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ABB SAMC11 POW
每一个单元为低压变频器，由整流桥，储能电容，H桥输出逆变器组成，由于输入侧为二极管整流，功率流只能是从整流侧输入从H桥逆变器输出，如果从H桥向功率单元有功率流入的话，只能使储能电容电压不断升高而损坏。故此，应防止变频器所驱动的电机进入发电状态向变频器回馈能量。
变频器运行时，三相交流电源通过功率单元内整流二极管桥进行整流，电容阵列对脉动直流进行滤波，变为恒定的直流。电容阵列同时作为PWM输出的能量中继池，提供给输出回路稳定的电压。
每相6个功率单元的H桥逆变器，其PWM输出控制信号由公共的正弦波和6个三角载波比较生成，6个三角载波按其自身周期的1/6互相错开，使6个单元相互串联叠加后输出电压为13阶梯波，如图3所示。其中UA1…UA6分别为A相6个功率单元的输出电压，叠加后为变频器A相输出电压UA0。图3中显示出了生成PWM控制信号时所采用A相参考电压UAr，可以看出UA0很好地逼近UAr。UAF为A相输出电压中的基波成分。
对6kV变频器，功率单元的输入电压为三相600V，当变频器输出频率为50Hz时，功率单元输出为单相577V，单元相互串联叠加后可输出相电压3464V。
图3 相电压回路叠加波形
由于变频器中点与电动机中性点不连接,变频器输出实际上为线电压，由A相和B相输出电压产生的UAB输出线电压可达6000V,为25阶梯波。图4所示为输出的线电压和相电压的阶梯波形,谐波成分及dv/dt均较小。
图4 线电压叠加波形
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