CHAPTER XX 



THREE-PHASE TRANSFORMERS 



Special Tests 



The order of tests on three-phase air blast transformers 

 is the same as for the single-phase type. The mechanical 

 construction of the coils is identical with that of the 

 single-phase type ; but as the air paths through the 

 iron are longer, the air pressure required is slightly higher. 



Cold Resistances 



The general instructions given for measuring the 

 resistance of the single-phase type apply to three-phase 

 transformers, but since the primary circuits are opened 

 for the heat run, the resistance of each set of coils should 

 be measured. If the secondary coils are permanently 

 connected in delta, the resistance between each set of 

 leads should be measured. If the secondary coils are 

 arranged for diametrical connection, measure the resist- 

 ance of each set of coils. Note on the record sheet how 

 these readings are taken, so as to avoid confusion in 

 measuring and recording hot resistances. 



Polarity 



The determination of polarity on three-phase trans- 

 formers requires much care. The diagrams allow a 

 comparison to be made of the various standard connec- 

 tions. Figs. 53, 54 and 55 are three-phase connections 

 for three single-phase transformers, and Figs. 56, 57, 58 

 and 59 are connections for three-phase transformers. 



In determining the polarity of three-phase air blast 

 transformers, see that the primary and secondary coils 

 are connected as shown. (Figs. 56 and 57.) Supply 

 direct current to primary lines No. 1 and No. 2 to give 



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