Carbon and commutator wear

WITH most theories on commutation the sparking voltage between commutator segments or the average voltage across the brush is used as the principal criterion for the commutation. More recently the idea has been advanced that the problem of commutation is very much the same in d-c. and a-c. machines as long as in both types of machines the sparking voltage is properly taken into account. It has also been shown by extensive data collected under practical operation that certain sparking voltages will have very similar effect in both a-c. and d-c. machines if other conditions are kept alike.² This should not be construed, however, to mean that the sparking voltage is the only important factor which enters into the commutating problem, especially into the wear of the carbons and the commutator. In other words, while it is true that the sparking voltages are of the same brand no matter whether they are induced in an a-c. or d-c. machine, the same sparking voltage may give widely different results under different conditions in either type of machine. The fact that very often the comparative data obtained with the same sparking voltage give rather consistent results for the carbon wear with different types of machinery is simply due to the fact that a good many factors, as for instance, current density, temperatures, commutator speeds, etc., are similar in a great many practical machines regardless as to whether they are a-c., d-c., or whether they are for industrial or traction purposes. Nevertheless it seems to the writer that the importance of these various factors upon the commutating problem should not be overlooked and that a detailed knowledge of the various factors as affecting brush wear is of utmost importance. If it is considered how important the brush and commutator wear is in connection with all kinds of commutating machinery it seems surprising how little investigation has been made along this line. While there are quite a number of tests available giving brush friction and contact resistance, practically nothing is known as to how sparking voltages, current densities, peripheral speeds, etc., affect the brush and commutator wear. It might, therefore, be in place to give here a few tests along this line with which the writer happens to be familiar.