THE TRANSFORMER 297 



threads both the secondary and primary. The amount 

 of leakage depends directly upon the load, because the 

 secondary current produces a back m.m.f. on the magnetic 

 circuit, forcing out of the secondary more and more flux 

 as the load increases. It is disadvantageous to have much 

 leakage in a transformer because of the poor regulation 

 which results; the two coils, primary and secondary, are, 

 therefore, placed as close together as possible. 



73. All-day Efficiency. The " all-day " efficiency of a 

 transformer is a very important factor for the central 

 station manager to consider; in fact it is much more im- 

 portant than the efficiency as ordinarily denned. 



Meaning of " All-day Efficiency." By all-day efficiency 

 is meant the ratio of watt-hours output to watt-hours input 

 during a whole day's operation. An example will make 

 this clear. Suppose a 5-kv-a. transformer is supplying a 

 lighting load; it would, in a normal day's run, operate at 

 full load 1J hours and perhaps at half load 1J hours; during 

 the rest of the day there would be no load on the trans- 

 former. Suppose the iron loss is 200 watts and the full- 

 load copper loss (I 2 R) is 200 watts; the copper loss at half 

 load would be equal to (J) 2 X200 watts = 50 watts. 



The transformer is connected to its supply line all 

 day so that the core loss for all day = 24 X 200 = 4800 watt- 

 hours. 



The copper loss for the 1J hours full load = 1JX 200 = 300 

 watt-hours and during the 1J hours half load = 1^X50 = 75 

 watts, so that the total copper loss for the day's run is 

 375 watt-hours. The total loss in the transformer for 

 24 hours = 4800+375 = 5175 watt-hours. 



The energy output in one day = 1JX 5000+ 1JX 2500 = 

 11,250 watt-hours. 



The energy input in one day =11,250+5175 = 16,425 

 watt-hours. 



The all-day efficiency = = 68.5%. 



