Stir Plate throws bar, help needed.

[RichSib]

Being an inveterate tinkerer I decided to build my own magnetic stir plate using a 3" fan and a rare earth magnet. The fan used is a 24v motor, the power supply is 12v 1000mA. The magnet is a 1 x 1/8 x 1/8 rectangular magnet which sticks, by it's own power, to the fans' hub. The speed is controlled by a Radio Shack potentiometer. At its lowest setting its still pretty fast, it will create a top to bottom vortex, with a corresponding rare earth magnet, in a 1000ml flask within a couple of seconds of power on.

It worked perfectly through the trial stages. It spun a corresponding rare earth magnet fast as a tornado and steady as a rock, it does the same with a 1" square section teflon stir bar in an empty flask. If there is any liquid in the flask however the stir bar makes a few revolutions and then shoots to the side of the flask and vibrates, (this doesn't happen with a rare rare earth magnet). I think the problem is that the fan is spinning too fast but before I try to build a speed control circuit for it, I'd love to have an expert opinion. I tried powering it with a 9v 200mA power supply but got the same results.
Thanks for your help, Rich.

 

[Photopilot]

Two potential issues
Speed as you mentioned.
Magnet placement: The magnets on the fan should correspond to the length of your stir bar. I believe the standard design has two rare earth magnets on a fan one with negative facing up and the other positive. The placement of the magnets should be such that they fall on the poles of the stir bar.

 

[Ovidsmuse]

+1 Photopilot

Also, you'll want to get the magnet away from the fan if you can, people usually use some pvc of the correct diameter and glue the magnet to that, this way you can get the right height for you enclosure and the magnet won't shorten the life span of your fan. I have a similar setup and when using one magnet find that I need to run the fan at 5-6v to stop it from shooting the bar out, with two magnets you can go faster because there is more stability and a stronger magnetic field.

 

[twigboy2000]

Mine did the same with the 1" bar but works flawlessly with a 1/2" bar.

-chuck

 

[Photopilot]

+1 Photopilot

Also, you'll want to get the magnet away from the fan if you can, people usually use some pvc of the correct diameter and glue the magnet to that, this

Please clarify this as the motor on my stir plate is dying and I have to make a new one. Do you mean you glue a PVC pipe to the fan and add magnets to it to separate the magnets from the fan?

 

[Ovidsmuse]

Exactly Photo, glue the pvc to the fan and the magnets to the pvc. The weight of the pvc is a lot less than the stress from the magnetic field, so don't worry if you need a tall piece of pvc to get the right height.

 

[smokinghole]

My guess is that it's the voltage running to the fan causing it to spin entirely too fast when you have the resistance of the wort to deal with. You need to get a resistor of some sort to further drop your voltage beyond what the potentiometer controls. I had to do that on my stirrer when it was fresh. Now that I've been using it a year it has slowed down a little for some reason. I can still throw my bar if I turn it all the way up but I just keep it below half speed and it's fine.

 

[RichSib]

It was the 24v fan powered by the 12v 1000mA power supply that was the problem, it spun the fan way too fast. The square section stir bar was too unstable at speed in the liquid and it just vibrated off into a corner and sulked. The potentiometer couldn't reduce the speed enough for the bar to stabilize.
Last night I hooked the stir plate up to a 6v 300mA supply and it has been running fine ever since.
As far as spacing your magnets away from your fan hub, a piece of PVC pipe will work but, if it is cut unevenly it may cause too much vibration and still shorten the life of your motor, not to mention make it noisy. When I started my build, intending to use magnets from a hard drive, I disassembled the hard drive into all its' components. Separating the two actual discs in the drive was an aluminum spacer ring. This ring is precision machined to spin at 7200rpm. It is almost exactly the diameter of my fans' hub and when epoxied in place with the magnet on top (I ended up using one 1" long R.E. magnet) it works perfectly.