202 



ELECTRICAL ENGINEERING 



(2) When varying the speed by field control the full line voltage 

 is impressed on 'the armature and a resistance R is connected in 

 series with the field winding, Fig. 167. As the resistance R is 

 increased the field current // decreases according to equation 



E 



and the flux $ decreases with the field current as shown by the 

 saturation curve of the machine in Fig. 172. Since the speed 

 varies inversely as <, Fig. 173, the variation of speed with field 

 current will be represented by a curve of the shape shown in 

 Fig. 174. 



By this method the speed can be increased to any required 

 value, and it tends to approach infinity when the field current is 



zero. 



In machines of ordinary design the speed can be increased sat- 

 isfactorily only about .70 per cent above normal speed by field 

 weakening. Beyond this point it is not possible to get sparkless 

 commutation of full-load current, since the armature m.m.f. is 

 strong enough to overcome the weak field m.m.f. and wipe out 

 the commutating field and at the same time the time of commuta- 

 tion is decreased. 



By using interpoles which neutralize the effect of armature 



