Evaluation of current density in DC motor brushes for mining machines based on air gap field measurement

Due to robustness and simplicity of control, DC machines continue to be widely used in many applications including mining. Though a mature technology, DC machines still require research attention to accomodate for heavier loads and harder operation conditions dictated by production needs. Recent advances in sensoring, data acquisition and signal processing technologies allow for direct measurement of quantities that used to be theoretically modeled or indirectly measured in the past. Within a research project with industry partners, the research team obtained the measurements of flux densities inside the motor air gap under both static and dynamic conditions. A variety of valuable information about the motor performance can be derived from these measurements. This paper focuses on one particular aspect of the commutation performance of DC machines, namely, the magnitudes and uniformity of current density through a brush, which to a large degree dictate wear rates of the DC motor commutating group. The paper revisits the classical commutation theory, presents a mathematical model for current density estimation across a brush, and applies this model to the experimental results obtained. The results are then discussed and related to the commutation quality of the machine.