Economical speed-variator

Does anyone know how I can build a simple and economical speed-variator for motors of A.C. using available common parts?

Simple AC Motor Speed Control

Assuming you are only using small motors (500 Watts ?) then you can use a domestic light dimmer - these are available with ratings upto 1kW.

The only problem from slowing the motor is the loss of efficiency of the cooling fan. You must allow the motor time to cool by either running for short bursts (a few minutes) followed by several minutes rest or several minutes full speed-low load or you must ensure there is only a small load at all times.