212 ELECTRICAL ENGINEERING 



If the induction density B is expressed in lines per square inch 

 and / is the number of cycles of magnetism per second, then the 

 hysteresis loss per cubic inch of iron per second is 



/ B Y- 6 

 W h = ri ((254)2) ( 2 -54) 3 / ergs per second; 



but 1 erg per second = 10~ 7 watts, 



therefore, the loss in watts per cubic inch for a frequency of / 

 cycles per second is , 



W h = 0.83 Y^/IO- 7 watts (221) 



In transformers where the induction density is nearly uniform 

 throughout the volume of the iron this value multiplied by the 

 volume of iron in cubic inches would give the hysteresis loss very 

 closely; but, in the case of dynamos the induction density is not 

 uniform, but varies from a maximum at the roots of the teeth 

 to almost zero near the shaft, as shown in Fig. 179 and, therefore, 

 the hysteresis loss cannot be calculated accurately but must be 

 estimated from experience with similar machines. 



FIG. 179. Flux distribution in the armature core. 



The hysteresis loss varies directly as the speed of the dynamo 

 which is proportional to the frequency of the reversals of mag- 

 netism and it increases to a slight degree under load due to the 

 distortion of the flux. In the regions where the density is in- 

 creased the loss is increased more than it is decreased in the regions 

 where the density is decreased. 



(e) The eddy current loss is due to electric currents set up in 



