【单选题】【消耗次数：1】
交流发电机的转子中有一个线圈绕组其作用是
①
产生磁场的，且磁场大小可调
②
产生磁场的，且磁场恒定
③
产生交流电的
④
产生直流电的
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相关题目
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能耗制动是在电动机脱离交流电源后，迅速给定子绕组通入_____产生恒定磁场，利用转子感应电流与恒定磁场的相互作用达到制动的目的。
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