【单选题】【消耗次数：1】
改变电动机所接电源的频率以改变n1，即所谓（）。
①
变频调速
②
转差率
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在电动机切断正常运转电源的同时改变电动机定子绕组的电源相序，使之有反转趋势而产生较大的制动力矩的方法称为（ ）。
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