426 ELECTRICAL EQUIPMENT 



drop from no load to full load is the same in all the units, both in 

 magnitude and phase. 



The impedance of a transformer is generally expressed as the 

 voltage drop at normal load in percentage of normal voltage. It is 

 the resultant of two components; the resistance drop, which 

 depends only on the ohmic resistance of the windings and is in 

 phase with the current, and the reactance drop, which depends 

 on the magnetic leakage between the high- and low-tension wind- 

 ings and is 90 out of phase with the current. 



Thus per cent IZ = V(per cent J#) 2 +(per cent IX) 2 , 



where IZ = total impedance drop; 



IR = resistance drop of high- and low-voltage windings; 

 IX = reactance drop of high- and low-voltage windings. 



The value of per cent IZ is easily obtained by short-circuiting 

 one winding and measuring the e.m.f. which must be applied at 

 the terminals of the other winding to force full-load currents 

 through the winding at normal frequency. The impedance may, 

 therefore, be measured directly. 



The resistance e.m.f. is equal to the high-voltage current mul- 

 tiplied by the equivalent resistance of the transformer, which may 

 be obtained by measuring the resistance of both the high- and low- 

 voltage windings, and, adding to the resistance of the high-voltage 

 windings that of the low-voltage multiplied by the square of the 

 ratio of transformation. 



The reactance e.m.f. may be calculated from the known values 

 for the impedance e.m.f. and resistance e.m.f. Thus 



IX = V(IZ) 2 -(IR) 2 . 



In the majority of power transformers, the total resistance 

 drop is small compared to the reactance drop, in which case the 

 per cent impedance drop (per cent IZ) can be taken as approx- 

 imately equal to the per cent reactance drop (per cent IX). In 

 many lighting transformers, however, where the reactance is 

 made as small as possible, this cannot be done without introducing 

 considerable error. 



The following formulae may be used for finding the division of 



