Abstract :
Contact voltage losses are caused by resistance of contact between brush and slip ring or commutator. Their values are affected by composition, pressure, temperature, humidity, current density, etc. Test apparatus should control and accurately record all these variables. Two surface variations, which may cause large errors, have been investigated: (1) The change in the surface of the brush due to continued passage of current causes large momentary fluctuations in the values of contact voltage loss. (2) The thin coating of carbon which adheres to the commutator surface decreases contact voltage loss values by forming a conducting medium between brush and commutator. The complexity of the problem is shown by the large number of variables, each of which materially affects the contact voltage loss. It is proposed to record the effect of these variables for each brand of brush on standard tabular and diagrammatic forms. 2. The value of the power loss due to brush friction is affected by composition, angle of brush, surface conditions, speed, pressure, etc. Commercial tests are not sufficiently accurate to be of use in getting standard values. Such values can only be obtained when test apparatus is used which controls and records all the variables. Curves and formulas are proposed which record the effect of the principal variables on each brand of brush. In conclusion it is urged that tests to determine comparative values for losses due to contact and friction should be made under ideal conditions and that the values thus obtained are more dependable than those of commercial tests now being made, and should supersede them.
