260 ELECTKICAL MACHINERY 



winding is to neutralize the other. By properly propor- 

 tioning this second differential winding it is possible to 

 obtain any degree of compounding desired. 



There are several auxiliary features to the regulator 

 which we have not considered, such as condensers across the 

 contacts, reversing switches for the contacts, the amount 

 of resistance required in the field rheostat for proper opera- 

 tion, etc. These may be found in trade publications 

 describing the regulator. 



64. Alternators Running in Parallel. Electrical power 

 for a city or community can be generated most efficiently 

 when all the generators supplying the power are located 

 in one central power station. The cost of electrical power 

 may be divided into several parts; coal and water, oil, 

 station attendance and repairs, line maintenance, interest 

 on investment and various other costs grouped under the 

 title of " overhead " charges. These are made up of office 

 rent, salaries of clerks, executive officers, taxes, etc. 



Combination of Stations Reduces the Overhead Charges. 

 Practically all of these charges may be reduced if one 

 power station, instead of several, supplies all the power. 

 But one staff of executive officers need be employed, only 

 one set of books need be kept, and one office; the cost 

 for station attendance will not be as great as if several 

 stations are used, etc. Also larger generators would be 

 used in the larger station than in the smaller ones and we 

 know that large generators have a higher efficiency than 

 small ones, i 



Necessity of Large Stations. ~The use of electrical 

 power is becoming so general that for large cities perhaps 

 a hundred thousand killowatts of power may be required 

 to supply all the demands. A generator to supply 100,000 

 kw. has never been built; the largest built so far has a capac- 

 ity of 20,000 kw. Hence to supply 100,000 kw. it would 

 require at least five of these big machines and they would all 

 be operated in parallel on the same bus bars. In some large 



