212 ELEMENTS OF ELECTRICAL ENGINEERING. 



This loss is nearly zero when the transformer is not loaded ; it 

 increases with the square of the current, and becomes excessive 

 when the transformer is greatly overloaded. 



104. Efficiency of transformers. The ratio power output -r- 

 power intake is called the efficiency of a transformer. The accom- 

 panying table shows the full-load efficiencies of various sized 

 transformers of a recent type. 



TABLE OF TRANSFORMER EFFICIENCIES. 



Rated Output Per cent. Efficiency 



Kilowatts. Full Load. 



I 94-8 



2 95-75 



3 96.2 



4 96.45 



5 96.65 



6 96.73 



7 v 96.8 



8 96-85 



9 96.9 



10 96.95 



15 97-2 



The efficiency of a given transformer is very low when the 

 output is small ; it increases as the output increases, reaches a 

 maximum, and falls off again when the output is very great. 

 This falling off of efficiency when the output is great is due to the 

 great increase of copper losses. Figure 178 shows the efficiency 

 of a transformer at various loads. 



All-day efficiency. A transformer is usually connected to the 

 mains continuously, although current may be delivered by the 

 secondary for a few hours only each day. In such a case the 

 iron loss is continuous and may represent a very considerable 

 loss of energy each day. The copper loss on the other hand is 

 not appreciable except when the transformsrjisxlafasidefclafi TtereJ- 

 fore a transformer wMdtfris^alrways^^rai^ 

 which is loaded only aicdhtertijtimQadawhsdi^teoialdobe'Jdeiign^d 

 to have as small a power loss as possible in the iron, even 

 the design may involve a considerable increase 

 in the copper. VV \ U SV + ^VSV = <\ 



