TESTS OF SYNCHRONOUS MOTORS 



139 



ranged in such a way that their spirals are pointed in opposite direc- 

 Owing to the form of these curves, the distance from their 

 jints of intersection to the center is proportional to the phase-angle 

 ;tween the two disks, or the lag of one with respect to the other, 

 "he point of intersection of two slits forms a small opening traversed 

 by rays of light which, in turning, make a circle that expands or 

 mtracts in proportion to the phase-angle. The radius of the circle 

 be measured on a fixed scale which is graduated directly in 

 igles. 



FIG. 70. 



Bedell and Ryan have used this method to determine the phase- 

 angle between E.M.F.'s, by bringing this phase-angle to zero when the 

 poles are in line. The currents are measured by means of an electro- 

 dynamometer. The E.M.F.'s are deduced from the excitation- 

 curves. Their vectors are then drawn with an angle between them 

 equal to the measured phase-angle, and, from these, the resultant 

 vector is obtained, in magnitude and in phase, by completing the 

 parallelogram. The current is known, in magnitude. Its phase, 

 with respect to the resultant E.M.F., is deduced from the reactance, 

 which is known by means of the expression 



reactance 



tan r= , 



resistance 



