158 COMMERCIAL ELECTRICAL TESTING 



and turns agree, repeat the ratio with the same meters; 

 and if still out, repeat with an entirely different set of 

 meters and potential transformer. If the ratio is still 

 out, the transformer is wrong. Try the ratio of another 

 transformer. If, however, the ratio should be correct 

 when the second set of meters is used, a third set should 

 be used and the ratio checked again. If the second and 

 third sets of meters give a correct ratio, record both sets 

 of readings. 



In taking ratio on transformers with taps or dial switch 

 note whether or not full windings are used. If the 

 transformer has more than one primary or secondary 

 coil, note whether the coils are in series or multiple. 

 The ratio must check within one per cent of the ratio 

 of turns. It is not necessary to take ratio on more than 

 one transformer of a group, as the parallel run will 

 determine whether they all have the same ratio. 



Checking Taps 



Nearly all transformers are provided with taps in one 

 or both windings, so that a slight change in ratio or a low 

 voltage for starting duty may be obtained. Before check- 

 ing taps, procure the proper winding specification and 

 sketch. Taps are easily checked by applying a certain 

 voltage per turn to the low tension winding and reading 

 the voltage between the terminals of the winding and the 

 first tap; then between successive taps on the same coil. 

 In some cases it is equally satisfactory to apply full 

 potential to one winding and read the voltage between 

 adjacent taps. This is done on dial switch transformers. 



The method can best be explained by an example. 

 TakeanAB-25-400-6300/6195/6048/59855/835/5600-170. 

 The primary winding has six coils connected in series, 

 of 43 turns each, with inside and outside ends. This is 

 called a single section coil. The secondary winding 

 consists of six coils connected in multiple of 7 turns 

 each. Taps are brought out of the primary coils P-l and 

 P-6 at the ending of the 29th, 34th, 38th and 41st turns 

 from the inside end. This gives tap turns of 43 41 =2, 

 41-38 = 3, 38-37 = 1, 37-34 = 3, 34-29 = 5 turns. 

 Since the secondary winding has four coil terminal 



