274 



ELECTRICAL MACHINERY 



is feeding power to the primary coil. The terms " high- 

 tension coil " and " low-tension coil " are often used instead 

 of the terms " primary coil " and " secondary coil." 



The power input to the primary depends directly upon the 

 amount of power being supplied by the secondary. It is not 

 at once evident how the power gets from the primary to 

 the secondary and we will first consider that point. 



Equation of the Transformer. Fig. 168 represents an 

 elementary transformer, having a laminated, closed magnetic 

 circuit, core A, a primary coil, B, and a secondary coil, C. 



FIG. 168. View of Elementary Transformer. 



Let 1 1 = the primary current; 



/2 = the secondary current; 

 NI =the number of turns in the primary coil; 

 N2 = the number of turns in the secondary coil ; 

 .Ri = the resistance of the primary coil; 

 R2 = the resistance of the secondary coil; 

 eiandl?i=the primary impressed voltage (instan- 

 taneous and effective) ; 

 <j> = the magnetic flux in the core at any time. 



The impressed e.m.f. of the primary must be balanced 

 by the reacting forces in the primary coil, so we may write 

 (for an open-circuited secondary) 



ei Riii-{-NiX(t^e rate of change of flux) . (64) 



