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ELECTRICAL ENGINEERING 



the reactance. The primary and secondary currents are the same 

 in both cases and the secondary impedance is the same. 



The only voltage generated in the secondary is the impedance 

 drop and therefore the flux and the exciting current are the same 

 in both cases. The angle measures the phase shift of the trans- 

 former; it is the amount by which the primary and secondary 

 currents are out of line. Referring to the diagrams; with the large 

 value of reactance the phase shift is small but the ratio of currents 

 tends to differ from the ratio of turns; with the large resistance 



EV e' 



FIG. 294. Vector diagram of a cur- FIG. 295. Vector diagram of a cur- 

 rent transformer with large sec- rent transformer with a large sec- 

 ondary reactance. ondary resistance. 



the phase shift is increased but the ratio of currents is more nearly 

 constant and equal to the ratio of turns. In order to fulfil both 

 the required conditions the exciting current and the core loss 

 should be as small as possible and therefore the core should have 

 high permeability. In order to reduce the flux to a minimum the 

 impedance of the secondary should be as small as possible. The 

 impedance of the primary winding does not affect the instrument 

 except in so far as the primary leakage reactance is a measure of 

 the secondary leakage reactance. 



The primary winding usually consists of a single turn in series 

 with the line and the secondary may have any number of turns 

 depending on the reduction in current required. 



The current in the primary is the load current of the circuit and 

 does not depend in any way on the transformer. It sets up a 

 magnetic flux in the core which links with both the primary and 

 secondary windings. When the secondary is open circuited all 

 the primary m.m.f. is magnetizing and a very large flux is pro- 

 duced in the core. This may produce dangerous voltages in the 



