346 PRINCIPLES OF ELECTRICAL DESIGN 



o-i ccr\ 



y - = 36,800. An excitation slightly in excess of this amount 



U.Ooo 



will probably suffice, 1 because if the average density over the pole 

 pitch is raised from 6,010 gausses to 6,010 X T = 6,300 



gausses, the average effect of increased tooth reluctance, as shown 

 by Fig. 139, is small, and we shall try 37,000 ampere-turns on the 

 field. This full-load field excitation is represented by the curve 

 Mo of Fig. 142. Now add the ordinates of M and M a , and ob- 

 tain the resultant m.m.f . curve M . Using this new m.m.f . curve, 

 we can obtain from Fig. 139 the corresponding values of air-gap 

 flux density, and plot in Fig. 141 the full-load flux curve C of 

 which the area, as measured by planimeter, is found to be 113.3 

 sq. cm. This checks closely with the calculated area (113.5 sq. 

 cm.) and it follows that a field excitation of 37,000 ampere-turns 

 will provide the right amount of flux to give the required terminal 

 voltage when the machine is delivering its rated full-load current 

 at 80 per cent, power factor. 



Items (55) to (57). When the shape of the flux curves of Fig. 

 141 is considered in connection with the fact that a distributed 

 armature winding tends to smooth out irregularities in the result- 

 ing e.m.f. wave (see Fig. 118, page 293), it is evident that we need 

 not expect any great departure from the ideal sine curve in the 

 e.m.f. waves of this particular machine either on open circuit or 

 at full load. At the same time it will be well to illustrate the 

 procedure explained in Arts. 100, 101, and 102, by plotting the 

 actual e.m.f. wave resulting from the full-load flux distribution 

 curve, C, of Fig. 141. 



The average flux density corresponding to any given position 

 of the four slots constituting one phase-belt is obtained as ex- 

 plained in Art. 100, and the instantaneous values of the "ap- 

 parent" developed e.m.f. are calculated by formula (107). The 

 results of these calculations have been plotted in Fig. 143 to 

 rectangular coordinates, and in Fig. 144 to polar coordinates. 

 The mean ordinate of Fig. 143 is 3,605 volts, and the r.m.s., or 

 virtual value of the e.m.f., is the square root of the ratio, twice 



1 This method of estimating the full-load field ampere turns is not scientifically 

 sound, especially in the case of salient-pole machines, because the m.m.f. 

 distribution over the armature surface, due to the field-pole excitation, is 

 rarely sinusoidal as here assumed. The correct increase of field excitation 

 to obtain a given full-load flux must, therefore, be obtained by trial; but the 

 method here used indicates the approximate increase of excitation required. 



