PeterH:
Big electric motors typically use something called a 'contactor' rather than a relay. They seem to be fundamentally the same thing (solenoid operated switch) but are presumably optimised for switching high current inductive loads.....
Grainger has a helpful PDF entitled "Choosing Relays, Contactors and Motor Starters" and clarified some points for me. here is the link: Redirect Notice
Hope that is helpful. It clarified some stuff for me.