THE SYNCHRONOUS MOTOR 313 



of a synchronous motor is weakened, the motor takes a lagging 

 current which strengthens the field by armature reaction and tends 

 to annul the effect of the weakening of the field. A lagging current 

 when a machine operates as a motor is a leading current when the 

 machine is considered as operating as a generator. It will be 

 QBmembered that a leading current in a generator strengthens 

 the field through the effect of armature reaction (see Par. 63, 

 page 135). That is, a lagging current in a motor has the same 

 effect on the magnetic? field as a leading current in a generator. 



A synchronous motor under any given operating conditions 

 requires a certain excitation. If its field is weakened, its excita- 

 tion becomes inadequate. This deficit is in part made up by 

 the motor taking a lagging current from the line. A lagging 

 current is ordinarily associated with inductance and therefore 

 with the excitation of a magnetic field. When the motor takes a 

 ig current, some of its excitation is therefore obtained from 

 the alternating-current line. In this respect it is similar to an 

 induction motor except that the induction motor takes all its 

 excitation from the alternating-current line. This lagging 

 current required by the synchronous motor to help excite its 

 own field weakens the field of the alternators supplying it, and 

 as a result their field excitation must be increased to maintain 

 the line voltage. Therefore, when the field of a synchronous 

 motor is weakened, a part of the excitation which it requires 

 is supplied indirectly by the fields 

 of the alternators supplying the 

 system. 



On the other hand, when a syn- 

 ch n> i ioi is motor is over-excited it 

 has a surplus of excitation. It 



Fio. 290. Induced arniaturr 

 a leading current . As Jl lead- v .lta K o U-ss than terminal volt:i K , 



urrent will neutralize a por- wl "; "> "<;i'">"""> ",..tr "'"t 



. u-ads terminal v<- 

 (ion ot the lagging current ot 



inductive apparatus (see Par. 131, page 322) connected to 



.<tem. or el-e will strengthen the fields 1. 1' the generators 

 supplying the system, the synclmmou< motor under these con- 

 ditions indirectly -upplie- on to other parts of the -\ 



lie vector diagram when the motor I 

 it. The //,' and the I \ drops are laid off with 



