AIR-GAP FLUX DISTRIBUTION 



271 



slots as shown in Fig. 104 are sometimes used. The calculation 

 of windings for the cylindrical type of field magnet will be taken 

 up in Art. 93. 



FIG. 104. Rotor of two-pole turbo-alternator, with parallel slots. 



91. Variation of Permeance over Pole Pitch Salient-pole 

 Machines. The curve representing the variations of per- 

 meance between pole face and armature core over the entire 

 pole pitch, can be drawn exactly as in the case of the D.C. 

 design. This was explained in Arts. 39 and 41 of Chap. VII. 

 The pole shoe in Figs. 42 and 43 is shaped in accordance with the 

 cosine law as explained in the preceding article, and the 

 flux lines in Fig. 43 are therefore such as would enter the 

 armature of a salient-pole alternator on open circuit. The 

 practical construction of Fig. 45 has been carried out on a pole 

 of similar shape, and nothing more need be added here concern- 

 ing the manner of plotting a permeance curve similar to the one 

 shown in Fig. 44. 



The effect of tooth saturation may be dealt with also as in the 

 D.C. design (Art. 42), and a set of curves such as those of Fig. 

 49 should be drawn. On account of the higher frequencies, 

 the tooth densities are lower in A.C. than in D.C. machines, 

 and the effect of saturation of the armature teeth is therefore 

 less noticeable. The "apparent" flux density, calculated for a 

 point half way down the tooth, can generally be used without 

 introducing any appreciable error in A.C. designs. 



I 



