JO-B] SYNCHRONOUS MOTOR. 3 J 9 



tion of the armature due to hunting or variation in operating 

 conditions would cause the motor to develop less power, insuffi- 

 cient for the demand, so that the motor would continue to drop 

 back and would eventually stop. 



4. Fig. 2 shows the loci of E' and Ez, which are arcs of cir- 

 cles each with a radius E'. The numbers o, I, 2 . . . 10 indicate 

 corresponding points on the different curves. For this particular 

 case, o and 10 are points of zero mechanical power beyond which 

 the machine acts as a generator and not as a motor ; 5 is the point 

 of maximum power. Stable operation is from o to 5; unstable 

 operation for 5 to 10. 



Practically E' must lie in the third quadrant,* for between o 

 and I the power developed by any motor is not likely to be suffi- 

 cient even to supply the iron and friction loss. 



Maximum power occursf when E' lags behind E by an angle 

 1 80 + Oz. For stable operation, therefore, E' lags behind E by 

 an angle that is less than i8o+0z and is (practically) more 

 than 180. The larger the value of Oz the wider is the range of 

 stable operation, which means that the reactance of the arma- 

 ture circuit should be large compared with its resistance. 



5. Current Locus. Since the locus of Ez is the arc of a circle, 

 the locus of / which is proportional to Ez must likewise be the 

 arc of a circle. The center C is on a line OC making an angle Oz 

 with E ; the length OC is E -*- Z, the radius CH is ' -*- Z. 



For different excitations the current loci consist of concentric 

 circles with different radii, determined by the relation CH:OC 

 =iE':E. When E' = E, the current locus passes through O. 

 For under-excitation (as in Fig. 2) the radius is less than OC; 

 for over-excitation the radius is greater than OC, the point H 

 falling on OC prolonged to the left. 



* (43). For zero power, the lag of ' behind E is 180 when E' = E; 

 it is more (or less) than 180 when E' is more (or less) than E. 



t(4b). This may be proved analytically, as in Alternating Current 

 Machines by Sheldon, Mason and Hausman, or graphically, as in Elements 

 of Electrical Engineering by Franklin and Esty. 



