The router control that Craig Smith suggests is only good for a
universal brush-type motor.  It is not likely that this is what is in
the device Marvin Horovitz has.

If the motor is an antique induction motor, there is really no perfect
speed controller for it.  Variable frequency drives (inverter
controllers) should ideally be used only on inverter rated motors.  It
might work or it might burn up your antique motor.

Changing to a DC motor is the best solution, period!!!  You can control
a brush-type motor (series wound) with the Harbor Freight router speed
controller, but it will not keep a fixed speed if the load varies.  To
do this you need a more serious speed controller and a suitable motor.

Bodine makes excellent motors and controllers but they are a little
expensive.  I have found them in surplus stores and online.  I have
dozens of them in 90-volt DC versions and have several controllers
including Bodine and other brands.  The last DC speed controller I
bought cost me about $50 on eBay and is running the spindle motor on
my CNC mill.  It will hold any speed quite accurately with widely
varying load.

If perfectly accurate speed control with varying load is not necessary,
the cheapest and most reliable solution is a variac and a bridge
rectifier.  If the motor is universal (AC/DC), you do not need the
rectifier.

One other suggestion was to use a ceiling fan speed controller.  This
is not likely to work.  The range of control will be very limited and
the power of the motor will be reduced.  Also, the continuously
variable fan speed controllers seems to be very difficult to find, as
they have been replaced with ones having two to four fixed speeds.  If
you can find one of the old continuously variable types, you can give
it a try and it might work, if you just need to change the speed a
little.

Spencer Chase 
Garberville, CA
http://www.spencerserolls.com