MAGNETIC AGEING 



171 



In Fig. 124 are shown the results of a heating test for an oil- 

 cooled transformer, the upper or full-line curve giving the temperature 

 list- of i he windings as determined by the increase of resistance 



-<to 



-so 



-10 



HOURS 

 FIG. 124. Showing Temperature Rise in Transformer. 



method, and the dotted curve giving the temperature rise of the oil as 

 measured by a thermometer. 



In carrying out this test, the most convenient and economical 

 method is to employ the arrangement of connections used in the 

 Sumpner efficiency test, and shown in Fig. 122. 



94. Ageing of Transformer Core 



The magnetic ageing test is one of great importance in connection 

 with transformers. It was originally noticed by Mr. Partridge that 

 the core loss of certain transformers gradually increased in course of 

 time. The effect has since been dealt with by a large number of 

 investigators.* The slow increase in the core loss, or the ageing of the 

 core stampings, has been shown to be due to the maintenance of the 

 core at a moderately high temperature by the losses taking place in 

 it, and a precisely similar effect, it was found, could be brought about 

 by simply baking the core in an oven maintained at the proper 

 temperature. 



Manufacturers of transformer sheets have now succeeded in pro- 

 ducing a iwn-ageing material, i.e. one whose hysteresis loss is un- 

 affected by long-continued baking. The only method of applying the 

 ageing test is to go on measuring the core loss periodically for some 

 time after the transformer has been installed. If after a few months' 

 use no appreciable increase has taken place in the core loss, the 

 transformer may be regarded as fulfilling the non-ageing requirement. 



* The most complete investigation of the ageing effect is that carried oat by Roget, 

 sec The Electrician, vol. xli. p. 182 (1898), also Roy Soe. Proc., vol. Ixiv. p. 150 (1899). 



