I am making a mixer for a mash tun for all grain beer brewing how ever i need to slow the motor down so as to not make a mess all over the kitchen (again). I am using the electronic controler that came with the treadmill but even the lowest setting is too fast. The motor is a 90vdc 15 amp motor. What size potentiometer do I need and how it that calculated? I would like to be able to control the motor from as slow as a few revolutions a minute and up to full speed (although i will never have to drive it that fast)> thanks for your help. If you need anymore info please ask. (I am an electronic noob).