350 ELECTRICAL EQUIPMENT 



their output will be kept constant and the load fluctuations will 

 be taken care of by the synchronous generators. 



Places of Utilization. The foremost use of induction generators 

 is, therefore, to be expected in stations where no storage is pro- 

 vided and where the entire output must be utilized or wasted. 

 Such stations will need very little attendance, due to their sim- 

 plicity; probably only once or twice a day. Means must, how- 

 ever, be provided for disconnecting the unit from the system and 

 shutting the gates should the power for some reason or other go 

 off the line. This would, of course, mean that the generator would 

 be unloaded and the unit reach an overspeed which must be 

 automatically guarded against. 



General Construction. The construction of an induction gen- 

 erator is identical to that of an induction motor with a low- 

 resistance squirrel-cage secondary winding. The machine requires 

 a very small air gap and careful consideration must be given to 

 the ventilation. 



4. EXCITERS 



One of the problems in connection with large generating sta- 

 tions which has been given comparatively little attention until 

 lately, is that of excitation. It is, however, of the greatest im- 

 portance, as upon it depends, to a large extent, the successful 

 operation of the plant. The capacity of the exciter units, the 

 proper division of the required exciter capacity into several 

 units, the method of drive, whether by separate prime-movers, 

 by individual motors, or whether direct connected to the main 

 generating units, the arrangements and connections of the dif- 

 ferent units, the proper system of automatic voltage regula- 

 tion, etc., are all factors which demand a careful consideration 

 when designing a power plant. 



Separate Excitation. With very rare exceptions all synchron- 

 ous machines are separately excited, the excitation being obtained 

 from some direct-current supply source. Generally, separate 

 direct-current generators are provided for this purpose, and when 

 so utilized are termed " exciters." 



A separately excited generator has no inherent tendency 

 toward regulation, this being either effected by a rheostat in the 

 field circuit or by means of different systems of automatic voltage 

 regulation, as treated more fully in the next section. 



