THE MAGNETIC CIRCUIT 



197 



smooth core armature would cut through the tooth, and also at 

 a point midway between these two extremes. These sections 

 are shown in Fig. 38, page 122; and if the assumption is made that 

 no flux lines either enter or leave the sides of the tooth, the densi- 

 ties at the middle and top of the tooth can readily be expressed 

 in terms of the root density since they will vary inversely as the 

 cross-section of the tooth. If, now, the iron loss in watts per 

 pound is read off the curves of Fig. 34, page 102, for the three 

 selected values of the tooth density, the average loss per pound 

 multiplied by the total weight of iron in the teeth, will represent, 

 within a reasonable degree of accuracy, the total loss in the 

 teeth of the machine. 



A high tooth density is an advantage from the point of view 

 of field distortion. It can easily be understood that a high flux 

 density, by saturating the teeth, will have a tendency to resist 

 the changes in the air-gap flux distribution, brought about by 

 the cross-magnetizing effect of the armature currents; and it is 

 therefore advisable to check the approximate estimate of tooth 

 losses with the results obtained by the more exact method of 

 calculation. 



In predetermining the efficiency, it is important that all the 

 power losses in the machine be taken into account. These must 

 include the losses in rheostats or diverters, which may be con- 

 sidered as part of the machine. A complete list of the losses to 

 be evaluated when predetermining the efficiency will be found 

 in the following chapter, where a numerical example will illus- 

 trate in detail the steps to be followed in designing a dynamo. 



As a check on calculations, the efficiencies in the following 

 tables may be referred to. They represent a fair average of 

 what may be expected in a machine of modern design. 



FULL-LOAD EFFICIENCIES OF DYNAMOS 



Kw output Efficiency, per cent. 



10 86.0 



20 87.6 



30 



40 



50 



75 

 100 

 200 

 500 and larger 



88.8 

 89.7 

 90.3 

 91.2 

 91.6 

 92.0 

 93 to 9$ 



