他励直流电动机拖动恒转矩负载进行串电阻调速，设调速前、后的电枢电流分别为<img src="https://storage.tutusouti.com/pzst/question_bank/res/4f/04/4f04807a00530499ca2ce0354954c0450051d9b4.jpg?x-oss-process=image/sharpen,200"/>和<img src="https://storage.tutusouti.com/pzst/question_bank/res/2e/8e/2e8e45bd14806382cbab1fcb0839f2b10e8a363c.jpg?x-oss-process=image/sharpen,200"/>，那么( )。