CHAPTER V. 



THE EXCITING AMPERE-TURNS IN ELECTRICAL 

 MACHINERY 



33. The Exciting Current in a Transformer. The magnetic 

 flux in the core of a constant-potential transformer is determined 

 : ially by the primary applied voltage, and is practically 

 independent of the load (see Art. 25). When the terminal 

 voltage is given, the flux becomes definite as well. The ampere- 

 turns necessary for producing the flux are called the mag- 

 netizing or the exciting ampere-turns. When the secondary cir- 

 cuit is open the only current which flows through the primary 

 winding is that necessary for producing the flux. This current is 

 called the no-load, exciting, or magnetizing current of the trans- 

 former. When the transformer is loaded, the vector difference 

 between the primary and the secondary ampere-turns is practi- 

 cally equal to the exciting ampere-turns at no-load. 



The exciting current is partly reactive, being due to the 

 periodic transfer of energy between the electric and the magnetic 

 circuits (see Art. 16 above), partly it represents a loss of energy 

 due to hysteresis and eddy currents in the core. Some writers call 

 the reactive component of the no-load current the magnetizing 

 current, and the total no-load current the exciting current. 

 Generally, however, the words magnetizing and exciting are used 

 interchangeably to denote the total no-load current. The 

 components of the current in phase and in quadrature with the 

 induced voltage are called the energy and the reactive components 

 respectively. 



The no-load or exciting current in a transformer must usually 

 not exceed a specified percentage of the rated full-load current ; 

 it is therefore of importance to know how to calculate the 

 exciting current from the given dimensions of a transformer. 

 Knowing the applied voltage and the number of turns, the maxi- 

 mum value of the flux is calculated from eq. (28). We shall 



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