324 



ALTERNATING CURRENTS 



The resultant current 7 is laid off degrees behind 7, but 

 terminating on line I-Ii, since the power and hence the energy 

 current /i is fixed. The synchronous-motor current I 8 has the 

 value, Fig. 297, 



I 8 = I sin 0i /o sin = / 2 - /o sin (73) 



It will be noted that the resultant current 7 is the vector sum of 

 the load current / and the motor current 7. 



When a synchronous motor is operated without load for the 

 purpose of merely correcting power-factor, it is called a synchron- 

 ous condenser. Such a synchronous condenser should not be 



Load 

 P. F. 



L> 



f \ Synchronous 



( O J Condenser 



FIG. 297. Raising power-factor to cos by means of synchronous condenser . 



employed unless its investment charges and cost of operation 

 are considerably less than the increased charges occasioned by 

 the low power-factor. Other considerations, such as voltage 

 control, however, are important. When a user of electric power 

 buys on either a kilowatt-hour or a kilowatt basis, a low power- 

 factor is not detrimental to him, except possibly to increase 

 slightly the cost of his mains. This low power-factor is, how- 

 ever, detrimental to the power company, which must install 

 larger generators, conductors, transformers, etc. For this reason 

 many power contracts now penalize low power-factor. 



132. The Synchronous Motor as a Corrector of Power -factor. 

 The synchronous motor may correct the power-factor of a 

 system and at the same time deliver mechanical power. 



