290 



ELECTRICAL EQUIPMENT 



34567 

 Amperes Field 



FIG. 161. Alternator Characteristics. 



157 to 159). Knowing from the no-load saturation curve the 

 required net excitation at this voltage, and correcting it for the 



33ooj I - | | | - - - p , effect of the armature re- 



3ooo| I I I I I I L^iTJ action, the necessary total 



field ampere-turns are ob- 

 tained. The result of such 

 calculations for different loads 

 and power factors are repre- 

 sented by the load-character- 

 istic curves. Such a full-load 

 characteristic of an alternator 

 is shown by curved, Fig. 161. 

 In order to get the best 

 combination for automatic 

 voltage regulation an alter- 

 nator should preferably have 

 a range in excitation from 



no-load to maximum load, with approximately 80 per cent 

 power factor, of the ratio of not more than one to two. With 

 125 volts excitation, the voltage should, therefore, not be allowed 

 to exceed 125 volts at maximum load, 80 per cent power factor, 

 and the corresponding no-load excitation should be about 70 

 volts. Should the excitation voltage be 250, the same ratio should 

 hold true. 



The excitation required varies considerably for different 

 machines, depending upon the size, the number of poles, the speed 

 and the regulation. For alternators of different capacities, but 

 otherwise similar, the relative excitation naturally decreases as 

 the size of the alternator increases. High-speed machines gen- 

 erally require less excitation than slow speed, due to the smaller 

 number of poles. With a large number of poles, however, the air 

 gap is usually smaller, and this will somewhat offset the higher 

 excitation for slow-speed machines. In general, it may be said 

 that small machines with many poles require a proportionally 

 large excitation and large machines with few poles a small exci- 

 tation. The curves given in Fig. 162 give the approximate exci- 

 tation required by water-wheel driven synchronous generators. 

 The values given are per Kv.A. per R.P.M. of the generator capa- 

 city, and is based on a maximum continuous rating at 80 per cent 

 power factor. 



