Just a couple comments on the speed control disscussion. Its been
suggested that variacs or router speed controllers can be used. They can
if the motor is an AC/DC (universal motor) or a DC motor. Some
extractors such as  certain Dadant models come with DC motors and are
equiped with speed control. Most extractors come with the more common,
and less expensive induction type motor. To speed regulate an induction
motor you must change the frequency of the AC power. There are such
regulators available but are quite expensive. Its much cheaper to use a
mechanical method such as Allen suggested. Useing the wrong combination
of speed control and motor may cause damage to the controller, the motor
or both. So before you spend any money, be sure you know what type motor
you are dealing with and be sure to select the proper controller.