306 



PRINCIPLES OF ELECTRICAL DESIGN 



of the machine, referred to in Art. 103 ; it is the curve for the com- 

 plete machine, and can be plotted only after the magnetic circuit 

 external to the armature has been designed. 



Knowing the increase of flux in pole and frame the magneto- 

 motive force absorbed in overcoming the increased reluctance of 

 these parts can be calculated, and in this way the dotted curve 

 A' of Fig. 125 can be drawn. This is merely the open-circuit 

 saturation curve corrected for increased leakage flux due to the 

 additional field current required to balance the demagnetizing 

 effect of a given armature current. 



Assuming all the alternating quantities to be simple harmonic 



functions, the vector diagram 

 (Fig. 126) can be drawn as ex- 

 plained in Art. 96. It shows 

 the voltage components for one 

 phase of the winding ; the vector 

 PE t being the IR drop, while 

 E g P and E' E g stand for the re- 

 actance voltage drops due to 

 end flux and slot flux respec- 

 tively. The numerical value of 

 E g P can be obtained from for- 

 mula (99) page 266, and of E' E g 

 from formula (106) of page 288. 

 Ampere-twrns per pole The apparent developed volt- 



FIG. 125. Method of construct- age E' g obtained from Fig. 126 

 ing saturation curve for zero power . ,, . , ,, ,, 



factor. gives the point M on the cor- 



rected no-load saturation curve 



A' in Fig. 125, and the distance E' M or OM r shows what exciting 

 ampere-turns are required to develop this electromotive force. 

 The terminal voltage is, however, only E t , which gives the point 

 N of the triangle MNR. Now draw NR parallel to the horizontal 

 axis to represent the total number of ampere-turns per pole due 

 to the armature current, which, as previously explained, will be 

 entirely demagnetizing, and must therefore be compensated by 

 an equal number of ampere-turns on the field pole. Thus E t R 

 or OR' is the field excitation necessary to produce Et volts at the 

 terminals of the machine. If the load is now thrown off, the 

 terminal pressure will rise to E and the percentage regulation 

 for this particular current output on zero power factor will there- 

 in 12 

 fore be 100 ^^/ This simple construction enables the de- 



