We do some dumpster diving at a local college at the end of the school year and get some great finds such as a brand new bullet mixer in box, samsung digital camera, and too many items to list.
I found a gadget that says it's a router speed control, made by Wand Essentials. Did some research and there are other brands which look like the exact same thing. They sell for anywhere fromm $15 to $24. I tried it out quickly on an angle grinder and a dremel tool. It did control the speed. I didn't try under load. The innards don't show much: fuse, bypass for full power, small circuit board that I can't get at to see components if any. The power cord is only 16 ga.
My question to any on the forum who are electricians is, will this device do any damage to the motors?