126 THE DESIGN OF STATIC TRANSFORMERS 



15-kw or even a 12-kw transformer will suffice for his 

 particular purpose, but this is not within the province of the 

 manufacturer. 



Thus I strongly condemn such advice as that preferred 

 by the engineer of a well-known transformer-manufacturing 

 company in the following paragraph : 



" Perhaps the better course for an engineer to adopt is that 

 of leaving the matter of temperature limits to be filled in by 

 the individual manufacturers, who are specialists, and have their 

 reputation at stake when tendering. To arbitrarily fix some 

 apparently low figure, when in reality nothing standard for 

 comparison exists, is to cause those manufacturers who declare 

 their maximum temperature to put up prices in order to go 

 under what to them must appear a ridiculous limit. 



" For transformers working without cases under climatic con- 

 ditions such as those existing, for instance, in many a London 

 sub-station, the writer is of opinion that better and safer 

 running may result by permitting transformer coils to assume 

 a degree of warmth such as to obviate any risk of break-down 

 that might result from the depositing of moisture on the surface 

 of the coils. 



" The rise in temperature of the iron core of a transformer 

 should, however, be kept well under 80 C. if any appreciable 

 mechanical restraint is imposed upon it, as ageing may occur 

 if the temperature of highly annealed iron remains at 80 C. 

 for long periods, and at the same time is subject to mechanical 

 stresses. 



" There appears to be a very considerable difference of opinion 

 as to what temperature may be permitted in a transformer. To 

 begin with, this limit of temperature should depend upon the 

 class of insulation employed. With oil-varnishes, and insula- 

 tions that oxidise, the limit is necessarily a low one, and 

 wherever oil is employed, the temperature should be kept down 

 to the lowest possible limits. The writer has run transformers 

 of his own design at temperatures as high as 150 C. for con- 



