TRANSFORMERS 157 



at once, one near the middle of the group should be 

 chosen as it will be safer and more convenient when the 

 transformers are run in parallel. With direct current 

 flowing through one winding of the transformer, connect 

 the special voltmeter across the terminals so as to get 

 a positive deflection; then transfer the drop lines to the 

 corresponding terminals of the other winding and break 

 the current in the first winding. If the polarity is correct, 

 a positive kick will be obtained. When making this test, 

 have sufficient resistance in series with the voltmeter 

 to protect it from damage. 



It is not necessary to take polarity on more than one 

 transformer of a group, as the parallel run will show 

 whether or not they all have the same polarity. In taking 

 polarity on special transformers, a clear sketch should 

 be made showing the polarity. For tap polarity, see 

 headings "ratio" and "checking taps." 



Ratio 



The ratio of a transformer is the ratio of primary volt- 

 age to secondary voltage, and should be equal to the 

 ratio of primary turns to secondary turns. The usual 

 method of determining this value is to apply about one 

 hundred volts to the secondary winding and read the 

 primary voltage, stepping it down with a suitable poten- 

 tial transformer. The ratio of the potential transformer 

 should be as nearly that of the transformer in test as 

 possible The potential transformer must be operated 

 at normal frequency and voltage, otherwise the ratio 

 will be unsatisfactory. In very small transformers, the 

 voltage should be applied to the primary windings. 



When the ratio of the potential transformer is very 

 nearly that of the transformer in test, the voltmeters 

 should be interchanged after five readings have been 

 taken. When this is done it is not necessary to correct 

 voltmeter readings from curves, as the meter errors will 

 appear in both columns and be eliminated. In determin- 

 ing any ratio at least five readings should be taken, and 

 the result carefully calculated. If the ratio by test 

 varies more than one per cent from the rated ratio of 

 voltage, check the ratio of turns; if the ratios of voltage 



