298 ELECTRICAL MACHINERY 



Now the efficiency of this transformer in the ordinary 

 sense 



5000 



5400 



= 92.7%. 



Proper Design to Get High All-day Efficiency. If the 

 transformer were redesigned (so as to have the same full- 

 load efficiency) with 100 watts iron loss and 300 watts 

 copper loss at full load the all-day efficiency would be better 

 than before. 



Core loss, 24 hours = 24 X 100 = 2400 watt-hours. 



Copper loss, 24 hours = 1J X 300+ 1|X 75 = 562 watt-hrs. 



Output in 24 hours =11, 250 watt-hours (the same as 

 before) . 



1 12^0 



The all-day efficiency -^j 79.3%. 



It is therefore evident that if a transformer is to be 

 used for a service on which the full load demand exists for 

 a short time only, the transformer should be designed 

 with as low a core loss as possible, even if the I 2 R loss is 

 much increased thereby. This idea must not be carried 

 too far, however, because a transformer with high resistance 

 regulates poorly and it would be unsatisfactory for supplying 

 the power for lamps. 



74. Determination of Losses. The iron loss in a trans- 

 former is easily determined by reading the watts input 

 (at normal voltage and frequency), when the secondary 

 is open circuited; connections as in Fig. 188. Although 

 this wattmeter reading does include a small I 2 R loss due to 

 the exciting current, it is ordinarily not considered because 

 it is so small. The total wattmeter reading is reckoned as 

 core loss. 



To determine the full-load copper loss connections are 

 made as in Fig. 189, a very low voltage being impressed on the 

 primary. This voltage is increased until the ammeter in 

 the short-circuited secondary indicates the full-load cur- 



