no ALTERNATING CURRENTS 



in the core cross-section and an increase in the yoke cross-section 

 will reduce the mean length of a turn in each winding, and so reduce 

 the copper losses. Now, this reduction in the copper losses may 

 more than counterbalance the increase in the iron losses. It 

 has been found that in transformers of ordinary construction (Fig. 81) 

 the total loss (iron and copper) reaches a minimum value when the 

 yoke cross-section is about 2 times that of the core cross-section 

 inside the coils.* 



56. Heating of Transformers 



Transformers are generally enclosed in suitable cases, which may 

 be either entirely of cast-iron or partly of perforated sheet metal. 

 When the former are used, the transformer is frequently entirely 

 immersed in oil. The oil not only improves the insulation, but 

 facilitates the transfer of heat from the transformer to the case, and 

 maintains the transformer at a more uniform temperature. The 

 maximum permissible rise of temperature of the coils is about 50 C. 



The difficulty of preventing an excessive rise of temperature in 

 the case of very large transformers is a serious one. Where no oil is 

 used, and where the natural convection currents of air are insufficient 

 to carry off the heat rapidly enough, an air-blast supplied by a 

 blower driven by a small motor is forced through ventilating ducts 

 provided in both core and coils. With oil-insulated transformers, 

 the following method of cooling is adopted. At the top of the oil- 

 tank containing the transformer is fitted a coil of thin-walled brass 

 piping, and through this cold water is circulated. The hot oil as 

 it rises gives up its heat to the water, and sinks to the bottom of 

 the tank, a vigorous circulation of the oil being thereby maintained. 



A good deal of controversy has taken place as to the relative 

 advantages of air-blast and oil-insulated water-cooled transformers. 

 Difficulties have sometimes been experienced with oil insulation. 

 These have been mainly due to the presence of moisture, either in 

 the coils of the transformer or in the oil ; to the use of an unsuitable 

 oil, or unsuitable insulating compound in the coils, the compound 

 being chemically attacked by the oil. It has been urged against 

 large oil-insulated transformers that they constitute a serious fire 

 risk. It is, no doubt, advisable to keep such transformers in separate 

 fire-proof compartments at some distance from the generating station, 

 and to have arrangements whereby the oil may be rapidly drained 

 off if necessary. The oil should have a high flash-point (about 



* See A. Muller, Zeitschrift fur Elektrotechnils, (Wien), vol. xxii. p. 417 (1904); 

 vol. xxiii. p. 243 (1905). 



