Commutation of DC motors operated at reduced field current

DC motors have been the workhorse of variable speed drives in the paper industry for many years. DC motor speed is easily controlled by varying the supplied armature voltage and/or excitation (field) current. Many mills have inquired about operating motors beyond the original motor rating. With armature voltage limited by the drive, motor speed may be increased by reducing the shunt field current, but not without consideration of vibration, machine adjustment, brush performance, and possible commutation issues because the machine was not designed and tested at those conditions. This paper examines and presents test/field data displaying the effects of field weakening on DC machine performance and maintenance.