Till: TRANSFORMER 177 



lions which follow the application of load to the secondary, 

 enabling the primary to take from the line the increased power 

 demanded by the secondary. 



The rhamri' in the back electromotive force in the primary from 

 no load to full load is ordinarily about 1 or 2 per cent. As the 

 hack emf. is proportional to the mutual flux 0, the value of <f> 

 therefore does not change appreciably over the working range of 

 the transformer. If this flux does not change appreciably, the 

 ipere-turns acting on the core cannot change appreciably. 

 Therefore, the increased ampere-turns due the secondary load 

 must l>e just balanced by the additional ampere-turns due to the 

 increased primary current. It also follows that the exciting cur- 

 rent must remain substantially constant. 



The effect of any increase of primary ampere-turns, when not 

 opposed by equal secondary ampere-turns, would be to increase 

 the mutual flux. This would increase the back emf.. and might 

 cause the primary to deliver power back into the power source, 

 which is in violation of the law of the conservation of energy. 

 Therefore, any primary ampere-turns in excess of the exciting 

 ampere-turns must be balanced by equal and opposing secondary 

 ampere-turns. 



The exciting current is of small magnitude and differs con- 

 siderably in phase from the total primary current, as shown by 

 / in riir. 17L page 180. Therefore, it is usually neglected in 

 comparison with the total primary current. If it be neglected, 

 the primary and secondary ampere-turns are u/ual, and 



Tli< 



/'; - J; 



That is, ///< iiritnnru and 8econ<l<iri/ currents are inversely fix the 

 ' turns. 



Tli- hit ion also follows from the law of the c<n>er\ at ion 



of energy, If th ormer losses be neglected and unity 



power-fact of be assumed. 



r,/, - r / 



/, r, tf, 



/ " V, " 



