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单选题 恒压供水系统为了恒定水压，在管网水压降落时，要（ ）变频器的输出频率。
A、 降低
B、 保持
C、 升高
D、 先降低后保持
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2025-06-30
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单选题 4、VVVF（变压变频）控制的特点是（ ）。
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