METHODS OF TESTING ALTERNATORS 



277 



(a curve which generally is nearly a simple straight line); it is also 

 possible to separate the mechanical losses, first making this test without 

 field-excitation, and then exciting the field-magnets so as to obtain 

 the normal e.m.f. 1 



Summing up, this test permits of determining, with a fair approx- 

 imation, the losses on open circuit and then on load, without being 

 obliged to actually develop these losses by full-load in the alternator 

 as well as the corresponding heatings. 



From an electrical point of view the same test permits of deter- 

 mining the total values of the direct reaction as a function of the 

 reactive current, and the constant L' of the transverse reaction. If 

 it is desired to analyze these phenomena more completely, the value 

 of ojs may be determined as follows: 



Determination of (us. For this purpose the following consider- 

 ations are made use of (the basis of which is the method of calculation 

 of the short-circuit current given by Kapp, which does not lend itself 

 to experimental verification): 



Let ONM be the characteristic of excitation; that is to say, the 

 curve of the armature e.m.f. as a function of the exciting ampere- 

 turns. Suppose the excitation to be constant and equal to OA 

 ampere-turns, giving an e.m.f. represented by AM. Suppose 

 reactance us sought for to be known 

 and also the short-circuit current 7^. 

 If a point P is taken on the curve 

 whose ordinate is equal to wsl cc , the 

 corresponding segment of the abscissa 

 AH will measure the back ampere- 

 turns of the armature KNI CC . If the 

 calculated values of (os and of K are 

 taken, the direction of the straight line 

 AP is known, and the line may be 

 traced from which the value of /^ may 



be deduced. This is the construction of Kapp. We shall take up, on 

 the contrary, the inverse problem, supposing /<.<. determined by experi- 

 ment and seeking to deduce from it the two constants K and cos. 



The following observation may then be made. If the segment 

 AH be taken as the measure of the short-circuit current /<* and a 

 segment AB as the measure of any other reactive current Id to the 



The total efficiency of the engine and alternator unit may also be obtained 

 later at load by means of the engine diagram. 



the 



B_ljA Field 

 \ec ] Ampere-turns 



FIG. 26. 



