124: TREATISE ON ALTERNATING CURRENTS. 



less tendency to leakage when the magnetic circuit is closed ; also 

 by winding the primary and secondary circuits in sections placed 

 alternately side by side the difficulty is to a great extent overcome. 

 To test the regulation of a transformer the terminal secondary 

 voltage should be taken at all loads from no-load to full-load, the 

 primary terminal P.D. being kept constant. 



INSULATION TESTS. 



79. Having tested the efficiency and regulation of a trans- 

 former, the next step is to test its insulation. 



The primary coil should be well insulated from the secondary. 

 To test the quality of the insulation between the two coils requires 

 more than the ordinary measurements for the determination of 

 insulation resistance. The insulation should first be determined 

 by any of the known methods, and then the transformer should 

 be run continuously for two or three hours, with twice or three 

 times the voltage for which it is intended, and then insulation 

 resistance should be determined again. If the insulation has in 

 no way suffered, the test may be considered satisfactory. 



TEMPERATURE TEST. 



80. It is of great importance that a transformer should remain 

 sufficiently cool even if working continuously on full-load. If 

 the temperature exceeds about 60 C., there is danger of the 

 insulation being damaged. 



Before a transformer is passed as satisfactory it should, there- 

 fore, be run continuously at full-load for some hours, and tempera- 

 ture readings taken at regular intervals of time by means of 

 thermometers inserted into it at different places. When the readings 

 indicate that the temperature is stationary, the thermometers 

 should not register more than 60 C. 



The stationary temperature depends upon the size and con- 

 struction of the transformer. A small transformer has a greater 

 cooling surface per watt of output than a large one. There is, as 

 a rule, no danger of overheating small transformers by simply 

 running continuously at full-load. With large transformers, 

 however, special devices have frequently to be employed to get 

 rid of the heat, e.g. immersion in oil. 



