102 



PRINCIPLES OF ELECTRICAL DESIGN 



The calculation of the watts lost in the core below the teeth 

 is a simple matter provided the assumption can be made that 

 the flux density has the same value at all points of the section 

 midway between poles. Although incorrect, this assumption is 

 very commonly made; and, for the purpose of estimating the 

 rise in temperature, the flux density may be calculated by divid- 

 ing half the total flux per pole by the net cross-section of the 

 armature core below the teeth. A reference to Fig. 34 will give 



100 200 



300 400 500 600 700 

 Kilogausses x Frequency, or. - 



800 900 1000 1100 



FIG. 34. Losses in armature stampings. 



the watts per pound, and this, when multiplied by the total weight 

 of iron 1 in the core (excluding the weight of the teeth), will be the 

 approximate total loss due to hysteresis and eddy currents. 



In order to calculate the losses in the armature teeth, it, is 

 necessary to know exactly what is the flux density at all sections 

 of the tooth. This is not readily calculated, because some of the 

 flux from the pole pieces enters the armature through the sides 

 of the teeth and the bottom of the slots. Again, in the case 

 of armatures of small diameter having teeth of which the taper 

 may be considerable, the change of section alters the flux density 

 and the degree of saturation, so that it is almost impossible to 

 determine accurately the average value of the tooth density for 

 use in calculating the watts lost. The question of flux density 



1 If the iron is of good quality it is generally permissible to reduce these 

 figures by 25 per cent. 



