The value of contact resistance between copper sliprings and carbon brushes, in wound-rotor induction motors, has a profound effect on the performance of these machines, The increase of this resistance due to bad contact may be detrimental. In this paper, an important factor affecting the resistance is introduced. This is the PV factor (product of contact pressure and peripheral speed), It represents the frictional power density at the interface; and it is also an indication of the volumetric or linear wear rate. A relation between the operational life time of brushes and the PV factor is experimentally deduced.