CHAPTER XV 

 EXAMPLE OF ALTERNATOR DESIGN 



112. Introductory. The principles and features of alternator 

 design, as given in the foregoing chapters, will now be applied 

 and illustrated in the working out of a numerical example. A 

 steam-turbine-driven three-phase generator will be selected, be- 

 cause this design involves greater departures from the previously 

 illustrated D.C. design than would occur if the slow-speed type 

 of alternator with salient poles were selected. It is true that the 

 difficulties encountered in the design of turbo-alternators espe- 

 cially of the larger sizes, running at exceptionally high speeds 

 are of a mechanical rather than an electrical nature; but this 

 merely emphasizes the importance to the electrical engineer of a 

 thorough training in the principles and practice of mechanical 

 engineering. 



It is not possible for a man who is not in the first place an ex- 

 perienced mechanical engineer to design successfully a modern 

 high-speed turbo-alternator. These machines are now made up 

 to 30,000 k.v.a. output at 1,500 revolutions per minute (25 cycles) 

 and 35,000 k.v.a. at 1,200 revolutions per minute (60 cycles). 

 Larger units can be provided as the demand arises; it is probable 

 that single units for outputs up to 50,000 k.v.a. at 750 revolutions 

 per minute will be built in the near future. With the great 

 weight of the slotted rotors, carrying insulated exciting coils, and 

 travelling at very high peripheral velocities, new problems have 

 arisen, and these problems should be seriously studied by anyone 

 proposing to take up the design of modern electrical machinery. 

 Engineering textbooks may constitute a basis of necessary 

 knowledge; but, with the rapid advance in this field of electrical 

 engineering, the information of greatest value (apart from what 

 the manufacturing firms deliberately withhold) is to be found in 

 current periodical publications, including the papers and discus- 

 sions appearing in the journals of the engineering societies. 



Since it will not be possible to discuss the mechanical details 

 of turbo-alternator designs in these pages, a machine of medium 

 size (8,000 k.v.a.) will be chosen, and the peripheral speed of 

 the rotor will not be permitted to exceed 18,000 ft. per minute. 

 The mechanical difficulties will therefore not be so great as in 



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