CALCULATION OF EDDY CURRENT LOSSES. 101 



The iron losses are ei h where i h is the energy component of the 

 primary current on open secondary circuit. If we write 



ritV + ei h ...... (28) 



the amended expression for the efficiency becomes 



since i\ is known for any load and i h can be calculated from the 

 known iron losses, the value of TI is perfectly definite. 



61. To calculate the Energy lost in Eddy 

 Currents in the Core of a Transformer. Let Fig. 

 38 show a section of one of the 

 transformer stampings at right angles 

 to the magnetic flux, so that b is the 

 thickness and a the width of the 



stamping. Let L be its length. FlG 38 



Consider an elementary path, 



whose outside length is x -f dx and breadth y -\- dy, and let the 

 induction be given by 



B sin pt 



The flux through the elementary circuit is 



xyB sin pt 

 therefore the E.M.F. acting round the path is given by 



e = -AwyB sin pt) X 10 ~ 8 



_ xyBp cos pt 

 10 8 



The resistance of the elementary path is given by 



pLdy pLdx 

 p being the specific resistance of iron 



2x 



= rr> nearly 

 pLdy' 



since y is always small compared with x. 



