I am in the process of building my own sous vide machine and like to replace the pump typically found in most commercial machines with a magnetic stirrer unit. I figured the easiest way would be to use a motor to spin a permanent magnet (instead of creating a strong rotating magnetic field electrically). I first started experimenting with some 12V DC fans leftover from an old computer. By either lowering the input voltage or via PWM I was able to obtain appropriate rotational speed (don't ask me about exact speeds, I guess around 300rpm or so). With the project maturing I am not sure which motor (type and/or exact model) to use. I hoped some of you might point me in the right direction for the type of motor or even better, could suggest a specific model. I considered stepper motors or continuous servo motors. My criteria for the motor are:
- easy to control rotational speed between 0 and around 500 or so rpm (for ramp up and variation of speed to increase turbulent flow)
- ability to rotate both clockwise and counter clockwise
- enough torque at all speeds to spin a disk with magnets attached (weight and inertia probably negligible, the to be used alnico magnets create a field strong enough to lift 0,4kg each)
- small axial dimensions
- reasonably priced and long lasting
The following picture shows a commercial magnetic stirrer found in your average laboratory. The blurry white object in the glass beaker is the the so called stir bar or "fish", a PTFE coated permanent magnet. It spins due to the rotating magnetic field created by the actual device underneath the beaker.