CHAPTER II 



ARMATURE LAMINATIONS 



IT was formerly the opinion that the softest Swedish charcoal iron 

 was the most suitable material for armature laminations. But 

 some fourteen years ago attention was drawn to the phenomenon 

 of " ageing," l in virtue of which the hysteresis loss increases when 

 iron is subjected for long intervals to temperatures of 60 Cent, 

 and higher. The increase in hysteresis loss by "ageing" is 

 generally slow, but it is nevertheless liable to be ultimately 

 sufficient to be detrimental to the performance of the apparatus. 

 Although the disadvantage is most marked in the case of stationary 

 transformers, it has also been found desirable to alter the com- 

 position of the material employed in armature cores, partly in 

 consequence of the investigations of this phenomenon of " ageing," 

 and partly in consequence of the improvements which have been 

 made in the production of sheet steel. For the rate of deteriora- 

 tion through " ageing " may in general be said to be most marked 

 in soft Swedish charcoal iron, and to be quite low or even 

 altogether absent in certain qualities of sheet steel. It is often 

 the very cheapest qualities of sheet steel which are the most 

 suitable as regards freedom from " ageing.'' The permeability of 

 such sheet steels is, on the whole, inferior to that of sheets 

 rolled from charcoal iron, and the initial hysteresis loss may be 

 somewhat higher. But both of these disadvantages may be 

 greatly reduced by annealing the sheets from a suitably lii^h 

 temperature, i.e. from the highest temperature which it is 

 practicable to employ without causing in the adjacent plates any 



1 "Ageing" is defined by the German rules for iron as the percentage 

 variation of the figure of loss caused by keeping the sample at a tnujHTanm- 

 of 100 over 600 hours. The "figure of loss" is defined as the loss in watts 

 per kg. measured with a density of 10,000 lines per sq. cm. and a fre<jiu-n 

 50 cycles per second. (See " Selection and Testing of Material- f<>i < '-.nsti -u 

 tion of Electric Machinery," Prof. J. Epstein's paper read before 1 

 Engrs., Nov. 22, 1906.) 



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