274 ELECTRICAL ENGINEERING 



and when the field current is increased a component of current 

 90 degrees ahead of the e.m.f. flows and demagnetizes the field. 



164. Characteristic Curves. The most important character- 

 istic curves of the synchronous motor are (1) the compounding 

 curves, or the relation between field current and armature cur- 

 rent for given values of power factor, (2) the load characteristics 

 showing the relation between armature current and output and 

 power factor and output for a given value of field excitation and (3) 

 the phase characteristics or V curves showing the relation between 

 armature current and field current for given values of motor 

 output. 



To predetermine these curves it is necessary to know the resist- 

 ance r and synchronous reactance x of the armature and to have 

 the no-load saturation curve showing the relation between the 

 generated voltage E Q r and the field current // or field m.m.f. M/. 



The impressed e.m.f. E is constant. 



165. Compounding Curves. The voltage equation of the 

 synchronous motor is 



E = E Q + /?o, (261) 



or E Q = E - 7.o (262) 



If the current I is taken as the line of reference, the impressed 

 e.m.f. can be expressed in rectangular coordinates as 



E = E cos <f> + JE sin 0, 

 where <j> is the angle of lag of the current. 



