3-B] PREDETERMINATION. 9 1 



APPENDIX I. 

 MAGNETOMOTIVE FORCE METHOD * 



34. In the magnetomotive force method, instead of combining 

 vectorially various electromotive forces as was done in the electro- 

 motive force method, Figs. 3, 4 and 5 the corresponding magneto- 

 motive forces are so combined. 



35. The magnetomotive force corresponding to any electromotive 

 force is found by reference to the no-load saturation curve, and is 

 commonly expressed in ampere-turns. For a given machine, with 

 constant number of field turns, field amperes are proportional to 

 field ampere-turns and may be used as a measure of magnetomotive 

 force. In Fig. I of this experiment and Fig. 2 of Exp. 3~A, it is 

 seen, for example, that 627 volts corresponds to a field excitation of 

 7.33 field amperes, or 3,401 field ampere-turns, either of which may 

 be taken as a numerical measure of magnetomotive force. 



36. It is readily seen that a straight saturation curve gives mag- 

 netomotive forces proportional to electromotive forces, so that the 

 same results will be obtained from the use of either, if the procedure 

 is otherwise identical. On the other hand, a saturation curve which 

 is not straight gives values of magnetomotive forces not proportional 

 to electromotive forces, so that different results will be obtained 

 according to whether magnetomotive forces or electromotive forces 

 are used. 



37. Method. f The three magnetomotive forces Mo, Mz and MT 

 are combined vectorially, as in Fig. 8 ; cos is the power factor of 

 the load. 



These three quantities Mo, Mz and MT may be interpreted by their 

 correspondence^ to the three electromotive forces Eo, Ez and ET, 



* No additional data are required ; see 43 for the particular application 

 of the method to be made. 



t (37a). This is the common interpretation of the method (see Rush- 

 more, p. 740, Vol. I., St. Louis Elect. Congress, 1004). In Franklin & 

 Esty's Electrical Engineering, Mo is obtained as the resultant of two mag- 

 netomotive forces which correspond not to ET and Ez, but to Ep and EQ 

 (the in-phase and quadrature components of Eo). 



$(37b). If the saturation curve were a straight line and magneto- 

 motive forces were proportional to electromotive forces, the triangles for 

 magnetomotive forces and electromotive forces would be similar and each 

 side of one triangle would be perpendicular to the corresponding side 

 of the other. 



