378 ELECTRICAL EQUIPMENT 



used. In general it may be said that it is usually difficult to go 

 above an 8 to 10 per cent reactance in a 60-cycle moderate size 

 transformer (1000 to 2000 Kv. A.), without undue eddy-current 

 losses, and that the allowable maximum would be considerably 

 less than this in low voltage designs. For 25-cycle transformers, 

 a higher reactance may be obtained, since the eddy-current losses 

 are, of course, less at a given density. 



Regulation. The regulation of a constant-potential trans- 

 former is defined by the A.I.E.E. rules as the difference between 

 the no-load and rated-load values of the secondary terminal 

 voltage at specified power-factor (with constant primary impressed 

 terminal voltage), expressed in per cent of the rated-load secondary 

 voltage, the primary voltage being adjusted to such a value that 

 the transformer delivers rated output at rated secondary voltage. 

 All parts of the transformer affecting the regulation should be 

 maintained at constant temperature between the two loads, and 

 where the influence of temperature is of consequence, a reference 

 temperature of 75 C. shall be considered as standard. If a 

 change of temperature occurs during the test, the result shall be 

 corrected to the above reference temperature. 



For non-inductive load the regulation varies approximately 

 from less than 1 per cent for large sizes to around 3 per cent for 

 smaller units. For inductive load it is naturally higher. It can 

 be determined by loading the transformer and measuring the 

 change in voltage with change in load at specified power- 

 factor. 



The A.I.E.E. recommends the following method for comput- 

 ing the regulation for any specified load and power-factor from 

 the measured impedance watts and impedance volts. 



Let P = impedance watts, as measured from short-circuit test 

 and corrected to 75 C.; 



E z = impedance volts; 

 IX = reactance drop in volts; 

 I = rated primary current; 

 E rated primary voltage; 

 5 r = per cent drop in phase with current; 

 q x = per cent drop in quadrature with current. 



