THE SYNCHRONOUS MOTOR 327 



and reactance X, inserted between the supply mains and the 

 motor terminals. 



Assume first that the synchronous motor is under-excited and 

 therefore taking a lagging current. Along the vector/, Fig. 301 (a), 

 the IR drop is laid off in phase with 7; at right angles to / 

 and leading, the IX drop is laid off. The vector sum of the IR 

 and IX drops is equal to the IZ drop in the line. Obviously, 

 the motor voltage must be equal to the generator voltage minus 

 the IZ drop, vectorially considered. Therefore, IZ is reversed 

 and added to V , givirtg V m , the motor voltage. It will be ob- 

 served that numerically V m is considerably less than V . 



If the motor now be over-excited, / will lead the voltage V m . 

 By subtracting IZ from V , Fig. 301 (6), the motor voltage V m 

 becomes numerically greater than V a . 



i 



(a) Lagging current; motor voltage (6) Leading current; motor voltage 

 lees than generator voltage. greater than generator volt:. 



Fio. 301. Effect of line impedance on synchronous-motor volta 



This gives a method of controlling the voltage at the end of a 



Iran -minion line. If the voltage at the receiving end of the line 

 tend- to change because of a change in the generator voltage or 

 in the line drop, it may often be held substantially constant by 

 varying the excitation of a synchronous motor placed at the 

 .ing end of the line. In practice, synchronous motors are 

 often installed for purposes of regulation only. At the Los 

 Angles end of tin- LMO-mile Big Creek Line, two 15,000 kv-a. 

 synchronous condensers arc installed, their sole function being 

 to hold the voltage in Los Angeles at the proper value. If the 

 load Were removed and no such regulating devices existed, this 

 "iild rise to values con>ideral>ly in excess of that at the 

 general inn itotion iMo miles away, due to the line charging 

 (in rent flowing through the line reacta 



