202 



ALTERNATING CURRENTS 



There are two different types of artificially-cooled transformers, 

 air-cooled and oil-cooled transformers. The air-cooled or air- 

 blast type is ordinarily mounted on a platform under which 

 air pressure is maintained by means of blowers (Fig. 192). 

 The air is forced up through the transformer windings, keeping 

 them at the proper temperature. The advantage of air-cooled 

 transformers is that fire risk is reduced because the danger of 

 flooding the station with burning oil is eliminated. They are 



FIG. 190. General Electric 8,000 kv-a., 25-cycle, 4,400-6,600 /6,390/6,270-volt, 

 outdoor transformer. (Radiator tank.) 



therefore used extensively in sub-stations which are located in 

 congested districts. On the other hand, this type of transformer 

 does not have the dielectric strength which the oil adds to the 

 insulation. For this reason air-cooled transformers are seldom 

 manufactured for potentials in excess of 30,000 volts. 



The most common method of artificially cooling the oil type of 

 transformer is to place a copper coil in the top of the transformer 



