the first thing to do is to toss out your single phase motors and get 3-phase motors instead.
then you may be able to find motor drives at a reasonable cost.
http://www.automationdirect.com/adc/Ove ... Control%29
the link above is just one random example. do your own googling.
this morning I had a look at the type of motor and found that it is possible to control the speed by regulating the voltage. However, this requires a very special type of controller. There is a warning from the manufacturer of the motor (pasted below). The reason for the warning is that there is a capacitor in this type of motor and it is necessary to have a filter between the motor drive and the motor to avoid large current transients and terrible EMI noise.
"4.2.2 Voltage control
With open loop speed control using transformers or electronic
voltage regulators (e.g. phase angle control), excessive current
In addition, noises can occur with phase angle control
depending on the mounting situation.
4.2.3 Frequency inverter
Fit sinusoidal filters that work on all poles (live-live and liveearth)
between the frequency inverter and the motor for
operation with frequency inverters.
Depending on how the device is installed, noises may occur."
So my conclusion is that yes you can regulate the motor speed by regulating the voltage, but you should use a transformer with several taps on the winding, or a variable transformer that is turned by a motor. this way you can feed a sinusoidal voltage to the motor and avoid the issues.