ALTEKX. \TnR REdl'L. \TION AND OPERATI<>\ 



135 



Fro. 140. Vector diagram 

 showing effect of armature 



reaction with current lag- 

 ging 90. 



with respect to the induce.! electromotive force. When the coil 

 is in position (1), Fig. 139 (a), the electromotive force is a maxi- 

 mum, as in Kig. 136. The current is zero at this instant because 

 it higs the induced voltage by 90. The current does not reach 

 ixiniuin value until the coil has travelled 90 electrical space- 

 rs further and has reached position (2). The coil then lies 

 directly under a south pole. It will be 

 noted that the magnetomotive force of 

 this coil is downward and is therefore 

 in direct opposition to the magnetic 

 flux entering the south pole. There- 

 fore, when the current lags the induced 

 electromotive force by 90, it acts in direct 

 opposition to the main field. As a result the field is materially 

 weakened by a lagging current and this is accompanied by a 

 reduction of the induced electromotive force. 



This result is similar to that of moving the brushes forward 

 90 in a direct-current generator. All the armature ampere-turns 

 are then demagnetizing, tending to weaken the Held. 



It will be observed that this coil is acting direct ly upon a part 

 of the magnetic circuit where there is iron, rather than on an inter- 

 polar space. Therefore, for a given current in the armature, the 

 coil in this position has a much greater effect upon the magnetic 

 field of the machine than it had at unity power-factor, shown 

 in I'm. Io7. I-'iiiiire 140 shows vectorially the conditions which 



Field 

 Mmf. 





Mai.Emt. ~""'/ 0o 



() 



\nnature reaction due to ciirn-i 



Co, I Mini. 



tr ( ."i" 



Tin- ai mat ure 

 so 



exi-t at ierO power-factor 01 ( .) Migcurren 



reaction A act- in direct opposition to the impressed Held 



that the resultant ti-'ld /' is con>iderably -mailer than / . 



ire 141 shoWS the OOnditii ing when the current leads 



the induced elect' v 90. As before, the electro- 



