A Study for Heavily Saturated V-I Characteristics at Carbon Brush / Steel Collector Ring Sliding Contact, on the View Point of Parallel Connected Brush Current Sharing Unbalance

The subject of this paper is in association with parallel connected carbon brush current sharing unbalance problem in magnetic field current supply system for turbine generator at thermal or nuclear electric power plant. In case of large DC motors, in which copper commutator is applied, several hundreds of brushes are connected in parallel, where approximately 20% or 30% of current sharing unbalance are existent, according to years of field experiences. On the contrary, in case of turbine generators, in which steel collector ring is applied, such phenomena as some of brushes may share only small percent of brush current or may share almost no current, can be found as a rare case problem, according to the field experiences. On the other hand, we reported at several IEEE Holm Conferences that heavily saturated V-I characteristics at carbon brush / collector ring sliding contact, can be found in case of steel corrector ring. In this paper, V-I characteristics at carbon brush/steel collector ring sliding contact, are further investigated where brush contact voltages are measured precisely from very small current values which are in terms of current density of "mA/cm²", through rated condition value of 10A/cm². As a result of the investigations, very high contact resistance can been found at very small current approaching to "ZERO", which may cause current sharing unbalance.