THE SYNCHRONOUS MOTOR 



321 



current would have changed from 01 to 01" and the power 

 factor would have decreased from cos cj> to cos ". 



In Fig. 209 are shown the two V curves for the loads 

 assumed in Fig. 208. For the load OA the current and 

 power factor are shown by the full-line curves, and for the 

 load OA' they are shown by the dotted curves. Evidently, 

 when the load is changed from OAXOE to OA'XOE (Fig. 

 208), the current increasing from XI to XI' (Fig. 209), 

 the power factor increases from XC to XC f (Fig. 209). 

 But if the load current OA is left constant and the excita- 



FIG. 208. Vector Diagram, Showing Effect of Varying Either Load or 

 Excitation of the Motor. 



tion is increased from OX to OX' (Fig. 209), the motor 

 current is changed from XI to XT' and the power factor 

 from XC to X'D. 



84. Equivalent Circuit of Synchronous Motor. It is 

 evident that for any condition of load and excitation the 

 armature current of the synchronous motor can be con- 

 sidered as made up of two components, one in phase with 

 the line voltage and one 90 out of phase. 



A synchronous motor is never run in practice with an 

 underexcited field but practically always with an overex- 

 cited field, so that we may always consider the reactive 

 current to be leading with respect to the line voltage. 



