圖1—3是用交流接觸器控制異步電動(dòng)機(jī)的主電路、控制電路和有關(guān)的波形圖。接觸器的結(jié)構(gòu)和工作原理與繼電器的基本相同,區(qū)別僅在于繼電器觸點(diǎn)的額定電流較小,而接觸器是用來(lái)控制大電流負(fù)載的,例如它可以控制額定電流為幾十安至幾千安的異步電動(dòng)機(jī)。按下起動(dòng)按鈕SBl,它的常開(kāi)觸點(diǎn)接通,電流經(jīng)過(guò)SBl的常開(kāi)觸點(diǎn)和停止按鈕SB2、作過(guò)載保護(hù)用的熱繼電器FR的常閉觸點(diǎn),流過(guò)交流接觸器KM的線圈,接觸器的銜鐵被吸合,使主電路中的3對(duì)常開(kāi)觸點(diǎn)閉合,異步電動(dòng)機(jī)M的三相電源被接通,電動(dòng)機(jī)開(kāi)始運(yùn)行,控制電路中接觸器KM的輔助常開(kāi)觸點(diǎn)同時(shí)接通。放開(kāi)起動(dòng)按鈕后,SBl的常開(kāi)觸點(diǎn)斷開(kāi),電流經(jīng)KM的輔助常開(kāi)觸點(diǎn)和SB2、FR的’常閉觸點(diǎn)流過(guò)KM的線圈,電動(dòng)機(jī)繼續(xù)運(yùn)行。KM的輔助常開(kāi)觸點(diǎn)實(shí)現(xiàn)的這種功能稱為“自鎖”或“自保持”,它使繼電器電路具有類似于R-S觸發(fā)器的記憶功能。
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