The alternating-current commutator motor

The alternating-current commutator motor in its various forms has been developed principally for the purpose of obtaining adjustable or variable speeds, the usual induction type motor, both in the single-phase and polyphase forms, being inherently constant speed. In general, control of speed with the alternating-current induction motor implies variable or adjustable frequency in some form, and this, in turn, involves a commutator of some type. In consequence, the problem of commutating alternating current usually goes hand in hand with that of speed control. The simplest method, according to the author, for viewing both the a-c. and the d-c. commutating problems, is to consider primarily the actual e.m.fs. short-circuited by the brushes, and the resistance in the short-circuited paths, — or in other words, the problem of commutation is largely one of the permissible amount of short-circuited current. In the d-c. machine the e.m.f. short-circuited by the brushes is that generated by rotation of the short-circuited armature coils in the armature and external field fluxes. In the same way, in the a-c. commutator motor, there is an e.m.f. due to the armature flux or field, as in the d-c. machine, and in addition, there are other e.m.fs. due to the primary or field fluxes. These latter may be classified as primary rotational and transformer e.m.fs. One limitation in the design of a-c. commutator motors, in general, is that these rotational and transformer e.m.fs. are often larger in degree than the e.m.f. due to the armature flux. However, the author contends that the commutation problem in a-c. motors is the same as for d. c. when all e.m.fs. are taken into account. The major part of the paper covers the consideration of the different e.m.fs. which should be taken into account in the various types of a-c. commutator motors, and it is shown in a general way that the e.m.fs. involved in speed control also appear in the commutation problem. In the latter part of the paper certain general conditions of commutation and brush operation are treated and some figures are given for comparison of a-c. and d-c. commutating limits.