depends which component rheostat..if field rheostat is
there it should be at the minimum....the armature rheostat
should be at the maximum..it should be done to limit the
speed of the machine.....at starting due to very less
induced emf the speed increases terribly
actually rheostat means -diverting......if keep the rheostat in minimum position motor will run at rated speed....if rheostat in maximum position ,the motor will run above the rated speed ,so that speed make the motor damage....