DESIGN OF TRANSFORMERS. 117 



The surface of the transformer is 



= 9700 square centimetres 

 = 1497 square inches 



At no-load the number of watts lost per square inch of surface 

 is 



TWf = 01 

 and at full-load 



which would be sufficiently small. 



Actual tests show that a cooling surface of 2000 square centi- 

 metres allows a rise of temperature, under normal conditions, of 

 1 C. per watt dissipated in the transformers. The rise of tem- 

 perature in the present case will, therefore, not exceed 



300 x 



= 62 C. 



This, in fact, will be somewhat in excess of the truth, since, owing 

 to the shape of the windings, the cooling surface will be greater 

 than 9700 square centimetres. 



The total volume 1 of iron in the transformer is, by reference to 

 Fig. 39, seen to be 



= 50700 cubic centimetres .... (21) 



This gives a loss of only O'OOS watts per cubic centimetre, 

 which is a liberal allowance of iron. 



To find the weight per kilowatt of the transformer. 



Since iron weighs 017 Ib. per cubic centimetre, the weight of 

 iron is about 862 Ibs. 



Also the weight of copper is 80 Ibs. 



Thus the weight of the transformer per kilowatt of output is 



i> r 42 94-2 Ibs. per kilowatt 



EFFECTS OF MAGNETIC LEAKAGE. 



7O. If there is no magnetic leakage, the square of the coeffici- 

 ent of mutual induction of the primary and secondary coils is 



