Similar topics have been raised in other threads but thought some of you might be interested in the following.

A friend of mine bought a cheap router trimmer with no sped control and asked me how he could reduce the speed as he was using it on plastic and the cutter speed was too high and melting the plastic. I thought about it and then remembered the beefy type (2000W) dimmer switch I used in a DC motor controller circuit discussed in the following thread.
DC motor performance

The dimmer switch controller was purchased from ebay for a whopping $12 (https://www.ebay.com.au/itm/AC-220V-4000W-SCR-Controller-Electric-Voltage-Regulator-Speed-With-Fan-SR/282658437804?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649)

The controller has a quoted power rating of 4000W, although the fine print says that the 4000W is for intermittent/short bursts and for continuous use the max recommended power is 2000W.
In the above thread link I discuss (accidentally) drawning 20A or 4800W though the controller and it survived which is worth knowing although I do not recommend operating it this way.
The controller comes with a small cooling fan but no earth connections which means mounting it inside an insulated box and routing an earth wire through to the outlet..

Anyway, I bunged it into a plastic box, added a mains cable for input and a 10A GPO as as output and here it is.
Box was $8 from ebay, $5 GPO and cable was free from my endless stash of computer mains cables - all up $25.
SpedCOntroller.jpg

The mates router is only 400W and he's only using it for short bursts so it doesn't matter if the fan is enclosed inside the box.
For constant and/or higher power use the box should be vented. It could be as simple as a few sub finger size holes around the box or for HD use an additional fan could be added to draw air through the box. The GPO could also be replaced by a simple 3 pin socket.

My mate is really happy with and said his missus saw it and asked if she could try it out on her cake mixer and now she has commandeered it and the mate has to borrow it back from her.

A couple recommendations.
There is no provision for an earth wire connection so if you wire in a 3 pin socket or GPO as the outlet make sure you wire the earth wire from the mains GPO right through to the outlet of the box.
This controller only works on universal motors so it will not work on any synchronous motors as used on most lathes, bandsaws, table saws etc.