282 SYNCHRONOUS MOTORS. 



slightly increasing, with further divergence from the most 

 favourable excitation shown by the upward bend of the lower 

 branches of the curve. This must be explained by the fact 

 that the rapid increase of current which flows through 

 the armature when the excitation is increased above or 

 decreased below the correct value produces a considerable 

 loss of energy in the armature, due to its resistance and 

 the iron losses in it. These losses ultimately become 

 so great that the power spent in the motor armature 

 increases more rapidly than the current. Although, 

 therefore, the current increases and only does the same 

 work as the smaller current in driving the armature, it 

 does so much work in heating the armature that after a 

 certain point the power factor begins to increase, instead 

 of decreasing further, as more current is supplied. 



It is important to observe how the curves just described 

 are affected by an increase in load applied to the shaft of 

 the motor. In each case the figures show three curves 

 obtained from the same motor when unloaded and when 

 loaded by a brake, and giving out '38 and '81 h.p. 

 respectively at the same speed as before. It was found 

 impossible to obtain as great a range of excitation with 

 the motor loaded as when running light. In each case 

 the extreme readings shown are the extreme values of the 

 excitation for which the motor would run. 



The curves of current and power factor are much 

 flatter at the apex when the motor is loaded. This is 

 especially noticeable in the case of the curves of cos #. 

 From this it is evident that, in the case of a loaded motor, 

 a small variation in exciting current has only a very 

 small effect when this variation is near the point of most 

 favourable excitation. This has a most important practical 

 result in the steady running of a synchronous motor under 

 load. The extremely sharp point in the curves of .the 

 unloaded motor at the point of best excitation shows that 

 very small variations in the exciting current 011 either side 

 of the best value will produce considerable changes in the 

 armature current and in the phase angle at which the 

 motor will run. This illustrates one of the chief difficul- 

 ties to be met with in the running of rotary converters 

 from the alternating current side. A rotary converter is 

 really driven as an unloaded synchronous motor by an 



