3-B] PREDETERMINATION. 99 



Behrend (see his St. Louis paper, 55) has developed a method 

 for circulating power in one machine by dividing not the armature 

 but the field and reversing the excitation of one part of the field. 

 The armature acts as a generator with respect to one part of the field 

 and as a synchronous motor with respect to the other part. Each 

 part of the field has its own rheostat, one controlling the generator 

 and the other the motor action. Tests are made in much the same 

 way as though two machines were used, 52. For a later modifica- 

 tion of this method, see paper by S. P. Smith, 55. 



54. Arguments for and Against Specifying Regulation at Zero 

 Power Factor. The opinion is growing among engineers that regula- 

 tion should be specified at zero power factor. Tests at unity power 

 factor are objectionable, not only on account of the use of much 

 power which may be prohibitive, but also on account of errors in the 

 results. In Fig. 7 it is seen that the difference in regulation for a 

 small change in power factor is very small near zero power factor, 

 but is considerable near unity power factor. 



At unity power factor, therefore, any inductance or capacity in the 

 load introduces a large error. The use of a water rheostat as a load 

 causes an error for this reason, for it possesses a capacity which, 

 though small, is sufficient to give an alternator a better regulation 

 than it would have if the power factor were unity. 



Tests at zero power factor, on the other hand, have the advantage 

 that such errors are insignificant; furthermore, the tests are less diffi- 

 cult to make on account of the small amount of power required. 

 They can often be made when tests at unity power factor are not 

 possible. 



For these reasons, specification of regulation at zero power factor 

 (rather than unity power factor) has been advocated; such specifica- 

 tion can be checked by experiment and, furthermore, it gives the 

 regulation under the worst conditions. On the other hand, this is 

 objected to because, by itself, the regulation at zero power factor is 

 no positive .indication of the behavior of the machine at unity power 

 factor; two machines with the same regulation at zero power factor 

 may have very different regulations at unity power factor. This is 

 largely due to resistance drop, which is of importance at unity power 

 factor, but has practically no effect at zero power factor. Specifica- 

 tion of regulation at zero power factor is, therefore, insufficient 



