你需要先搞清楚电流的流向。
作为直流发电机,在原动机驱动下,电枢旋转产生感应电动势,在该电动势作用下向外输出电流,电枢电流Ia与电动势Ea方向相同,如图1.4.2(a)电枢电动势也称电源电动势。对于直流电动机。电枢导体中也产生感应电动势,但因与外部所加的电压方向相反,电动势Ea与电枢电流Ia方向相反,如图1.4.2(b)电枢电动势为反电动势。
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/d6ca7bcb0a46f21f0de5e8b3f8246b600d33ae79?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
这个是并励直流电动机![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/9825bc315c6034a855a80e61c513495408237683?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
电枢电流与电枢电动势方向相反,电枢电动势也称反电动势,用结点电流法看电流方向,I=Ia+If.
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/0df3d7ca7bcb0a4640b23dd96563f6246a60afba?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
作为直流发电机,用结点电流法看电流方向,Ia=I+If.