Hi Les,
This might be a bit late but if it's of interest, there are two basic forms of speed regulation. The 'cheap' option, in effect pulses the power on and off to the motor, known as Pulse Width Modulation. In effect the motor tries to get to full speed but as the pulse switches off, it then slows down again. However this is happening so fast, the result is the motor appears to be running at a certain speed. The shorter the pulse, the slower the 'speed'. It is the same principal used in filament lamp dimmers. The real downside of this is that you are losing power as well as speed. But these are very cheap controllers.
The other form of speed control uses three phase motors and the controller varies the frequency of the power supply. These motors are quoted to run at a certain speed e.g. 1425 for a four pole motor, but this assumes a 'normal' 50Hz supply. The controller is able to provide the voltage/current at variable frequency, but full voltage/ current, thereby allowing the speed to be changed with minimal loss of power. There are power losses due to other factors but nothing like as bad as 'dimmer' controllers.
Another advantage of frequency controllers is that you can run the motor faster than its nominal speed, as most controllers allow you to set an upper maximum frequency well above 50Hz. Again there are pros and cons of doing that, but it can be very useful by giving you an even wider speed range.
The downside of these is the cost, as you need a more expensive controller and a three phase motor but once you've used a proper variable speed lathe, you won't need much convincing about the benefits.
