Carbon-brush contact phenomena in electrical machinery

Attention is drawn to the fact that the change in the contact resistance when sliding occurs has not so far been satisfactorily explained, nor has any mathematical analysis based on physical principles, giving the contact drop in terms of the current density, yet been applied successfully to sliding contacts. The transfer of current across the contact without arcing must be due to either thermionic or field emission, or to a combination of these phenomena, and it can be shown that the mechanism of thermionic emission can play only a very minor role in practical brush contacts. The principles of wave mechanics applied to the problem of field emission give for the true current density: Since this can be written in the form i¿ = A0E2¿¿k/E amp/cm2 it appears reasonable to suppose that the generalized contact equation with a true emission area Ae is where A0 and k are constants for given materials and x is the effective contact spacing. It is shown that the properties of the contact, such as effects due to pressure, temperature, materials and surface condition alter the values of Ae and x, thus determining the value of the contact drop with a given current density. Conclusions drawn from the experimental results show that the generalized contact equation is not inconsistent with the postulated theoretical process, and some information has been gained about the emitting area and the effective contact spacing of practical brush contacts comprising a carbon brush sliding on a copper or cast-iron slip-ring. The transfer of relatively small currents across the contact is believed to be due to the quantum mechanical tunnel effect and is not discussed in the present paper.