There are two commonly used ways to power a DC motor from AC voltage: SCR and PWM.
In short, an SCR will essentially chop off half the AC input and feed that to the motor, with the speed being controlled by where in the AC waveform it starts to feed it to the motor. When the motor is set to full speed this is fine, but at anything less than full speed the motor won't be getting the full voltage. While with PWM the AC is rectified to DC, and then the DC is very rapidly switched on and off to control the speed, so the motor sees rapid, brief inputs of full voltage.
In real life this means SCR controllers are cheaper, more robust, and able to handle more current, but are louder (they produce an AC buzz), the motor runs hotter, and the brushes and commutator have a reduced lifespan. PWM controllers are more expensive, have lower current limits, are somewhat easier to break, but they run very silently and the motor runs cooler with a longer brush and commutator life.
I just recently upgraded my mill to a KBIC speed controller which uses SCR technology. However, I has able to get a KBWS PWM controller for a really good price, so the KBIC came out and the KBWS went in.
|KBWS installed in the mill.|
The quieter operation was immediately noticeable and very welcome. After an extended run the motor was still cool to the touch, unlike with the KBIC. Ultimately, I think it was a worthwhile upgrade.
Both the mini mill and mini lathe mostly use PWM controllers stock. However, the stock controllers are very limited and prone to die. If my choice was the stock controller or a KBIC/KBLC I would go with the KBIC/KBLC, even though they're SCR simply for the significantly greater performance and adjustability they offer. However, if you can get a KBWS for a good price, that's a better the way to go.
A quite note, KB Electronics makes a cheaper version of the KBWS called the KBWD. However, while the KBWD will work, it doesn't have an electronic stop like the KBWS has. On a mill/lathe the electronic stop is very handy, since you can set a speed and then use it to turn the motor one and off without disturbing the speed setting. That feature is useful enough to me to specifically seek out the more expensive KBWS.