94 SYNCHRONOUS ALTERNATORS. [Exp. 



41. Zero Power Factor. When cos0 = o, it is seen that, by the 

 construction of Fig. 8, Mz and MT are in one straight line; hence 



MT = MQ Mz ; or, Mo = MT -\- Mz. 



At no load MO = MT. Under load, if MT (and ET) is to have the 

 same value as at no load, the field excitation Mo is to be increased by 

 an amount Mz added in this case arithmetically* 



42. Determination of Full-load Saturation Curve. Given the no- 

 load saturation curve, Fig. I ; the full-load saturation curve for zero 

 power factor is found by adding the constant magnetomotive force 

 Mz = OG. The two curves (i) and (6) are accordingly a constant 

 distance apart, measured horizontally. 



43. Application. To illustrate the use of the magnetomotive 

 force method, it will suffice to apply the method, using observed data, 

 to the following typical cases : 



1. Using the Institute Method, 40, obtain Eo, corresponding to 

 rated voltage, ET, at full load, unity power factor. Plot this as the 

 point p, Fig. 6. Note that this point is a little lower than Eo obtained 

 by the electromotive force method, i. e., the regulation is better. 



2. Also, locate p by the method of 39. 



3. By the method of 38 and 41, locate the point q, Fig. 6, that is 

 Eo corresponding to rated ET at full load, zero power factor. Note 

 that this is considerably lower than Eo obtained by the electromotive 

 force method. 



4. Construct a full-load saturation curve (42) for zero power 

 factor. 



44. Justification of the Magnetomotive Force Method. The con- 

 struction of Fig. 8 shows that the armature ampere-turns are com- 

 bined with the field ampere-turns in such a way as to have the great- 

 est effect for power factor zero, cos0 = o; the least effect for cos 

 0= i ; and intermediate effects for intermediate values of cos 6. This 

 will be shown to be qualitatively correct, although quantitatively it is 

 only correct approximately or under certain assumptions. 



45. Fig. 9 shows two conductors of an armature coil, one midway 

 under a north pole, the other midway under a south pole. In this 

 position the electromotive force induced in the armature conductors 



* (4ia). The corresponding electromotive forces at zero power factor 

 are likewise added arithmetically; Eo = ET -\- Ez. (See 21.) 



