DIRECT-CURRENT MACHINERY 213 



the armature iron by the e.m.f.'s generated in it as it cuts across 

 the flux. 



In Fig. 180 abed represents a section of an armature punching 

 of thickness t in. If the flux density in the gap is B lines per square 



FIG. 180. Eddy current loss in the armature core. 



inch and the edge ab is moving with a velocity of S ins. per second 

 across the flux, then, the e.m.f. generated in the length ab is 

 e = BtS 10- 8 volts. 



This e.m.f. will cause a current to circulate through the iron as 

 indicated by the arrow; the value of the current will be 



e BtS 10- 8 



i = j- ; -- amperes, 

 kp kp 



where p is the specific resistance of the iron and & is a constant 

 depending on the dimensions of the section. 

 The loss in the section will be 



7 

 p = i z kp = - , 2 2 - = ki - watts, . (222) 



where fe, is a constant. 



The eddy current loss, therefore, varies as the square of the in- 

 duction density, the square of the thickness of the punchings and 

 the square of the speed; it also depends on the specific resistance 

 of the iron used but it cannot be calculated accurately in the case 

 of a rotating armature, where the induction density varies through- 

 out the section. It increases under load due to field distortion but 

 tends to decrease as the temperature rises and increases the spe- 

 cific resistance of the iron. 



Eddy currents are also produced in the pole faces due to local 

 variations of the induction density as the armature teeth move 



