196 PRINCIPLES OF ELECTRICAL DESIGN 



following figures indicate approximately the losses due to wind- 

 age and bearing friction in modern types of dynamos. 



Rated kw. output Friction losses (per cent.) 



10 3.0 to 4.0 



30 2.5 to 3.0 



60 2.0 to 2. 5 



100 1.7 to 2. 3 



200 1 . 3 to 2.0 



500 1.0 to 1.8 



Large machines 0.7 to 1.5 



Closer Estimate of Hysteresis and Eddy-current Losses in 

 Armature Teeth. For the purpose of calculating the armature 

 losses with sufficient accuracy to determine whether or not the 

 temperature rise is likely to be excessive, it was suggested in 

 Art. 31 (page 103), that the average value of the apparent tooth 

 density be used in calculating the iron loss in the teeth. When 

 the tooth density is very high, or the taper of the tooth consider- 

 able, this method will not yield very accurate results. It is the 

 actual tooth density which, together with the frequency, will 

 determine the losses per pound in a given quality of steel punch- 

 ings; and this actual tooth density may be read off a curve plotted 

 from the formulas derived in Art. 37, Chap. VII. 



The tooth density with which' we are concerned in the calcu- 

 lations of power losses, is obviously the maximum density, and 

 this will occur when the tooth is in the zone of maximum air 

 gap density, the value of which can be read off the full-load 

 flux distribution curve, C, derived as explained in Art. 43, 

 Chap. VII. 



When the tooth is not of the same cross-section throughout 

 its length, the question arises as to what particular value of the 

 actual tooth density should be taken for the purpose of calcu- 

 lating the iron losses. The tooth might be divided into a 

 number of imaginary sections concentric with the shaft, and the 

 watts lost in the elemental sections could be calculated and 

 totalled; but this would be a lengthy and tedious process, and 

 the following approximation will usually give results of suffi- 

 cient accuracy for practical purposes. 



First calculate the actual flux density at the root of the tooth 

 (see Art. 37, page 119), and then again, at two other sections, 

 namely, near the top where the circumference of the " equivalent" 



