CHAPTER XVIII 



SINGLE-PHASE TRANSFORMERS 



The following order of tests on transformers has been 

 found to be most convenient : Cold resistance ; polarity ; 

 ratio and checking of taps; impedance; core loss and 

 exciting current; parallel run; insulation tests; double 

 potential for one minute; one and one-half potential 

 for five minutes; and high potential test. 



Transformers built for potentials above 50,000 volts 

 should have the double potential test taken after the 

 high potential test. 



Since many of the tests on the different types are 

 almost identical, a complete discussion on the air blast 

 type will first be given, and then shorter discussions on 

 the others. 



Single-Phase Air Blast, Type AB Transformers 



The transformer should be properly placed over the 

 pit and no opening left through which air can escape 

 and influence the reading of the thermometer on the 

 transformer iron. When the transformer is in place, a 

 careful inspection should be made, making note of any 

 defect, no matter how slight, and if found, it should be 

 repaired immediately. 



The order of tests may be varied if found necessary. 

 Usually two or more transformers of the same rating 

 are tested at once. In the following discussion, two or 

 three transformers are considered. 



Cold Resistance 



If the temperature guarantee of the windings specifies 

 that the increase-in-resi.stance method be used, consider- 

 able care must be taken in measuring resistances. This 

 measurement is usually made as follows: Place a ther- 

 mometer on the coils of each transformer and send from 

 10 to 15 per cent full load current through the trans- 



[155] 



