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40 TAE MaRINE REVIEW A LARGE STEAM TURBINE GENERATOR. B, A. BEHREND. CHIEF ELECTRICAL ENGINEER, ALLIS-CHALMERS : co., IN The Electrical Age. _ It is only a short time ago that the installation of a number of 5,000-K. W., three-phase generators in the power houses 'of the Manhattan and Interborough stations, in the city of "New York, attracted considerable attention. A few years ago the capacity of these units was considered very large, _and_.the speed of the Corliss engines operating at 75 revolu- tions per minute made these generators some of the most gigantic machinery in the world. o _ Since that time the unwearying efforts of engineers have developed the steam turbine which, in regard to speed, rep- 'resents a position of greatest antithesis to the reciprocating steam engine. From 75 revolutions per minute with the Corliss engine, we are making the immense stride to 750 revolutions per minute in the steam turbine, increasing the speed tenfold. The diameter of the rotating field of the Man- -hattan and Interborough generators, turning at 75 revolutions per minute, is 32 ft., whereas the diameter of the rotating -ifield of the steam turbine generators operating at 750 revolu- tions per minute is only slightly larger than 6 ft. That such 'difference in operating conditions involves a radical departure from standard engineering practice and leads to machines of 'altogether different design, may be concluded without much investigation. A generator of 5,500 K. W., direct connected to a steam. tur- bine, was recently tested at the Bullock Works, at Cincinnati, of the Allis-Chalmers Co. This machine and its perform- ance are interesting on account of numerous novel features as well as on account of the high efficiency, perfect regulation, and low temperature, which have been brought out by tests. Although the. normal rated capacity of this generator is 5,500 K. W. at either 6,600 or 11,000 volts, the generator must be capable of standing 6,880 K. W. continuously without rising more than 45 degrees C., and 8,250 K. W. for three hours. without rising more than 55 degrees C. On the over- load this generator, therefore, will require a prime mover developing 11,500 H. P. The current developed by this machine has 25 periods, is three-phase, and is generated by a 4-pole magnetic field turning at 750 revolutions per minute. Fig. 2 shows the ma- chine assembled on the test floor; Fig. 1 shows the stationary core or armature; and Fig. 3, the rotating element. Fig. 2 illustrates well the compact design of this machine,*due to high speed and the small number 'of poles; Fig. 1 gives a good view of the thorough lamination and division of the stator core for the purpose of ventilation and cooling; while Fig. 3 shows the most interesting part of the machine, namely. the rotating field. When we stop to consider that the peripheral speed of the surface of this rotating field is almost 15,000 ft. per minute. we realize that we have before us a problem taxing the designing ability of the engineer to the utmost. If we could proceed, either on railroads or on the ocean, at such speeds as this, we should travel from New York to London in approximately 17 hours. Under such conditions of speed the centrifugal force as+ sumes tremendous magnitude and the careful distribution and calculation of the stresses and strains in the different members of the rotating magnetic field become a matter of the utmost importance and seriousness. The bursting of a mass of steel, of 75,000 lb. rotating at the rate of 3 miles per minute, would produce very disastrous effects, and the re- sponsibility of the designer and builder of such machines is, therefore, a very grave matter. A careful investigation in regard to the stresses and strains produced in rotating discs under the influence of centrifugal force was carried out by the writer several years ago, theoretically as well as experimentally, and it brought forth the interesting result that, in a plain rotating disc, the stresses are a maximum on the inside boundary of the disc and a minimum on the outside boundary. These results, obtained by mathematical analysis, were borne out by experiments on lead discs, the distortion of which was carefully studied by measuring the discs before the speed tests and after, and the