目前器件電壓*高6kC,在不串聯(lián)情況下變頻器*高電壓4.2kV。6kV變頻器必須串聯(lián),線路復(fù)雜,器件多,可靠性受影響:外很少做6kV變頻器,10kV也基本不做。從原理上說SDM通過H橋單元串聯(lián),變頻器輸出電壓不受器件電壓限制,可以較高,但提高電壓的代價是器件大量增加、可靠性降低。對于輸出同樣功率的變頻器,使用較高電壓、較多串聯(lián)單元所花的代價大于用較低電壓、較少數(shù)量、電流較大單元的代價,也就是說在器件電流允許條件下應(yīng)選用盡可能低的電壓等級。
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許多應(yīng)用場合都要求有旁路功能,即在變頻器故障時將電動機直接接入電網(wǎng)、恒速工作。為降低變頻器的造價,電動機電壓低于電網(wǎng)電壓后,如何設(shè)置旁路是一個需要解決的問題,可參考文獻(xiàn)[1]。
3、矢量控制和直接轉(zhuǎn)矩控制
凋速系統(tǒng)的任務(wù)是控制速度速度通過轉(zhuǎn)矩來改變,調(diào)速系統(tǒng)的性能取決于轉(zhuǎn)矩控制的好壞,矢量控制(VC)和直接轉(zhuǎn)矩控制(DTC)的任務(wù)都是實現(xiàn)高性能的轉(zhuǎn)矩控制,它們的速度調(diào)節(jié)部分相同異步電動機的轉(zhuǎn)矩等于磁鏈?zhǔn)噶亢投ㄗ与娏魇噶縤的矢量積。磁鏈不能直接測量,需要通過定子電壓、電流及電動機參數(shù)算得。
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