OUTPUT OF CONVERTER 251 



Using (5) and (6) we get the following relative outputs, that of a 

 continuous-current generator being taken as 100 : 



N = 2 3 4 6 12 



Kelative output = 85'2 133 162 193 219'5 



The output, it will be seen, increases with the number of slip- 

 rings, rapidly at first, then more slowly, tending ultimately to the 

 limit 230 as the number of slip-rings is increased indefinitely. 



As a matter of fact, the output would increase even more rapidly 

 with the number of slip-rings than would appear from the above 

 figures. For the practical limit of output should be determined, not 

 by the average temperature rise of the coils, but by the highest 

 temperature to which any one coil is raised. Now, although, owing 

 to the partial equalization brought about by the conduction of heat 

 along the coils and core, and the more powerful radiation of heat 

 from the hotter parts of the armature, the curves of temperature rise 

 in the various cases will not be quite so steep as the curves of 

 Fig. 153 (the end portions becoming depressed, and the middle 

 portion raised, by the combined effect of conduction and radiation), 

 yet especially in the case of the 2-ring (single-phase) converter 

 the coils close to the slip-rings will attain a considerably higher 

 temperature than those midway between the slip-rings. It might 

 be thought that a simple method of surmounting this difficulty of 

 excessive local temperature rise would be to increase the cross-section 

 of the conductors close to the slip-rings ; but this would introduce 

 a fresh difficulty excessive sparking in certain positions of the 

 armature, due to the destruction of the symmetry of the winding. 



Single-phase converters are of no practical interest ; on account 

 of their relatively small output, the enormous concentration of the 

 heating on the coils close to the slip-rings, and the further difficulty 

 due to their tendency to spark by reason of the fluctuating armature 

 reaction, they are never employed in practice, motor-generators being 

 invariably used instead. Most of the large power transmission 

 plants making use of either two-phase or three-phase transmission 

 lines, both two-phase (4-ring) and three-phase (3 -ring) converters 

 are largely used, and it might be thought that these two types are 

 the only ones practically available. Such, however, is by no means 

 the case, for on account of the important advantages presented by 

 six-phase and twelve-phase converters, the former has been coming 

 very largely into use * with three-phase transmission lines, while 

 quite recently an arrangement has been patented for using a twelve- 

 phase converter in connection with a three-phase transmission line. 



* The four largest rotary converters hitherto constructed (by the General Electric 

 Co., for the New York Edison Co.), each capable of giving an output of 2500 k.w. 

 continuously, are operated six-phase (Electrical World and Engineer, vol. xliii. p. 1207 

 (1904)). 



