THE TRANSFORMER 289 



and at least twice * the voltage for which the high-voltage 

 coil was designed. 



Higher Insulation on First Turns. In order to protect 

 transformers from the high voltages due to surges (electrical 

 oscillations in a transmission line) it is customary to insu- 

 late the first few turns of the high-voltage winding much 

 better than the rest of the turns. It is found in practice 

 that a transformer not so built, nearly always breaks down 

 hi these first turns. 



71. Methods of Cooling. Due to the losses which 

 occur in the core and windings of a transformer it heats 

 up the same as any other piece of electrical apparatus. 

 But the heating in a transformer would be very exaggerated 

 unless means were taken to prevent it as there are no moving 

 parts to create air currents and so dissipate the heat. Spe- 

 cial means are always used to keep down the temperature 

 of a transformer. 



Air cooling, oil cooling, or water cooling may be employed; 

 that most generally used for small transformers being the 

 oil cooling and for large transformers in power plants, 

 water cooling. 



Air-cooled Transformers. In the air-cooling method a 

 fan is used to force cool air from the bottom of the trans- 

 former, past the windings and core to carry off the heat 

 from the top of the transformer. Sometimes the fan con- 

 nects to a pressure chamber in the basement of the station 

 and the row of transformers is placed directly over this 

 chamber. The bottom of each transformer case is open 

 and connects to the chamber below; the air pressure used 

 in the chamber depends upon the load the transformers 

 are carrying, being sufficient at heavy loads to send a very 

 swift air current through the transformers. The air should 

 be well cleaned otherwise dirt and foreign particles will 



* This figure varies for different transformers being greater for low- 

 voltage than for high-voltage transformers. 



