METHODS OF TESTING ALTERNATORS 279 



back ampere-turns, and the coefficient of the stray field, but which 

 is not directly applicable to the ordinary problem of the calculation 

 of the ampere-turns necessary for constant voltage. 



This method is one of those which was devised by Mr. B. A. 

 Behrend, who recently published a number of applications of ic. It 

 is wholly different from the preceding, in which the voltage is main- 

 tained constant at the terminals instead of the current. The same 

 author employs also another method, which consists in dividing the 

 field magnets into two equal parts to which are given exciting currents 

 of opposite sign and slightly different strength, so as to develop in 

 the armature a certain current, which is necessarily reactive while 

 developing a mean flux-density sensibly equal to that in normal 

 operation. This test may appear equivalent to that which is obtained 

 by means of two alternators mechanically coupled and set at opposite 

 phase (see preceding Method No. i), with the simplification, however, 

 that only one alternator is employed; but as the current thus obtained 

 is only a reactive current, the conditions are not identical with those 

 of the preceding method, but only equivalent to method No. 2 above 

 described (see the case of the alternator operating as a synchronous 

 motor on no-load). It is not, besides, sufficiently rigorous except 

 for alternators having a large number of poles, because when there 

 are but few poles the presence of two poles of the same sign side by 

 side upon the field magnet at two points on the latter would seem 

 to modify notably the conditions of the magnetic circuit. Conse- 

 quently, Method No. i is preferable when it can be 

 employed. 



Method No. 3. For the Determination of Trans- 

 verse Reaction (coefficient L). Besides the preced- 

 ing methods, several others may also be pointed out 

 which are very simple for the determination of the 

 transverse reaction, and which avoid the objection 

 so often made to the diagram in which this reaction 

 appears. Moreover, the oscillograph (or ondo- 

 graph) permits the real angle of dephasing <p to 

 be measured between the internal e.m.f. E (Fig. 27) and the current, and 

 gives immediately, in consequence, the value of L, when the alternator 

 operates upon a non-inductive resistance = 0, by the equation 



ti)LI + U sin <f) ioL 

 ~ sf- ~~U' 



r+ 7 



