Alternating Current Commutator Motor.—The number of commutator motors used on alternating current is much smaller than other types as the problem of commutation is more difficult than with direct current.
However, the demand for an alternating current motor which would be suitable for railway work resulted in various forms of single phase motors (requiring a single supply wire and trolley) having a speed-torque characteristic equivalent to that of a series direct current motor.
These are known as series, repulsion and doubly-fed motors. A demand for a polyphase alternating current motor with adjustable speed characteristics resulted in several "alternating current shunt motors." The schematic diagrams for two of these are shown in fig. 7, A and B. In A the brushes on the rotor are fed with current from taps taken off of the stator winding and hence at varying voltage. In B the rotor winding has collector rings at one end of the rotor and a commutator at the other. The supply line connection is made to the collector rings instead of to the stator winding. On the commutator are two sets of movable brushes. Opposite ends of the three separate phase windings on the stator are connected to corresponding pairs of brushes. Due to the shifting of the brushes, a changing voltage is impressed on the stator winding and speed adjustment results. These types of motors are expensive and complicated and are little used. Three general applications have been made of single-phase alter nating current commutator-type motors. The first, as a means of getting better starting characteristics on a single phase indus trial motor. Unlike a polyphase induction motor, a motor which is supplied from a single-phase supply line has no starting torque unless auxiliary windings or so-called phase splitting devices are used. However, a single-phase commutator motor of either the repulsion or series type has very good starting characteristics.
Theref ore it is the practice of several manufacturers to build a single-phase induction motor which has a rotor similar to the armature of a direct current motor. In addition, the rotor has a device actuated by centrifugal force which short circuits all the commutator bars after the rotor is up to speed and it then con tinues to run as an induction motor. This motor is referred to as motor. The second application of alternating current commutator motors is in connection with heavy traction or the electrification of steam railroads. Here the motor has been used both as motive power for electric locomotives and in so-called motor cars. The third general application for alternating current commutator motors is as auxiliaries to large wound rotor induction motors where adjustable speed must be had and where the application of direct current is not feasible. This system has been applied to main roll drives in steel mills and to variable ratio frequency changers, where two alternating current supply systems are to be tied together for the exchange of power, but where there must be some flexibility allowed to each system in the maintenance of its exact frequency. Some typical applications of alternating current motors are shown in Table II. In comparing the uses of direct current motors with those of alternating current, it may be said that alternating cur rent is more widely used but that certain classes of work can be handled more satisfactorily by direct current motors. Broadly, these are : industrial cranes, high-speed passenger elevators, ad justable speed machine tools, heavy steel mill reversing rolls and city street railway service.
Aside from these applications, alternating current motors are preferred because the general distribution of energy by any other means than alternating current is practically prohibitive on ac count of cost.