3-B1 PREDETERMINATION. 73 



EXPERIMENT 3-B. Predetermination of Alternator Charac- 

 teristics.* 



i. Introductory. It is desirable to be able to predetermine 

 the performance of any machine without loading, and this is 

 particularly true of alternators; for, in the case of large ma- 

 chines, the regulation can not be conveniently found in any 

 other way. 



There are two simple methods for predetermining the per- 

 formance of an alternator approximately, the electromotive 

 force method and the magnetomotive force method. Although 

 other more complex methods are proposed for the more exact 

 determination, no one method has been found which is generally 

 accepted and gives correct results in all cases. It is well to first 

 thoroughly study the electromotive force method, on account of 

 the insight it gives into the general performance of the alterna- 

 tor and into other methods of dealing with the subject. The 

 magnetomotive force method should then follow; after which, 

 other methods (essentially modifications of these two) can be 

 made a special study by those who desire to pursue the subject 

 further. (See Appendices I. and II.) 



2. There are primarily two causes for the change in termi- 

 nal voltage of an alternator with load : 



1. The effect of armature resistance, which is small and defi- 

 nite; this causes a drop in electromotive force which is in phase 

 with the armature current and is equal to R I. 



2. The effect of the flux set up by the armature current, a 

 much larger and less definite effect, discussed in the next para- 

 graph. 



. 3. All the flux set up by the armature current encircles the 



*To be preceded by Exp. 4-A. See 9 for a statement of data to be 

 taken. For a short experiment, take 1-18 and 26-30, plotting curves 

 for unity power factor only. The curves used to illustrate this experi- 

 ment and Exp. 3-A all relate to the same machine. 



