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IN PAST years various attempts have been made to power locomotives by 25-cycle single-phase rectified voltage applied to d-c series motors. Early efforts were unsuccessful, mainly because of the faulty operation of the mercury-arc rectifier. Continued research and development, however, have now made the operation of d-c traction motors on mercury-arc rectifiers in locomotive service entirely feasible from a technical standpoint. In recent years, a new feature has been added. The development of the Diesel-electric locomotive has resulted in a standardized, low-cost d-c traction motor suitable for general-purpose road-locomotive applications. Hence, any present-day electric locomotive design employing d-c traction motors can benefit by the use of such standarized motors. An example of this is found in the rectifier locomotives recently built by the General Electric Company for the New York, New Haven and Hartford Railroad wherein the standard d-c traction motor shown in Fig. 1 was applied. The problem of operating these motors on rectified single-phase 25-cycle power resolved itself into the problem of smoothing the undulating current to a sufficient degree so that this motor would not be harmfully affected from either a stress or performance standpoint.