All motors are designed with a power factor rating usually expressed as 0.8pf . You can usually find it on the metal ident plate. The general idea thats simple to understand, is if you run a motor at 80% of its power rating, it will live longer, and not overheat and go into meltdown. Also an electric motor reaches it full potential conversion factor at 80% or its rating, and is able to take sudden surges without overlaoding. The stepper coils allow full voltage to be applied to steppers and servos while limiting current. S0 you want a motor 20% bigger not 20 times, than your controller coils limit. eg. 5 amp coils, 6amp motor.