为了说明心脏如何使血液在全身循环流动，老师描述了水泵的工作原理。
To illustrate how the heart circulates blood through the body, the teacher described how a pump works.
分析了变频器并联运行的原理，建立了变频器并联的数学模型，研究了变频器并联运行时的环流特性。
It analyzed the principle of parallel inverter, established the mathematical model of parallel inverter, and researched the circulating current of parallel inverter.
分析了静态转换开关（STS）的无环流切换控制的工作原理。
The control theory for static transfer switch(STS) without circulation current is elaborated.
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