64 THE DESIGN OF STATIC TRANSFORMERS 



transformers, and that the load factor on the transformer is 

 20 per cent., the cost pi-r annum \vith Transformer A amounts 

 to ouly .311, as against 321 for Transformer B. This 

 assumption for the cost per kw hr is not quite correct, hecause 

 the copper loss will necessarily correspond to a greater outlay 

 per kw hr than the core loss, the latter heing equivalent to a 

 load of 100 per cent, load factor and the former being equivalent 

 to a variable load, and therefore, as such, the cost per kw hr 

 will also be variable. For a load factor of 20 per cent, the 

 cost per kw hr of copper loss will amount to some 1,5 to 2 

 times the cost per kw hr of core loss. For our present purpose, 

 however, let us assume the same price per kw hr for both the 

 copper and the core losses. In fact, the circumstance that the 

 core loss at no load is associated with a very low power factor, 

 may largely or fully offset the advantage that it represents a load 

 of 100 per cent, load factor. Suppose that with both trans- 

 formers the customer pays 3d. per kw hr delivered from the 

 secondary ; then in both cases he pays 



85 OOP X 3 _ 

 ~240~ 



If Transformer A is employed, the electricity supply company's 

 annual gross profit is 126, as against only 116 when Trans- 

 former B is used. The " annual " efficiency may, as already 

 stated, appropriately be termed the "energy" efficiency, as it 

 is the ratio of the energy output to the energy input. This 

 distinguishes it from the conception to which the term efficiency 

 is usually applied. This usual conception may appropriately 

 be termed the " power " efficiency. 



Let us at this point bring together, as in Table 7, r the data 

 of losses and efficiency of these two designs, which we have 

 designated A and B. 



