Behavior Factors of Rectifier-Driven D-C Motors

In the rapid development and application of rectifiers to drive d-c motors, certain factors which influence the behavior of the motor may require unaccustomed consideration. The pulsating nature of currents which may be encountered introduces departures from conventional treatment of (1) heating value of the current waves, (2) stresses upon the insulation and commutator resulting from high peaks of voltage involved, and (3) torque pulsations tending to produce vibration and noise. Analysis is made from experimental and theoretical approaches of possible ratios of the heating values to be expected with typical wave shapes applied to electronically driven motors. The ratio of effective to average values of current varies with the load on the machine and with the number of tubes used in the rectification process. It is shown that the problem is most acute in machines of small rating where mechanical inertia is low and where tube capacities would permit the use of but one or two tubes for economy of materials. This situation may require ``overmotoring´´ an application in order to allow the required thermal capacity. Vibrations which will otherwise show up in undesirable effects on the driven machine, and noise, would also dictate the use of larger machines than necessary with conventional d-c practice. The conclusion is drawn that factors not at first apparent must be considered in the application of the rectifier drive, and that as many phases as practicable are desirable in the rectifier.