POWER-FACTOR CONTROL OF LINE VOLTAGE 17 



as "power-factor correctors." This, however, is their 

 most useful employment only in systems where the 

 energy is all used very close to the generators. The early 

 central station power systems were of this type. When 

 their loads gradually changed from a high power-factor 

 lighting load, to a low power-factor induction motor load, 

 the troubles which they experienced from low power- 

 factor, as described in Chapter IX, made a wide-spread 

 impression, and determined to an excessive degree the 

 method of looking at the application of synchronous 

 motors. 



In most systems, the transmission of the energy, even 

 across a city, plays a very important part and involves a 

 large share of the total expense. Synchronous motors 

 should in such cases be used as "voltage regulators," 

 since they can thus give commercial benefits, as described 

 in Chapter VI, much in excess of the benefits from mere 

 power-factor correction. The best part of the matter is 

 that the benefits from power-factor correction are not 

 sacrificed, but the synchronous motors fulfil both func- 

 tions as "voltage regulators," and as "power-factor cor- 

 rectors" at times of heavy load when power-factor cor- 

 rection is most valuable. 



The theory of how synchronous motors can be used 

 as voltage regulators is easily understood by considering 

 the vector diagram of an A. C. transmission or distribu- 

 tion line. See Fig. 4. This diagram is a very familiar 

 one in A. C. work. Let E s be the voltage at the source 

 of energy supply, and E the voltage at the other end of 

 the line where the energy is applied. Let the current / 

 be composed of two components, P in phase with the 

 voltage E, and Q in quadrature with E and lagging be- 



