276 ELECTRICAL MACHINERY 



former is very small compared to its full load current 

 (generally about 5%), we may put, without much error, 



..... (68) 

 or 



N 1 I 1 =N 2 I 2) ...... (69) 



which gives the approximate fundamental relation between 

 the currents in the primary and secondary. This may 

 be stated in words thus : The primary ampere-turns are always 

 just equal to the secondary ampere-turns (the magnetizing 

 current being neglected). 



Ratio of Voltages. If all the flux which the primary 

 produces threads the secondary coil (and this is approx- 

 imately true), the volts generated per turn will be the 

 same in each winding. If we call this voltage e, we must 

 have 



E 2 = N 2 e ........ (70) 



and 



(71) 



Equation (71) is true because the right-hand member is 

 the counter e.m.f. of self-induction and this must be equal 

 to the impressed e.m.f. (approximately). 



Substituting (70) and (71) in (69), we have 



#1/1 = 2/2 ......... (72) 



This is the approximate equation for apparent power in 

 the two coils. Also from (70) and (71) we get the relation 



Ei/E 2 =Ni/N2, ..... (73) 



which is the fundamental voltage relation in the two coils. 

 Expressed in words this relation is: The primary voltage 

 bears the same relation to the secondary voltage as the 

 number of turns in the primary coil bears to the number of 

 turns in the secondary coil. Of course, from equation (72) 



