Comparison of Brush Sliding Contact Characteristics between Structure Steel Ring & Stainless Steel Ring for Large AC Motors

In large AC motors, such as synchronous motors and wound rotor asynchronous motors for industrial applications, brush / collector ring current collection systems are applied. For these motors, bronze had been applied to the collector ring originally. However, in recent years, steel ring has been applied to the collector ring, due to mechanical strength for high speed collector rings. For the steel collector ring, structure steel is applied generally to turbine generators, for which maintenance labors can be expected and stainless steel is applied generally to large industrial motors, for which less maintenance are required and for which maintenance labors to prevent ring surface "rust", which will cause operation failure, can not be expected. In case of structure steel ring, more than 60 years of long time operation experience exists for turbine generators, while in case of stainless steel ring, only 30 years of operation experience exists for industrial motors. Brush sliding contact characteristics for the stainless steel ring, have not yet been enough studied or disclosed in technical papers. In this paper, following results have been obtained 1) V-I sliding contact characteristics for stainless steel are almost same as those for structured steel 2) Same behaviors have been confirmed at stainless steel / structure steel comparison test, in which voltage drop and electric resistance across small strip and maximum temperature at center of the small strip, are measured with same electric current and same strip size, as far as the maximum temperature exceeds "curie point" temperature. 3) Stainless steel ring contact characteristics and structure steel ring contact characteristics are considered to be same, as to reliabilities which are required to actual machine service operation, because a-spot temperature is always exceeding the "curie point" temperature, except for very low brush current density conditions, for both stainless steel ring and structure steel ring.