POWER-FACTOR CONTROL OF LINE VOLTAGE 19 



and it is the increase and decrease of this quantity as the 

 load changes which cause the troublesome variations 

 in line voltage to which reference has previously been 

 made. 



It has been stated that by changing the excitation 

 of a synchronous motor, a large lagging or leading quad- 

 rature current may be made to flow in the A. C. line 

 which is supplying power to the synchronous motor. 

 Thus, if the field current is weakened, Q, which is shown 

 as a lagging current in the diagram, is increased. Simi- 

 larly if the field current is strengthened, Q is decreased, 

 and may even be made negative, in which case it becomes 

 a leading current. The voltage drop is now 



PR-QX 



that is, the leading current reduces the drop and so tends 

 to increase the delivered voltage. Since reactance is 

 always prominent in A. C. lines, it is plain that changes 

 in Q produced by altering the field current of a synchro- 

 nous motor have a powerful effect in raising or lowering 

 the line voltage at the synchronous motor. This may 

 be tested by any one, by simply moving the field rheostat 

 of a large synchronous motor. In many usual cases a 

 variation of 10 per cent in the voltage at the motor may 

 be produced in this way, since reactance in the trans- 

 formers supplying the motor acts in the same way as 

 reactance in an A. C. line. It is thus seen that by con- 

 trolling the power-factor by means of synchronous 

 motors, it is possible to obtain a very effective control of 

 the voltage. 



