Recently there have been advertisements in many woodworking mail order magazines for an electronic speed controlling unit which is being primarily offered to vary the speed of routers. The difference between these and a plain rheostat is that they are able to maintain a nearly constant level of torque throughout the complete speed range. Motors that can be speed controlled are always of the universal variety, that is, brush type motors either for AC or DC current. Induction motors such as those in a household fan cannot be slowed by voltage changes. Only brush motors have that ability. Most of the little powered tool that can be shop built will incorporate a small sewing machine type motor (universal) and instead of having to provide multi step pulleys to achieve speed changes, an electronic speed controller can be used to very effectively regulate the speed range. I have installed one of these very units in line with my Micromark Drill press by first plugging the controller to the regular wall outlet and the drill press is then plugged to the controller. My particular controller has a switch that when set to the middle position, it is off, when set to the left it provides variable steeples range of speeds and when flipped to the right it gives a single full power high speed. I have set the pulley on the drill press to the slowest speed position which gives me a speed range of 0-2100 rpm. The second and third steps give 4500 and 6500 rpm. At these speed settings the apparent possible range of speeds seems really great but what actually happens is that the motor tends to bog down quite a bit at any speed setting unless you are drilling very tiny holes. If those really high speeds are needed, it is best to run it at full power without the controller. I have never had the need for a speed higher than the first range. It will provide a lot more torque with less tendency for the motor to slow down during a tough drilling job.
Sewing machine motors are available new for under $5.00 (normally $40.00) from American science and surplus, a great source for many electronic components and other really neat stuff. I consider them a great source for the home hobbyist and do it yourself types.
Do not try to run this controller on an already controlled tool or motor. I have and it simply will not work. If you think this useful little item is for you, they can be obtained for around $39.00 from many woodworking tool and supply mail order companies.