hamshie.k
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Magnetic Stirrer - selection of motor
please anyone help me regarding the mechanical calculations for selecting a motor for magnetic stirrer.
1. The forces acting in the magnetic stirrer rod while in motion
2. from the force applied in the magnetic stirrer rod - how to select the motor based on torque(power)?
i am using two similar magnetic rods one affixed in motor and another inside liquid.
Planned to use a CPU fan motor. but i need calculations to prove the selection.
[Edited on 15-6-2012 by hamshie.k]
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99chemicals
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My magnetic Stirrer has a 4.5 inch computer fan in it with a hard drive magnet. It has way to much power when plugged into 12 v It can over flow a
1000 ml beaker!
Just plug it in to the correct voltage and try to stir you biggest beaker. If it works you are in buissness!
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Swede
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I'm thinking a CPU fan motor is too high an RPM, and too low in torque. There are thousands of various motors available for a DIY project like this.
The big obvious... needs to be variable speed.
I'd suggest looking at various surplus outfits, depending upon where you live. Surplus Center is a good USA source for all sorts of motors.
http://www.surpluscenter.com/
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99chemicals
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I am using a speed ruducing circuit using an LM317. It works very well. The torque is really negligible because there is not that much friction on a
teflon stir bar.
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chemrox
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Oh you DIY guys! Go on ebay, go on ebay..
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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99chemicals
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Can you show me a good un-used magnetic stirrer for under 50$
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mineralman
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Dont they use POLE MOTORS?, My old mag stirrer had a motor that looked like a oblong w/rounded corneres layered core, wound on either end, and a pole
to one end that had the magnet on the top. it was also variable.
someone will know the name of this motor,
The only problem with the comp fan is heat close to it and something about high powered magnets with the fan motor IIRC, Never used one myself. hope
this helps MM
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hamshie.k
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there should a maximum limit to rotate the magnet inside the liquid.Considering that, the motor should be selected. Please give me some advice
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mineralman
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I guess a neodynium magnet would allow for more of a gap between the two, most of the older magnets are ferrous, no idea about the newer ones.
Ill just go take my mag stirrer apart and get you the name of the motor, ive got to feed the phesants anyway......back soon MM
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hamshie.k
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yes, i need mechanical calculations . if possible
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mineralman
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OK, first the down side, My camera battery just went flat, so im useing goog to find some pix.
On the up side, I just found my spare mag stirrer motors (cheers man).
my old hotplate/stirrer has a Squirrel-cage induction C shape shaded pole motor w/round magnet 10mil thick and 10mil groove down the middle, This sits
on top of the pole. mine is a MYCALEX MOTORS SOC 16
My £450 high tek hotplate/stirrer has a brusshless pancake motor (don't quote me on that one), but its just like a solid case centeral pole motor
from a tape recorder/player.
Im not taking my BIBBY apart for love or money, but the hotplate is about 1ft square & ceramic, shes ma baby.
sorry I cant be more spasific then that.
Im reliably informed that an electric car window motor can be used as a v-good motor to power a mag stirrer. ill post some pix l8r, but wikkapedia
has a good section on lab equipment IIRC MM
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dann2
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What is below is from this thread 24 Feb 2008
http://www.sciencemadness.org/talk/viewthread.php?tid=3056&a...
Roscoe Bodine said:
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 .
End
___________________________
Unless you are trying to make a magnetic stirrer that used as small a motor as possible (going into production) . Just pick a strong enough motor that
will do the job. The harder part will be coupling the torque produced by the motor to the stirr bar (they need to be close together) and arranging for
speed control. You need a motor that will allow speed control (obviously).
I have seen cheap 'shaded pole ' motors in the more cheaper magnetic hot plate stirrers. They allow (somewhat crude) speed control using a triac. You
can get a suitable shaded pole motor from a domestic washing machine water pump (dumpster or purchase).
The more up-market magnetic stirrers have an optical arrangement to sense speed of motor and adjust accordingly to keep speed controlled accurately.
Are you actually building a stirrer? or trying to obtain some design formula for selecting motors/magnets/seperations/speeds/torques for a school
exercise (or something like that)?
Dann2
[Edited on 16-6-2012 by dann2]
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