169 



reading of W 2 will remain unaltered, and be the same in Fig. 122 as 

 in Fig. 123. The sum of the readings of Wi and W a * gives the total 

 loss of power in the two transformers, and half this sum represents 

 the loss in each transformer. From the loss and the load (as deter- 

 mined liy tin- reading of AM) the efficiency is easily determined.! 



One of the great advantages of this method lies in the fact that it 

 enables a full-load test of two large transformers to be carried out 

 with the expenditure of only a small amount of energy the power 

 dni\vn from the mains simply representing the losses in the trans- 

 formers. 



The efficiency test just described may be conveniently combined 

 with the leakage reactance test explained in 90. 



92. Separation of Core Losses 



In many cases, it is interesting to separate the core losses into 

 their hysteresis and eddy-current components. Such a separation 

 may be effected by a method due to Carhart.J The principle of the 

 method is based on the fact that, for a given constant value of the 

 maximum induction B, the eddy-current loss varies as the square of 

 the frequency (see 54), while the hysteresis loss varies in simple 

 proportion to the frequency. Hence the total core loss at any 

 frequency n may be written 



w - En 2 + Hn 



w denoting the total watts lost in the core, E?i 2 the eddy-current 

 watts, and Hn the hysteresis watts ; E and H being two constants 

 for a constant value of B. 



If, then, we carefully determine w at two different frequencies, n\ 

 and Tia, using the same value of B, the two equations 



enable us to find E and H, and thus to calculate separately the 

 hysteresis and eddy-current losses occurring at any frequency 

 always assuming B to remain constant. 



Instead of determining only two values w\ and w$ of w at two 

 different frequencies, it is advisable to take an entire series of 



* Corrected, if necessary, for any loss of power in the connecting wires and 

 instruments. 



f Efficiency = ; ^-r- . The load may be practically taken to be VA, where 



V = p.d. of mains and A = reading of the ammeter AM. 

 \ Electrical World, vol. xxxi. p. 30G (1898). 



