REGULATION OP ALTERNATORS 



317 



to modification in matters of detail and may be elaborated if 

 desired. 



In the writer's opinion, a further advantage of the method of 

 flux distribution and wave-form analysis lies in the fact that the 

 designer obtains thereby a clearer conception of the factors enter- 

 ing into the problem of regulation than he is ever likely to obtain 

 if he confines himself to the use of formulas and vector diagrams, 

 which are always liable to be abused when familiarity with their 

 purpose and construction leads to forgetfulness of their meaning 

 and limitations. 



111. Efficiency. In estimating the efficiency of an alternating- 

 current generator before it is built, the same difficulties occur as 

 in the case of the dynamo. There are always some losses such 

 as windage, bearing friction, and eddy currents, which cannot 

 easily be predetermined, and it is therefore necessary to include 

 approximate values for these in arriving at a figure for the total 

 losses. Very little need be added to what has already been said 

 in Art. 60 of Chap. IX, to which the reader is referred. He should 

 also consult the working out of the numerical example under 

 items (148) and (149) in Art. 63; and make a list of all the losses 

 occurring in the machine at the required output and power factor. 



In the ratio efficiency = - r~JT\ - ; 



the generator at a given power factor with which we are con- 

 cerned, and not the rated k.v.a. output. 



Windage and bearing friction losses are never easily estimated; 

 but the following figures may be used in the absence of more 

 reliable data. 



actual output of 



APPROXIMATE WINDAGE AND FRICTION LOSSES EXPRESSED AS PERCENTAGE 



OF FULL-LOAD OUTPUT 



K.v.a. output 

 f 50 



Self -ventilated A.-C. generators. 



Turbo-alternators: forced ventilation 

 (exclusive of power to drive fan) .... 



200 



500 and 

 larger 



2,000 



5,000 

 10,000 

 15,000 

 20,000 



Windage and 

 bearing friction 



1 . 5 per cent. 

 1 . per cent. 



. 5 per cent. 



1 . 8 per cent. 

 1 . 5 per cent. 

 1 . 2 per cent. 

 1 . per cent. 

 0.9 per cent. 



