318 ELECTRICAL MACHINERY 



From these curves it may be seen that although a varia- 

 tion in the field current (from normal) is accompanied by 

 an increase of the armature current, the power input to the 

 motor is practically unaltered because of the decreased 

 power factor. 



Reactive Current Caused by Improper Field Current. We 

 must conclude that field currents other than normal cause, 

 in the armature circuit, either a leading or a lagging cur- 

 rent, the amount of lead or lag depending upon how much 

 the field has been changed from its normal value. If a 

 power-factor meter had been used in the V curve test, it 

 would have indicated that, with fields greater than normal, 

 the motor took a leading current from the line, while with 

 less than normal excitation the motor took a lagging cur- 

 rent from the line. This idea is generally expressed by 

 saying that an overexcited synchronous motor draws a leading 

 current and an underexcited motor a lagging current. 



Reason for Reactive Current. The reason for these facts 



becomes apparent when we 



1 Line r consider the circuit made up 



of the synchronous motor 

 armature, the line, and the 

 armature of the generator 

 supplying the line, as shown in 



~. ..ATI x Fig. 206. Eg represents the 



FIG. 206. Circuit Made up of 



Generator Armature, Motor generator voltage and E m the 

 Armature and Line. e.m.f. generated in the arma- 



ture of the motor. Any current 



which flows in this circuit must then be caused by the 

 resultant of E m and E a . 



In Fig. 207 are shown the possible relations of these 

 vectors. First, assuming no load and neglecting the stray 

 power, we see that when E m and E g are equal and opposite 

 the resultant e.m.f. in the circuit in zero, hence no current 

 will flow. But now suppose the motor is overexcited so 

 that the motor voltage is shown by OE m ' in Fig. 207. The 



(Q) Generator ftS>Motor 



