CHAPTER XXIII 



SERIES TRANSFORMERS 



Series transformers designed to supply current for 

 operating meters and relays are generally tested in groups 

 of ten to twenty at a time. The tests made are: Cold 

 resistance on the secondary winding (one out of every 

 five), polarity, ratio, heat run and insulation. To test 

 the insulation between layers, the transformers are run 

 for one minute at full load primary current with the 

 secondary open. The primary and secondary windings 

 must be carefully distinguished. In series transformers 

 the winding that is to be connected in series with the 

 circuit is called the primary, and the primary leads are 

 nearly always brought out through much larger bushings 

 than the secondary leads. 



Cold Resistance 



When several transformers are tested at the same time, 

 a measurement of cold resistance on the secondaries 

 need only be taken on about one-fifth of the transformers 

 in the group. The primary resistance is too low to be 

 measured accurately, and this test is usually omitted. 

 The same precautions must be observed on these trans- 

 formers as on large transformers. 



Polarity 



Polarity should be carefully taken. If the polarity 

 is not correct, trouble will be experienced in making 

 connections for polyphase meter circuits. 



Ratio 



Instead of actually determining the ratio, the trans- 

 formers are checked with a standard. The one selected 

 as standard must be sent to the standardizing laboratory 

 to be carefully checked for ratio at proper load and 



[194] 



