272 PRINCIPLES OF ELECTRICAL DESIGN 



92. M.m.f. and Flux Distribution on Open Circuit Salient- 

 pole Machines. The procedure here is still the same as in D.C. 

 design, and the reader is referred to Arts. 40, 41, and 42, of 

 Chap. VII. 



In deriving the curve of m.m.f. from the open-circuit flux 

 distribution curve (A), the modified method, as explained in 

 Chap. X (page 221) under items (72) to (76), may be used. This 

 short cut is often permissible in predetermining the air-gap flux 

 distribution of an alternating-current generator, because, as pre- 

 viously mentioned, the effect of low flux densities in the teeth 

 is to discount their influence on the distribution of the flux 

 density over the armature surface. 



By carefully shaping the pole face, a sinusoidal distribution 

 of flux density over the armature surface can be obtained on 

 open circuit; but the design of a salient-pole machine to give 

 a sine wave of e.m.f. under all conditions of loading involves 

 other factors, and is by no means a simple matter. The effect 

 of the armature m.m.f. will be considered after taking up the 

 special case of the cylindrical field magnet. 



93. Special Case of Cylindrical Field Magnet with Distributed 

 Winding. In the case of a slotted rotor carrying the field coils, 

 and an air gap of constant length due to the fact that the bore 

 of the stator or armature is concentric with the (cylindrical) 

 rotor the shape of the m.m.f. curve due to field excitation alone 

 can readily be found without resorting to the somewhat tedious 

 process of getting the permeance between pole and armature 

 points, as described and recommended for salient-pole machines. 



If the whole surface of the rotor is provided with equally 

 spaced slots, the average permeance of the air gap between 

 stator and rotor will have a constant value for all points on the 

 armature periphery. This condition is represented in Fig. 105, 

 if the center portion of the pole, of width W, is slotted as indicated 

 by the dotted lines. This constant average air-gap permeance 

 can be calculated within a close degree of approximation by mak- 

 ing conventional assumptions in regard to the path of the mag- 

 netic lines, as was done in the case of the salient-pole machines 

 when deriving formula (57) (page 117) giving the permeance at 

 center of pole. The flux lines can be supposed to be made up 

 of straight lines and quadrants of circles; and if the permeance 

 over one tooth pitch is worked out for different relative positions 

 of field and armature, very satisfactory results can be obtained 



