278 ELECTRICAL MACHINERY 



be transmitted at high voltage (perhaps 50,000-120,000 

 volts), and must be supplied to the customer at low voltage 

 (110-440 volts). The transformer raises or lowers the 

 voltages where required and, without the transformer, the 

 successful development of a-c. transmission systems would 

 have been impossible. 



Fig. 169 shows how the transformer forms links in a 

 transmission and distribution system. For railway sub- 

 stations the voltage is changed, in one step, from the line 



Generator 



FIG. 169. Diagram of Power Transmission System, Showing Service 

 Performed by Transformers. 



voltage to that required in the substation; for lighting work 

 the high voltage of the transmission line is generally stepped 

 down by a few large transformers to 2300 volts, and is then 

 distributed throughout the city at 2300 volts to small 

 transformers located on the poles which step it down again 

 to 110 volts. 



Each of these small transformers (generally of less than 

 15 kv-a. capacity) supply perhaps 25 to 50 customers. 

 Fig. 170 shows one of these small transformers located on 

 a pole, just under the cross arms; in the cut only the out- 

 side containing case of sheet iron can be seen because the 

 coils and core are always put inside cases to protect them 

 from moisture, dirt, mechanical injury, etc. The small 

 wires going into the case at the back (next the cross arms) 

 are the 2300-volt wires and the larger wires coming from 

 the front are the 220-volt wires which connect directly to 

 the lighting wires in the customer's house. 



