7 SYNCHRONOUS ALTERNATORS. [Exp. 



APPENDIX I. 

 MISCELLANEOUS NOTES. 



12. Tests on Polyphase Generators. The tests described above 

 may be made on polyphase generators in the same manner as on 

 single-phase machines. The polyphase generator when loaded should 

 ordinarily be given a balanced load, i. e., one that is divided equally 

 between the several circuits. Tests may also be made by loading 

 down one phase only and taking measurements on the unloaded as 

 well as the loaded phases. 



In plotting curves, plot voltage and current per phase (the more 

 usual way) ; or, line voltage and equivalent single-phase current. See 

 Exp. 6-A, particularly 28-30. 



13. Power Factors Less than Unity. The characteristics of an 

 alternator under load vary with the power factor of the load. With 

 a power factor less than unity and current lagging, the regulation 

 will be poorer, the full-load saturation curve will be lower, the exter- 

 nal characteristic lower and the armature characteristic higher than 

 at unity power factor. The reverse is true when the current is lead- 

 ing (instead of lagging), as it may be when there is capacity in the 

 line or in the load, or when the load consists in part of over-excited 

 synchronous motors or converters. 



These facts may be fully shown by calculation (Exp. 3~B), or 

 by a complete series of runs made with loads of different* power 

 factors. If such runs are to be made, it will be more profitable to 

 make them after Exp. 3-6. At present, it will suffice to illustrate 

 these facts by a few readings only, as in the next paragraph. 



14. Tests to Compare Effects of Inductive and Non-inductive 

 Loads. The difference between inductive and non-inductive loads 



composite winding is not, however, being extensively used, for it can not 

 give constant voltage under all conditions e. g., varying power factor 

 and the rectifying commutator is liable to spark. The Tirrell regulator 

 (33, Exp. i-B), applied to the exciter of an alternator, can maintain 

 constant voltage under all conditions of load. 



*'( I3a). This will require special facilities for adjusting power factor; 

 for an inductive load, this can be done by means of an adjustable resist- 

 ance and adjustable reactance in parallel. Runs should be made at one 

 high power factor, one medium, and one as low as can be obtained. 



