274 



PRINCIPLES OF ELECTRICAL DESIGN 



If the depth of the slots in the rotor has been decided upon 

 and the number of ampere-conductors in each slot determined, 

 the distribution of m.m.f. over armature surface due to the 

 field winding can readily be plotted as in the lower sketch of 

 Fig. 105. Thus the ampere-turns in the coil nearest the neutral 

 zone are represented by the height AB, those in the middle coil 

 by BC, and those in the smallest coil by CD. The broken straight 

 line so obtained is best replaced by the dotted curve, which takes 

 care of fringing, and represents the average effect. This curve, 

 being the open-circuit distribution of m.m.f. over armature 

 surface for a given value of exciting current, may be combined 

 with the curve of armature m.m.f. to obtain the resultant 

 m.m.f. under loaded conditions. The flux curves for open- 

 circuit and loaded conditions can then be derived by proceed- 

 ing exactly as in the case of the salient-pole designs. 



A BE C D 



FIG. 106. Distribution of air-gap permeance in turbo-alternator 

 when unwound portion of pole face is not slotted. 



With salient-pole machines variations in the open-circuit 

 flux distribution can be obtained only by shaping the pole shoes 

 so as to alter the air gap over the pole pitch in a manner which 

 is not easily determined except by trial, but with the field wind- 

 ing distributed in slots it is not difficult to arrange the coils to 

 give any desired distribution of m.m.f. over the pole pitch. 

 Thus, if the desired flux curve is known (a sinusoidal distribu- 

 tion is usually best for alternating-current machines), and if the 

 average permeance per square centimeter of the air gap has been 

 calculated, an ideal m.m.f. curve can easily be drawn, since, at 

 every point, the m.m.f. is the ratio of the flux density to the 

 permeance per square centimeter. The spacing and depth of 

 slots can then be arranged to produce a magnetizing effect as 

 nearly as possible the same as that of the ideal curve. 



94. Armature M.M.F. in Alternating-current Generators. 

 In a continuous-current machine the current has the same value 



