104 



PRINCIPLES OF ELECTRICAL DESIGN 



32. Usual Densities and Losses in Armature Cores. The 

 flux density in the core below the teeth will be determined by 

 considerations of heating and efficiency. The same may be 

 said of the tooth density, but in this case the total weight of iron 

 is relatively small, and higher densities are permissible. It is 

 desirable to have a high flux density in the teeth because this 

 leads to a "stiffer" field and reduces the distortion of air-gap 

 flux distribution caused by the armature current. Better pres- 

 sure regulation is thus obtained, and also improved commutation, 

 especially on machines without interpoles where the fringe of 

 flux from the leading pole tip is used for reversing the e.m.f. 

 in the short-circuited coils under the brush. If the density in 

 the teeth is forced to very high values, the losses will be excessive, 

 especially if the frequency is also high; another disadvantage 

 being the large magnetizing force necessary to overcome the 

 reluctance of the teeth and slots. 



The accompanying table gives flux densities in teeth and core 

 that are rarely exceeded in ordinary designs of continuous-current 

 machines. 



UPPER LIMITS OF FLUX DENSITY IN DYNAMO ARMATURES (GAUSSES) 



As a guide to the permissible losses in the armature punch- 

 ings of D.C. machines, the following figures will be useful. 

 They are based on modern practice and should not be greatly 

 exceeded if the efficiency and temperature rise are to be kept 

 within reasonable limits. 



Output of machine 



Core loss, expressed as percentage 

 of output 



10 kw... 



20 kw... 



50 kw... 

 100 kw... 

 500 kw. . . 



2.8to3.3 

 2.5to3.0 

 2.0 to2.4 

 l.Stol.8 

 1.3tol.5 



1,000 kw 1.2 to 1.4 



