SYNCHRONOUS MOTORS. 277 



excitation still further, the angle will change sign i.e., 

 the current supplied will begin to get greater again for 

 the same load, but the current will lead the voltage in 

 phase instead of lagging behind it. 



An undue increase of excitation will increase the 

 current required to maintain the rotation of the armature, 

 on account of the great difference of phase between the 

 armature current and voltage. This can only be clearly 

 seen from a diagram. The current will always lag behind 

 the resultant electromotive force of the circuit by a 

 definite amount on account of the self-induction of the 

 armature. The difference of phase between the terminal 

 voltage of the motor and the current will, however, vary, 

 and consequently the amount of the current must vary 

 also, since it is only the component in phase with the 

 motor volts which is effective in driving. For every load 

 there will be a particular value of the excitation which will 

 give the lowest value of the armature current, i.e., the highest 

 power factor and smallest idle component of the current. 

 This is obviously the most favourable condition. The 

 excitation applied to a motor under working conditions 

 should be such as to give the lowest armature current at 

 the normal average load. 



We may now observe how the effects of varying the 

 excitation just discussed are illustrated by the curves 

 obtained from the experiment. 



Figs. 130 and 131 show curves of current and watts 

 obtained by varying the excitation of a small 4-pole 1 h.p. 

 motor having a ring-wound rotating armature, when run- 

 ning at a speed of 1,500 revs, per minute. The three curves 

 correspond to three sets of readings taken each at a 

 constant output. The loads were : No load, '38 h.p., and 

 '81 h.p. respectively. 



It will be seen that the curves of current consist of two 

 inclined portions, which in the case of no load are nearly 

 straight lines, meeting at the lowest portion of the curve 

 corresponding to 1*47 amps, excitation. 



It therefore appears that with 1*47 amps, excitation the 

 armature current is most nearly in phase with the back 

 voltage, and the current supplied to the motor is conse- 

 quently a minimum. With a lower excitation the current 

 lags behind the back electromotive force, and the amount 



