IMPEDANCE OF TRANSFORMER 165 



test should bo applied to the machine when hot, after a load run of 



nil hours' diiratiuii. 



The value of the testing voltage is determined by the inirni.il 

 working voltage of the winding under test. Now, since the factor >f 

 safety in the case of low-voltage windings is much higher than that 

 which it is possible to attain with high-voltage windings, the ratio 

 testing voltage , , , , , , 



working voltage y g volta g es > as 



otherwise there may be serious danger of damaging the winding. 

 Again, the application of a moderately high voltage over a prolonged 

 perind must be regarded as furnishing a more satisfactory and less 

 dangerous test of dielectric strength than that of a veiy much higher 

 voltage during a shorter period. 



For voltages up to 5000 volts, the testing voltage may be double 

 the working voltage. Between 5000 and 10,000, the testing voltage 

 may be 5000 volts in excess of the working voltage, while for any 

 voltage exceeding 10,000 the testing voltage should not exceed 1 

 times the working voltage. The testing voltage should be applied 

 for hour.* 



90. Experimental Determination of Equivalent 

 Impedance of Transformer 



For the purpose of determining the regulation t of a transformer, 

 we have to find its equivalent reactance. This may be done by short- 

 circuiting the secondary, and applying a p.d. to the primary sufficient 

 to produce the full-load currents in the windings. In order to 

 eliminate any error due to the resistance and reactance of the 

 ammeter, it is preferable to place this in the primary circuit. If 

 V = primary p.d. required to produce the full-load current Ii in the 



V 

 primary of the short-circuited transformer, then . gives us the 



equivalent impedance of the short- circuited transformer, the equiva- 



xg \ 2 

 lent resistance being >'i + ( ^ ) r *> where /*i, r. 2 are the resistances, 



and Si, 8-2 the turns in the primary and secondary respectively. The 

 transformer behaves as if there were no magnetic leakage and no 

 resistance in its coils, but as if connected in series with its primary 



there were an impedance T *, the resistance component of this 



* The above rules ure those adopted by the German Klektrotechnidcher Verciu. 

 t See 80-8:1. 



