Shunts for High-Current Density Brushes

Abstract

The conventional shunt size cannot be maintained with brush body current densities in the range of 1000A/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (1.55 MA/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and higher. The large shunts which are necessary to preserve low-voltage drops and acceptable temperatures ate found to be heavy and stiff. Brush body voltage drop may also become a significant contributor to electrical power loss. With bulky shunt leads, it is difficult to avoid adverse mechanical restraint of the brush by the shunt, and this in turn prevents achievement of the contact load control necessary for good brush performance. In some machines, the contact load is also modified by electromagnetic forces on the brush body associated with conventional shunting techniques. A sliding contact shunt is being developed to replace the conventional shunt. The new shunt greatly reduces the electrical power loss and the effect of magnetic forces. Friction interference with brush movement is made acceptable through the application of multiple-point contact configurations. Increasing the number of contact points reduces the total contact force required to achieve a given value of resistance. This force reduction associated with well-defined multiple contacts presents itself as a useful design tool in electrical machinery development. The concept of the multiple-point contact shunt has been demonstrated experimentally, and test results are found to be consistent with the theoretical predictions.