This solid-state thyristor (SCR) switch circuit is perhaps the simplest means of reversing a 3 phase motor. “Static” is a catch-all term that essentially means without moving (mechanical) contacts ...

I have been trying to fix a little old milling machine I bought (and here), and needed a replacement motor controller. It is a Clarke DCM10, a variant of Sieg’s X1, sometimes knows as a ‘micro mill’.