单选题：对笼型异步电动机能耗制动附加电阻值进行计算，设电动机的功率为7．5kW，空载电流，Ikz=9．5A，额定电流Icd=15

题目内容：
对笼型异步电动机能耗制动附加电阻值进行计算，设电动机的功率为7．5kW，空载电流，Ikz=9．5A，额定电流Icd=15．9，每相定子绕组电阻Rd=0．87Q规定能耗制动用直流电源电压为220V，能耗制动电流为电机空载电流的3倍，指出其附加电阻值最接近下列哪一项数值？(忽略电缆连接电阻)（　　） A．1．74Ω
B．7．7Ω
C．6．0Ω
D．9．8Ω

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