DIY stir plate

As long as it doesn't throw the stir bar off... I'd say no. I've never heard of a stir plate stirring yeast too vigorously, but I could be wrong. I had that same issue when I built my own a few years back, and it kept throwing the stir bar.

 

No risk to the yeast, but you might have some real problems with high speed only. Normally, you start out low and slowly increase the rpm's to prevent throwing the bar. But in your case you'll be running like Kyle Busch at Daytona. When making a 2L starter (in 3L flask) I don't even use top speed, but really depends on how many rpm's you're cranking out.

Suggest you test all of this in a flask of water to work out the bugs. Just be aware that starter wort is thicker than water (will slow down things a little bit). In worst case you can always go back to shaking until your rheostat shows up.

 

I use this fan. https://www.amazon.com/Thermaltake-...696&sr=8-2-spell&keywords=thermatake+desk+fan

 

I have been using my home made one for years, all different size flasks, only one speed, never had an issue.

 

I don't think you could hurt the yeast with a stir bar. Put your yeast in a blender however...
https://www.beeradvocate.com/commun...e-using-liquid-yeast-making-a-starter.565719/

It's not the best solution, but you can use an a/c dimmer switch to reduce the fan speed to about 80%. A dimmer is not meant for a fan but it will work, sort of. If you just need to shave off a bit of power. Look for a portable device dimmer switch, it is for the floor and designed made to plug in with the slider built right in. There is no fine control.

Cheers.

 

I use some old papst fan with HDD magnet. Controlled via ebay led-strip dimmer and it even has remote controller. As the dimmer uses PWM and the papst is voltage controlled fan the low speed noise is kinda funky but it works very well. Full speed is quite low because this is very slow speed fan.

My second plate is ran straight with 12V, using this I play with stir bar size to get correct speed. This usually used for first step on little quantity of harvested yeast.

 

Sounds like that would be kind of stressful to the motor.