THE TRANSFORMER. 179 







Instructions. Place the transformers to be tested in a 

 row with the high-tension terminals to one side and the low- 

 tension terminals towards the operator. Connect the high- 

 tension side of the transformers to the correct voltage 

 derived from the mains or from an auxiliary transformer. 

 Connect a pair of long leads in series with an ammeter to the 

 low-tension side of the standard transformer. Connect 

 these leads in turn with the secondaries of the other trans- 

 formers, taking care to connect the left-hand terminal of 

 each to the right-hand terminal of the standard, &c. When 

 connection is made in this way, differences in secondary 

 voltage of less than -2 per cent, produce a readable deflection 

 of the ammeter. 



The necessary voltage may be obtained from one of the 

 series of transformers ; but, in this case, the voltage applied 

 to its secondary winding must be slightly higher than the 

 secondary voltage given out of the remaining transformers, 

 since it has to supply the magnetising current for the whole 

 series. 



This method compares the ratio of transformation of the 

 transformers at no-load only, but if the resistances of the 

 windings do not vary by more than 20 per cent., the differences 

 in the voltage drop will have a maximum variation of -4 

 per cent, if a total copper loss of 2 per cent, is allowed. 



A heating test under load should also be carried out to 

 make sure that the normal load can be carried without 

 excessive rise of temperature. 



The Auto -Transformer. Transformers, which are em- 

 ployed for low pressures only, sometimes have their primary 

 and secondary windings connected in series. 



Since the changes of voltage in the primary and secondary 

 windings of a transformer go through their maximum and 

 minimum values at the same time, the result of connecting 

 the two windings in series is to produce a voltage which is 

 either the sum or the difference of the voltages of the 

 windings, according to the mode of joining them. 



If the windings of a step-down transformer are joined in 

 series so that their resultant voltage is the sum of the vol- 

 tages of the two windings, the source of supply may be con- 

 nected to the terminals of the composite winding, instead of 

 to the terminals of what was originally the primary winding. 



