A lot of the problem with older extractors is the ability to control the speed... I have two extractors with different methods of controlling the speed on them, and neither method makes me overly happy, so I started to look for a better way..
I have been a mechanic for well over 20 years, but not an electrician.. so I always understood that changing the voltage on an electric motor would burn it up... However, in my searching, I found that the universal motors, or the ones that had brushes COULD be regulated with a rheostat. So I am making some modifications to try it and see how it works..
http://www.turbineproducts.com/servlet/Detail?no=30 this is the rheostat I am going to try. I am going to modify my extractors somewhat, and attempt to use the rheostat to control the speed, rather than the unreliable mechanisms they currently have in place. I will let you know how it works out.. If you have experience, I would appreciate comments and suggestions!!!