THE SYNCHRONOUS MOTOR 



317 



below its normal value, it will be noticed that the armature 

 current rapidly increases; for very low or very high values 

 of the field current the armature current may be much 

 greater than the full-load current although the motor is 

 running light. 



Variation of Armature Current with Field Current. If 

 the locus of the armature current is plotted, it forms a 

 V-shaped curve, as shown in curve 3 of Fig. 205. If, now, 

 the motor is loaded (mechanically) up to its rating and the 

 field current again carried through as wide a range as 



01 2345678 9 10 11 

 Pi&ld Current 



FIG. 205. Phase Characteristics or " V " Curves of a Synchronous 



Motor. 



possible, curve 1 will be obtained. For half load, a curve 

 similar to that one numbered 2 would be obtained. These 

 curves, showing the relation between the armature current 

 and the field current for a fixed load are called the phase 

 characteristics or " V " curves of the synchronous motor. 



Variation of Power Factor with Field Current. If the 

 power input were measured as well as the armature current, 

 the power-factor could be calculated, the line voltage being 

 known. For the different loads the power-factor curves 

 would resemble those shown in dotted lines in Fig. 205. 



