Stir plate issue

I'm having issues with my DIY stir plate. No matter what speed I have it going (I have one of those multiple voltage bricks, from 3 to 12 volts at 1.5 v increments) the 1'' stir bar always just rattles in the flask but won't spin. The fan has 2 hard drive magnets of opposite polarity attached to it and I use 4 screws to set the distance from the bottom of the flask.

When I start out with the fan off and the bar right over it, I notice that the fan won't spin (light up, but not spin) unless the voltage is 9, 10.5 or 12. This high of a speed normally makes the bar jump off.

Any thoughts what I should do to try and fix this?

You're not getting enough torque to turn the motor at low speeds. You can replace the fan with one that draws more current, or you can try to position your magnets differently. Are you getting spin without the stirbar?

I am getting spin easily without the stirbar. At 12 volts it spins so hard and fast it will actually move with a full 1l flask on it since I don't have an enclosure. I'll try upping the distance between the magnets and the flask, see if maybe that helps. I had it work once back when I was using a 12v brick and 10k pot (bad I know, switched methods). But I also had only 1 magnet. After a couple of test runs it kept throwing it (added a 2nd magnet before going to this multi voltage brick).

Erroneous said:

I am getting spin easily without the stirbar. At 12 volts it spins so hard and fast it will actually move with a full 1l flask on it since I don't have an enclosure. I'll try upping the distance between the magnets and the flask, see if maybe that helps. I had it work once back when I was using a 12v brick and 10k pot (bad I know, switched methods). But I also had only 1 magnet. After a couple of test runs it kept throwing it (added a 2nd magnet before going to this multi voltage brick).

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Yes, increase the gap between the magnets and the flask. The problem is at start up, the fan must have enough power to overcome inetia and the initially high friction between the bar and the glass bottom of the flask. If the magnets are too close, too strong or both, the downward pull of the magnets increases this friction too much You will have to do some trial and error testing to determine the optimum gap. The pivot ring design stir bars substantially reduce the force required to get the bar moving initially. Unfortunately, widening the gap also lessens the magnetic coupling effect and this can cause the bar to be more prone to getting tossed.

I fixed it by stacking the magnets on top of each other and placing them in the middle. I also decided to add a pwm fan controller circuit to my arduino fridge controller so now I have complete control of the speed with a slide pot. I love multitasking devices

As for the start up issue it seems I could only get it when the voltage was under 6 so I guess I was remembering wrong.