328 



ALTERNATING CURRENTS 



Even without the adjustment secured by altering the field 

 current, a synchronous motor tends to maintain constant volt- 

 age at the end of a transmission line having reactance. If the 

 voltage at the motor terminals drops, its back emf. tends to 

 exceed the terminal voltage and the motor must then take a 

 leading current in order to operate. This leading current, flowing 

 through the line reactance, tends to maintain the motor voltage, 

 as a leading current flowing through reactance tends to produce a 

 rise of voltage from generator to load. On the other hand, a rise 

 of voltage at the motor terminals tends* to cause the motor to 

 operate under-excited. This increases the drop from generator 

 to load and tends to cause the voltage at the load to decrease. 



FIG. 302. Synchronous motor for controlling voltage at end of transmission line. 



The effect of the synchronous motor on voltage control may 

 be shown by a laboratory experiment, the connections for which 

 are given in Fig. 302. A synchronous motor, running either 

 light or partly loaded, is supplied from constant potential mains 

 through three series reactances, one in each main. A lamp load 

 or an induction-motor load is connected in parallel with the syn- 

 chronous motor. Vary the lamp load or the induction-motor load 

 and maintain the synchronous motor terminal voltage V m con- 

 stant by varying its field current. It will be found that the 

 field current must be materially increased as the load is increased. 

 Figure 303 shows the general trend of the curve giving the rela- 

 tion between the field current and the load. 



