Stir plate problems

[GORILLA]

Hello,

I'm a new poster but avid reader and I was hoping someone might be able to shed some light on why my DIY stir plate is doing this. I started this project a few weeks ago and got frustrated and left it, but I really want a stir plate!
Initially when you plugged it in the stir bar would spin a little and throw itself to one side of my flask. Now as you will see it starts to spin and then slows down and repeats this until after a little while finally throws itself to the side.
The power supply is a 12v 300mA unit, the fan is an 80 mm unit, and I do not have a potentiometer on it. Could the lack of control be the reason it slows and then ultimately throws itself? I have two HDD magnets on this and it's separated by a piece of 1/4 inch wood. I will say it behaves better with 1000 mL vs 1500 mL. Lastly the top of the magnets is as close as I'm going to get them without hitting the lid.

Thank you for your help.
<iframe width="420" height="315" src="https://www.youtube.com/embed/khRHhRh350c?rel=0&amp;controls=0&amp;showinfo=0" frameborder="0" allowfullscreen></iframe>

https://youtu.be/khRHhRh350c

 

[max384]

It looks to me like your RPM is too high. It creates a large vortex that knocks the stir bar around. I would add a potentiometer so that you can adjust your speed.

 

[mongoose33]

I just made one myself and have a 1" stir bar. I have to run it at the slowest speeds to work correctly--at high speed it throws the stir bar.

 

[pshankstar]

I agree with the others above. If mine is too fast it throws the stir bar. If you can adjust the speed it should help or solve your issue.

 

[GORILLA]

Thank you...off to get a potentiometer

 

[IchLiebeBier]

When I made my first stir plate, it threw it off all the time. The magnets weren't centered on the fan so it didn't stay on very long. It wasn't until I remade it like this one that it started working correctly:

https://www.homebrewtalk.com/showthread.php?t=109587

I also added a potentiometer to mine. I start it out full to get it spinning, then turn it down to about a quarter power to do its thing.

 

[BlueHouseBrewhaus]

My first stir plate used a computer fan, HDD magnets and a dimmer switch all spliced to a 12V charger cord. My recollection is that I had to put the dimmer almost as low as it would go so as not to throw the bar. It wasn't terribly consistent so I eventually bought a Stir Starter for about $30.

 

[bosster01]

Make sure your two magnets are polarized correctly. I had a similar issue and it was fixed when I flipped one of the magnets. Mine was attracted to one magnet and repelled by the other. It would spin for a minute than get thrown off at higher speed.

Also make sure your spacing of your magnets is good. Might need to experiment with closer and further apart.

 

[GORILLA]

I've got a rheostat on the way

 

[p_p]

I have a different take on your problem. Having a way of regulating the speed is very handy as it allows you to make different volumes of starter yet keep the level of movement the same.

The problem you have is that the magnets are not strong enough to keep hold of the stir plate at higher speeds. You want strong magnets on the stir plate and ideally the stir bar. Make sure the location of the magnets in the stir plate suit the size of stir bar you have and finally, construct the box such the distance between the stir plate magnets and stir bar is minimum.

 

[pshankstar]

I have a different take on your problem. Having a way of regulating the speed is very handy as it allows you to make different volumes of starter yet keep the level of movement the same.

The problem you have is that the magnets are not strong enough to keep hold of the stir plate at higher speeds. You want strong magnets on the stir plate and ideally the stir bar. Make sure the location of the magnets in the stir plate suit the size of stir bar you have and finally, construct the box such the distance between the stir plate magnets and stir bar is minimum.

Good point!! I ended up cutting a hole with a hole saw in the center of my box to move the magnets up higher and closer to the stir bar. By doing this it removed the wood from the cigar box and now the only thing between the magnets and stir bar is the flask.