lo-A] SYNCHRONOUS MOTOR. 315 



and the curve for power factor would reach a corresponding 

 maximum of 100 per cent.,*but in practical operation these ideal 

 limits cannot be reached: Is'mO never falls quite to zero and 

 power factor never reaches 100 per cent. 



The wattless current /sintf acts as an exciting current which 

 strengthens the field when the motor is under-excited and the 

 current is lagging and weakens the field when the motor is over- 

 excited and the current is leading. 



24. 0-curves. For each load plot a polar curve for I, by 

 laying off each reading of current to scale and with the proper 

 phase angle. This gives a series of O-curves (each curve being 

 like the bottom part of the letter O) corresponding to the series 

 of F-curves. For constant power output and constant losses, 

 with armature resistance zero, these curves would be parallel 

 straight lines perpendicular to E; for an armature resistance R 

 the curves are arcs of circles about a common center located in the 

 direction of at a distance E-^-2R from the origin. 



*(23a). This would mean that the electromotive force and current 

 are simple sine waves and that there is no hunting; otherwise currents of 

 other than fundamental frequency would flow and the relations of plane 

 vectors, upon which the derivation of the expression 7sin0 depends, 

 would not hold. (See 47, Exp. 6-A). Dissimilarity in the line and motor 

 electromotive force waves will cause wattless currents of higher frequency 

 to flow. 



