SYNCHRONOUS MOTORS. 279 



the back electromotive force in phase, so that a smaller 

 and smaller component . of the current supplied to the 

 armature reacts on the fields so as to produce the rotation. 

 Thus, in spite of the fact that the field is strengthened, 

 the current supplied must be increased in order to keep 

 the component of the current which is in phase with the 

 back electromotive force at a sufficiently high value to 

 produce the required constant torque. 



It will be seen that the right-hand part of the current 

 curve is somewhat less steeply inclined than the left-hand 

 branch, since, as just explained, in this case the field is 

 strengthened by the change of excitation, and makes it 

 unnecessary for the value of cos </> to increase so rapidly 

 as is the case for points on the left-hand portion. 



From the slight difference between the inclination of 

 the two branches of the curve, it is evident that the variation 

 in strength of the field produces only a small direct 

 change in the armature current, and that nearly the whole 

 of the variation is produced indirectly because of the 

 alteration in the lag of the motor armature which follows 

 the change of field. The lack of symmetry in the two 

 limbs of the curves will depend also on the magnetisation 

 curve of the machines, since excitation, and not flux, is 

 plotted horizontally. In the present instance the mag- 

 netisation curves were practically straight lines within the 

 range here employed. The other curves follow generally 

 the form of the no-load curve, but are more curved and 

 show a more gradual bend at the lowest point, indicating 

 a less sudden transition from lagging to leading current, 

 and consequently more stable running. 



The curves of watts in Fig. 131 will be seen to follow 

 generally the shape of the curves of current, but show a 

 less proportional variation. At first it might be thought 

 that the power supplied to the motor would only vary 

 very slightly, since the motor runs at a constant speed 

 and exerts the same turning effort. From the curve it is 

 evident that this is not the case, at anyrate when the 

 motor is lightly loaded, the power supplied varying from 

 a minimum of 165 watts to a maximum of 625 watts. 



This variation is specially high in the present case, 

 since the motor was small and had a high armature resis- 

 tance and low efficiency. A variation in power necessary 



