【单选题】
一台他励直流发电机的输入功率P1等于()
①
<imgsrc="http://wljy.whut.edu.cn:80/uploadfiles/kindeditorAttached/image/20191203/20191203180904_934.png"alt=""/>
②
<imgsrc="http://wljy.whut.edu.cn:80/uploadfiles/kindeditorAttached/image/20191203/20191203180938_647.png"alt=""/>
③
<imgsrc="http://wljy.whut.edu.cn:80/uploadfiles/kindeditorAttached/image/20191203/20191203181005_927.png"alt=""/>