Physical limitations in D-C. commutating machinery

In direct-current machines, there are a number of apparently distinct limitations, such as sparking at brushes, flashing at the commutator, burning and blackening of the commutator face, picking up of copper, etc. which, in reality, are very intimately related to each other. The principal object of the paper is to bring out such relationships and to show that all these actions are special cases of well known phenomena. The theory of commutation is considered only in its relation to the e.m.fs. generated in the coils short circuited by the brush; and the limiting e.m.fs. per commutator bar and per brush are shown to be fixed principally by brush contact resistance. The effects of the negative coefficient of the contact resistance is also referred to briefly. Flashing due to various causes is next taken up, and the relations between the maximum volts per bar and flashing conditions is indicated, both from test and general experience. Flashing due to various other causes, such as interrupting the circuit, etc. is also considered. Burning and blackening of commutators, high mica, picking up of copper, etc. are treated in detail, and these actions are shown to be very closely related to the commutation limits derived in the earlier part of the paper. Noise, vibration, etc. are also considered as limitations in design of d-c. apparatus. In approaching the ultimate design, these limitations become increasingly prominent. Flickering of voltage and winking of lights are two well-known actions in direct-current practise. A simple explanation of the winking of lights (not original with the author) is given with the results of tests on a generator with well proportioned compensating windings in the pole faces. Apparently the difficulty is a fundamental one, and is liable to occur in all types of d-c. machines. In conclusion, a brief chapter is given on design limitations as fixed by commutator peripheral speed. This applies particularly to large capacity high-voltage machines. An appendix is added covering a method for determining the maximum capacity of d-c. machines in terms of the short circuit volts per commutator bar when the various constants in the machine are given certain limiting values. The results show that in large high-speed machines, the maximum capacity is considerably above present practise.