382 



ELEMENTS OF ELECTRICAL ENGINEERING. 



of the iron in cubic centimeters, / is the number of magnetic 

 cycles per second, / is the thickness of the laminations in centi- 

 meters, cB is the maximum flux density, and e is a constant 

 depending upon the electrical conductivity of the iron. The 

 value of e for sheet iron is about 1.6 x icr 11 . The value 2.5 

 X io~ n is more frequently used for e so as to allow for incom- 

 plete electrical insulation between the laminations. 



Armature core loss. The total loss of power in an armature 

 core is due partly to hysteresis and partly to eddy currents. 

 Neither of these parts can be accurately calculated, because the 

 flux density in the core of a toothed armature is far from uni- 

 form. This is especially true when the armature flux <$ is 

 greatly distorted by the armature current. The eddy current 

 loss, as calculated by equation (18), is all the more uncertain be- 

 cause of the indeterminate character of the insulation between the 

 sheet iron laminations, especially if the edges of the laminations 

 are burred by filing or other machining. 



If it is desired to predetermine the armature core loss of a 

 newly designed dynamo, it is less satisfactory to calculate the 

 loss by equations (17) and (18) than it is to estimate it from ex- 

 perimentally determined values of the armature core loss of other 

 machines. The following table gives the average results of ex- 

 perimentally determined core losses in toothed armatures. These 

 values are tabulated for various values of f a , the product of 

 the number of magnetic cycles per second and the flux density 

 in the armature core below-the slots. The value of/" is pnJ2 

 where / is the number of field magnet poles, and n is the speed 

 of the armature in revolutions per second. 



TABLE. 



Armature core losses. 



