Stir plate magnets reversing polarity

[aiptasia]

I'm working on a DIY stir plate using a small PC fan and a 3-12 watt adjustable wall wort. I have the fan mounted to a project box by four #6 screws with bolts and washers with a rubber grommet on each screw to serve as a shock absorber and a rattle baffle.

I'm using a 2" stir bar and button sized metallic rare earth magnets. I've got two super glued to a pog (circular cardboard circle) and the pog glued to the fan blade in the center of the fan. Each magnet is on opposite sides of the pog to attract the ends of the stir bar. I've repeatedly assembled the stir plate with the magnets in place. Everything seems fine until I plug the unit in. The stir bar seats properly on both poles until I apply the power.

When I plug in the unit, the 2" stir bar goes whizzing off in random directions. Again, it seats just fine over the magnets when it's assembled, but the moment the unit is plugged in, the magnets somehow change polarity and the stir bar gets flung. When I disassemble the unit and test the magnets with the stir bar directly, the poles repel instead of attract, even though they were mounted and glued correctly before the power was applied.

I'm not quite sure what's going on to reverse the polarity or what to do to fix the problem.

 

[evanmars]

Slow down your starting speed. I have read that 1" bars are easier to keep from spinning off.

 

[fartinmartin]

You must have a fan from the wrong hemisphere, a northern hemisphere fan will run clockwise, take it to the Southern Hemisphere it will run anticlockwise.
Do you live on the equator ?

 

[TNugent]

You must have a fan from the wrong hemisphere, a northern hemisphere fan will run clockwise, take it to the Southern Hemisphere it will run anticlockwise.
Do you live on the equator ?

I figured it had more to do with the name of his brewery... Paranormal Brewing!

 

[GrizzAndFish]

I figured it had more to do with the name of his brewery... Paranormal Brewing!

In Them Scary Woods. It's probably screwing magnetic fields that make them so scary!

 

[aiptasia]

Evan, tried that. It does help but I think the problem is that the magnets are too close to the fan. Another part of the problem is that at lower wattages (3-5 watt), the pull of the magnets is strong enough to stop the fan entirely.

Also, I think a 1" stir bar will help.

You other non-helpful b*tches.....

 

[Takuie]

Sounds like your magnets may be a tad too far apart. Try bringing them in just a little.

 

[evanmars]

I had to have a pretty thick spacer so the magnets didn't overpower the fan.
I haven't used anything but a 1", so I don't know if that'll help or not.

 

[aiptasia]

I had to have a pretty thick spacer so the magnets didn't overpower the fan.
I haven't used anything but a 1", so I don't know if that'll help or not.

I added a 1" plastic bottle top from a 2 liter and re-glued the cardboard pog to that. Then, I glued on the magnets by attaching them to the ends of the 2" bar and glued the undersides of the magnets directly to the pog. This also didn't really work too well, so I decided to replace the stir bar. I found some at an alternate LHBS a little ways out of town and they were the 1" pill sized stir bars with the rubber core ring center. These bars work brilliantly, a nice tight vortex spinning away at even 12 volts speed in a two quart jar.

Sounds like your magnets may be a tad too far apart. Try bringing them in just a little.

Didn't have to. I let the magnets center themselves where they wanted to attach to the 2" bar, which as it turns out, was all the way out on the ends of the bar. If I slid them purposefully closer together, they'd migrate out back towards the end of the stir bar. I figured that was their natural sweet spot and just left them stuck to the stir bar, applied glue to the base of each magnet, then flipped it over and glued them down to the pog.

I think the part that really, really helps the spin is the rubber ring at the center of the 1" stir bar. Super tight spinning vortex at low and high speed both.