all the time , and kept the
facts confidential and proprietary .....just the same as they will not acknowledge the speed control capabilities of PSC motors themselves , much less what factors
such as bearing preload are determinants in that area
of motor performance . And some people say they
think I am an asshole , hey shop around , there are bigger ones to be found ! Check some big corporations
for quick disvovery on that score .

If it's " not invented here " or how many thousand units
do you wish to have shipped ...they are not much help .

attached is the dimensional drawing for the X269

It is about a $140 retail motor . I scrounged a new one
for about 130 less than that :D


[Edited on 17-5-2006 by Rosco Bodine]

Attachment: Marathon X269 motor.pdf (100kB)
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[*] posted on 17-5-2007 at 13:06


I got a commercial magnetic stirrer from ebay today, the principle it uses is way more clever than any PWM circuit that only regulates the power that goes to the motor.

There is a plastic disk with many slots in it on the stirrer shaft, and this disk goes through an optocouple sensor (LED + photodiode), giving feedback about the actual stirring speed to the circuit. Using a special IC and an OP-Amp, this feedback is used to keep the stirring speed always exactly constant, regardless of load on the motor due to e.g. viscosity changes in the reaction mix.
If the motor speed goes down, the circuit sends more power to the motor so that the speed remains the same.

I can scan the schematics and make a pic of the inside of the stirrer if wanted.




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[*] posted on 17-5-2007 at 19:13


Yeah go ahead and scan 'em and let's see what it is they are doing nowadays on the electronics . I can make pretty good guess .

Recently I aquired a couple of old tape reel direct drive PM 1.2 to ~42V DC servo motors in new old stock condition for cheap . They will start and run from a single flashlight battery at about 40 rpm , with torque strong enough that you can barely grab the 1/2" shaft and slow it by hand , even at under a half amp current . You can slave one unpowered motor to the other with test clips , and manually turn the shaft on one motor and the other motor shaft will turn to follow it like a synchro , from the generated voltage . I think they are old Electrocraft motors
probably made for one of the old Univac? government computers from fifty years ago . Ball bearings , new brushes
they look brand new ....cost me about ten bucks each :D
They have encoder shafts on them but no encoders .
They are so torquey that really just give 'em a regulated voltage and they hold speed open loop good enough for any stirring application short of maybe stirring setting plaster .


I have a couple of hotplate stirrers based on the servo motor/encoder wheel tachometer feedback design , one was commercially manufactured having a C-frame shaded pole motor and the other was a custom aftermarket add-on servo-lock that I put in a stirrer which before had only a triac on a 3 3/8 frame shaded pole motor for speed control .

BTW , I also have a couple of overhead stirrers with encoder signaled Dart digital industrial controls which are giant sized versions that work on the same principle to control 1/6 hp continuous duty series motors from 30-7000 rpm , and have programmable torque and speed profiles , process timers , and parallel cable interfaces for remote operation and monitoring from a PC :D IIRC these were limited manufacture by Talboys from twenty years ago . The point being that the same manufacturer also makes puny little
mini-scale laboratory hotplate stirrers that look like everybody elses cheap stuff , great for a couple of liters or less and that's about it . But you know very well from looking at their " industrial scale " equipment that they understand the technology , and deliberately underbuild cheap stuff strictly because of the bottom line . You couldn't buy a good true heavy duty hotplate stirrer for two or three hundred bucks and get the manufacturers interested , until raising that price range by a factor of ten because that amount would barely cover just the motor and magnet .

The tachometer controlled speed regulation is the best setup for constant speed operation . However accurate and smooth performance is only delivered if the motor and magnet is pretty hefty and entirely adequate to the job also , as the speed control only compensates for speed variations which the motor torque can easily manage .
The problem is that the motor itself must have plenty of dynamic range " reserve torque" in order to be smoothly responsive to throttling by the "cruise control" . The sin that most stirrer manufacturers are guilty of is using way too small a motor ( cheap little C-frame shaded pole fan motor ) and then add an encoder and a tachometer (frequency to voltage converter) output to an error amplifier op amp driving a vactrol ( high voltage optocoupler ) to control the triac output to the little shaded pole motor . A signal voltage
from the wiper on a control pot represents a selected speed and the power to the motor is simply increased or attenuated
until the tachometer output voltage matches the voltage from the wiper on the speed control pot . It's like a standard servo-lock DC motor speed regulator , the only difference
being with the Vactrol optocoupler AC biasing for the Triac ,
to enable high voltage AC output , whereas an ordinary
optocoupler would be used in a DC motor control circuit .

Really to do it best , they would need to use a precision ball bearing permanent magnet brush type DC servo
motor with an encoder , like an old computer tape reel
drive motor of 1/10 hp or so , and use a DC supply to a power op amp error amplifier . That would give a good high torque low speed performance even down to 40 rpm or so
and upwards to 1600 rpm .....a 40:1 usable torque speed control range .

And if they are going to use an AC motor then something
like the Marathon X269 PSC motor would be ten times better than anything like a C-frame or a 3 3/8 frame shaded pole motor . A specially made permanent split capacitor 3 3/8 frame ball bearing motor would do it as a something of a compromise at about 1/20 hp and would still be at least five times better than a C-frame shaded pole POS motor .
But what the manufacturers are presently building in terms of both medium and heavy duty mag stirrers , with regards to not only the base units but the design of stirrer magnets themselves proves that they either just don't get it , or aren't about to spend the bucks on the components to put into the equipment what they ought to put into it .

[Edited on 19-5-2007 by Rosco Bodine]
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[*] posted on 30-5-2007 at 18:19


Has anyone looked into possibly using induction? A series of electromagnets in a ring (say, 6 of them) and a controller turning them on and off and reversing polarity in the proper sequence to spin the magnet bar in the flask. It would be like an axial brushless DC motor. No moving parts except for the stir bar. Maybe it could be more reliable and more compact then a mechanical one. Not less complicated, may use more power, and heat could be an issue if it was embedded in with a heating mantel. Anyway, Just a thought...
What do you guys think?

-Alan
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[*] posted on 30-5-2007 at 18:59


You want not to run a DC motor with a variable voltage, but with pulses of full voltage and control the rotation rate by pulse width and pulse rate modulation. That way you get full torque at low rotation rates, the concept is used in various variable rate motor setups.

I tried the induction route some years ago, effectively making a stepper motor where the armature is the stir bar. It looked like I would need 10 to 16 poles, this could be because I was lousy at winding electromagnets or because my controller design wasn't good. Whatever the cause the fewer pole prototype didn't work well, the stir bar tended to lose lock and hop about, and was stuck up on the shelf. I still like the concept, though.
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[*] posted on 30-5-2007 at 19:21


PWM averages out to variable DC in the winding's inductance and resistance. The whole point of PWM is to provide a variable DC voltage without dissipating the power of a linear regulator.

A stirrer is an AC motor. A magnetic field is created which is rotating. Two windings can be used, on an appropriate armature, to produce a rotating magnetic field. Tesla's original distribution system was four wire, two phase (quadrature). Later, it evolved into today's triphase system, with a phase of 120 degrees between legs. Three windings produce mutually rotating fields which, because each is a sine wave, sums vectorially to the same magnitude of field (producing a smooth torque output) rotating versus time. Two phases, one following a sine and the other following a cosine wave, accomplish the same goal; triphase only has the advantage that three wires can have the same voltage between legs.

In practical terms, we need a core to distribute the magnetic field as close to the stirbar as possible. It is utter foolishness to attempt any sort of motive device, generator or motor, without iron. Air gaps must be minimized. A piece of laminated iron in the shape of two "U"s merged at the bottom of the "U" and having 90 degrees between the two would be suitable. Windings go on the legs, say. The legs should be round for ease of winding, spreading to nearly intersecting at the top (to minimize air gap between the windings).

The best way to excite such a motor would be a low frequency quadrature oscillator. This could be done with a clock generator, variable divider, an EPROM programmed with a sine table and a DAC, but somehow I doubt three hundred thousand transistors are necessary. For instance, a switched-capacitor filter is used in Figure 5.38 (p. 292) in Art Of Electronics, 2nd ed., Horowitz and Hill; mention is made (on pp. 293-4) of linear-to-trigonometric conversion chips by AD; and a fixed-frequency quadrature oscillator is shown on page 304 (c.).

Tim




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[*] posted on 30-5-2007 at 20:52


Actually a plain permanent magnet servo motor like I mentioned , the old tape reel drive motors or huge aluminum platter hard drive motors , is the cheapest substitute for something like a Kollmorgen servo motor
you will find . And they have plenty of torque to run open loop and maintain good virtually constant speed with
nothing but a constant voltage applied , plenty accurate enough for any usual stirring application . PWM is for the most part an energy conservation method for high power
usage or battery powered equipment where the efficiency
is more critical than any concerns about insulation on the wiring or the compatability of the motor with that sort of
chopped power delivery . A piece of bench equipment
which is going to draw maybe 150 watts at most and generally much less , and is powered from a wall outlet
is not exactly begging to be powered from a PWM controller , especially if you are using a scrounged motor
that was designed for smooth pure DC . A regulated DC supply , or the rectified and lightly filtered output of a control transformer energized by a light duty variac is
kinder to the windings and easy enough to buy or build .

I have an electromantle stirrer mantle which uses the magnetic drive that is produced from sequentially energized electromagnets , and it's cute but nothing
so powerful nor smooth as a conventional motor drive
with a hefty rotor .

Variomag is one of the companies that makes the solid state type stirrers and there are others , but they don't have the performance of motor drive stirrers .

Really it isn't engineering the control means that is the
challenge , nearly so much as choosing the right motor and other components , matching the parts that work together well .....the control becomes greatly simplified .
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[*] posted on 23-2-2008 at 21:31
Magnet arrangement


Hello,

Can anyone with a purchased (or a good working homemade) magnetic stirrer tell me what is the magnet arrangement. Do theses things have 1, 2, ....4 magnets?

I have made a magnetic stirrer out of a CPU cooling fan(quite a big fan, about the size of what is in a computer PSU) with one hard disk magnet stuck to it with epoxy. Don't know rotation speed. I am going to guess about 400revs.
I have tried a number of different shapes/size of Magnetic stirrer bars but most are not stable. They will 'come off' and go the the edge of container and sit there jittering. One short thick cylindar shaped bar is OK. An egg shaped one is bad. Other cylindar shaped ones, both bigger and smaller than the one I am using will work sometimes for a while but are totally unpredictable.
I have the magnet as close to the bar as is possible. Perhaps if I had two magnets?
Why are the bars magnetic? Would a piece of Iron work OK?(plastic coated).
If I could slow the moter down it would work, I guess, with any bar but that is not possible.

TIA,

Dann2
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[*] posted on 24-2-2008 at 00:11


About the minimum technology that was ever serviceable here is described as follows.
For an older Cimarec 12" stirrer hotplate the drive magnet is two high grade ceramic block magnets 1" wide by 1 1/2" long by 1/2" thick which are superglued to a piece of cold rolled 1" barstock 3/8" thick and four inches long . But that design would have improved coupling and centering for smaller stirbars if the length and center gap between the blocks was reduced by 1/4" to 3/8" . The block magnets poles are on the 1 by 1 1/2 faces , each having the opposite pole upwards . An aluminum plate provides an eddy current loading which acts as a crude speed regulator , for the shaded pole motor , whose power is controlled by a 50 watt 250 ohm 0.45A rheostat .
The motor is a fasco 1/125 hp 1500rpm 1/4" shaft
sleeve bearing motor 0.42A impedance protected 3 3/8 open case cooled only by the draft from the magnet rotor.

Open loop control only works well when everything is carefully matched together for the task so that you have a pretty linear response without any active speed regulation .

The bars have magnetic cores to increase the coupling force between the drive magnet and the driven magnet .
When stirring a thick slush of crystals or a viscous solution , the stirring magnet will decouple and hop around when the torque limit is exceeded .
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[*] posted on 24-2-2008 at 03:42


Hello,

Thanks for that.
The stirrer/magnet bar combination that I have has worked fine for about 3 days but today the bar was back sulking over at the edge. I am running a tall chlorate cell. Stirring is essential as the top of the liquid will reach 73C without stirring (the bottom staying cold).

Will put a second magnet on the fan.

Dann2
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[*] posted on 24-2-2008 at 07:21


On something having a light duty motor like that , a single rod magnet with poles at the ends laid horizontal with
the axis at the middle would work better and it needs no
backing plate so it will be lighter in weight . But it will only
couple well with driven magnets which are close to the same length .
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[*] posted on 3-5-2008 at 19:14
measuring rotation rate


I'd like a reasonably simple way of measuring rotation rate of a lab stirrer. I have in mind the over head motor type as this gives me the torque I need.



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