THE TRANSFORMER. 



159 



The following curves should be plotted on a load base, 

 either of secondary current or watts. 



(1) Efficiency. 



(2) Secondary voltage. 



(3) Power-factor. 



Fig. 73 shows the results of an experiment carried out 

 on a small 1 kw. transformer having a transformation ratio 

 of 100 to 50 volts. The efficiency curve shows a characteristic 

 which will be generally noticed in lighting transformers 

 which are only occasionally required to work at full load, and 



100 



90 



5O 8O 



70 

 J 



a> c 



^ 40 g 60 



* 50 

 | | 



8 30 | 40 

 w = 



^ 30 



2O 2O 



10 



10 o 



O 2 4 6 8 1O 12 14 16 18 2O 



Amperes. 

 FIG. 73. EFFICIENCY OF A TRANSFORMER. 



are often connected to the mains for many hours to supply a 

 light load only. This feature is the comparatively high 

 efficiency at light loads, where the iron losses are almost the 

 only ones felt, with a decided fall of efficiency towards full 

 load on account of the comparatively large copper losses. 



A transformer designed to have low iron losses (and 

 consequently a high efficiency at light loads) and heavier 

 copper losses will show a higher annual efficiency if working 

 on a circuit which is only occasionally fully loaded, than a 

 transformer designed with smaller copper losses and greater 

 iron losses. A transformer intended to work usually at 

 or near full load should, of course, have low copper losses. 



