About DC motor

hi

in my plant i have baumuller made 97.6kw dc motor (armature voltage 480vdc.field voltage 220vdc) has rewinded. it was driven by eurotherm made dc drive,and it uses tachogenerator feedback to measure speed.But when i connected after rewinding its armature +/- was interchanged and drive feeds full voltage to the motor and its commutator was little damaged.(this is because tacho and motor direction was not matched,so drive given its full o/p)

then i came to know that when we are not sure about the direction,in such case we can put series of resistance with armature,now please can anybody explain me how to choose that series resistor ohms value and its wattage according to our motor rating.