I have a Porter Cable 690LR single speed router that I would like to use a speed controller on. It seems like some people say it will harm the router and others say it is fine. Porter Cable said not to and a guy at a local tool shop also said it would burn out the router. Everything I have read online seems to say it is okay. So what is the real story/ Do add on speed controllers burn out single speed routers? If they do, why do they even make them?