TRANSFORMERS 179 



Repeat this process for the two other phases and if 

 they all agree with the first one, the polarity is correct. 

 A sketch should be drawn on the record sheet, showing 

 how the polarity test was made. 



The polarity test on three-phase transformers also 

 determines whether there will be a change of rotation 

 of phase in transforming from one potential to the other. 

 To determine the polarity of transformers connected as 

 shown in Fig. 57, supply current to primaries 1-2 and 

 take deflection on secondary 1-2; this should show 

 reversed polarity. With current on 13 primary the 

 deflection on 3-4 secondary should be positive. With 

 current on primary, the deflection on 5-6 secondary 

 should be reversed. 



Ratio and Checking of Taps 



Whenever possible, a three-phase transformer should 

 have ratio and taps checked on each phase separately. 

 If this is not practicable, care must be taken to see that 

 both primary and secondary voltages are read on the 

 same phase. When these measurements can not be 

 taken single-phase, three-phase voltage must be applied. 

 If the ratio appears to be wrong, connect both windings 

 in delta, apply full voltage to the secondary, and read 

 the exchange current in the primary delta. This current 

 should not exceed 6 per cent of full load current. 



Impedance 



For this test, connect the transformer according to 

 Fig. 60 and follow the same general instructions as 

 given for the single-phase type. Calculate the current cor- 

 responding to the primary voltage of the transformer as 



c n capacity in watts 



follows : primary current = 



line voltageXV3 



Connect in two wattmeters of the same capacity, as 

 shown in Fig. 60. The ratio of the two potential trans- 

 formers or multipliers should be the same. The alge- 

 braic sum of the readings of the wattmeters will 

 represent the impedance loss of the transformer. The 

 sign of the readings must be carefully noted, since the 

 reading of one wattmeter may be reversed. To test for 



