V-CURVES OF SYNCHRONOUS MOTOR 133 



71. V-Curves of Synchronous Motor 



By assuming any constant value for the load, and drawing a 

 straight line such as AB in Fig. 99 corresponding to this load, we 

 can determine, from the intersections of this line with the consecutive 

 power curves, the values of corresponding to various motor e.m.f.'s. 

 By then referring to the curves of Fig. 98, we can find the values of 

 the armature current corresponding to the different values of 0, and 

 so obtain the relation connecting the armature current with the e.m.f. 

 when the load is maintained constant. One such curve, correspond- 

 ing to a load of 250 k.w., is shown in Fig. 101. It will be noticed 

 that with this load the motor will not run at all unless its excitation 



00 1000 1*00 4000 



MOTOR E.M.F. 

 FIG. 101. Relation connecting Excitation and Armature Current. 



is such that the e.m.f. exceeds about 500 volts. As the exciting 

 current increases, the armature current decreases to a minimum value, 

 and for still higher excitations again increases. This variation of the 

 armature current with varying excitation is an important characteristic 

 of the synchronous motor, first pointed out by Mr. Mordey.* The 

 variations in the armature current for a given range of variation in 

 the excitation are greater at lighter loads, and the curve obtained 

 when the motor is running light is more nearly V-shaped than that 



* Journal of the Institution of Electrical Erujineert, vol. xxii. p. 128. 



