230 



PRINCIPLES OF ELECTRICAL DESIGN 



The ampere-turns per inch are read off the upper curve of 

 Fig. 3 (page 17), and the lengths of the various parts of the 

 magnetic circuit are taken from Figo 86. The length of the 

 iron path in the armature core is taken as one-third of the pole 

 pitch, as suggested in Art. 57. The ampere-turns for the air 

 gap, teeth and slots are read directly off the curve a, b, c, d t 

 e, /, of Fig. 82 (page 219), for the density corresponding to the 

 maximum value of the flux curve A of Fig. 84. It is assumed that 



4000 



5000 6000 7000 



Ampere -Turn s per Pole 



8000 



9000 



Fia. 87. Open-circuit saturation curve (numerical example). 



the shape of this curve remains unaltered, and that the maximum 

 ordinate is directly proportional to the developed voltage. 



The curve, Fig. 87, is plotted from the results of these calcu- 

 lations. It shows the connection between developed e.m.f. (or 

 open-circuit terminal voltage) and the corresponding ampere- 

 turns of excitation per pole. The shunt ampere-turns on open 



230 

 circuit are 5,930, and at full load, 5,930 X 220 = 6,200. The 



ampere-turns in the series winding are 7,850 6,200 = 1,650, 

 which includes the ampere-turns to compensate for armature 



