描述
當(dāng)異步電動機(jī)定子繞組中的三相電源相序改變時,可以產(chǎn)生與轉(zhuǎn)子轉(zhuǎn)動方向相反的轉(zhuǎn)矩,反接制動也是利用這一原理工作的。
當(dāng)交換任意兩相順序時,就可以產(chǎn)生制動轉(zhuǎn)矩,起到制動作用。不難想象,反接制動的過程為:當(dāng)想要停車時,首先將三相電源切換相序,然后當(dāng)電動機(jī)轉(zhuǎn)速接近零時,再將三相電源切除。控制線路就是要實(shí)現(xiàn)這一過程。
我們知道,電動機(jī)正在正方向運(yùn)行時,如果把電源交換相序(稱為反接),電動機(jī)轉(zhuǎn)速將由正轉(zhuǎn)急速下降到零。如果反接電源不及時切除,則電動機(jī)又要從零速反向起動運(yùn)行。所以我們必須在電動機(jī)制動到零速時,將反接電源切斷??刂齐娐酚盟俣壤^電器來判斷電動機(jī)的停與轉(zhuǎn)。
使用速度繼電器時,速度繼電器的轉(zhuǎn)子是與電動機(jī)的轉(zhuǎn)子同軸連接,電動機(jī)轉(zhuǎn)動時,速度繼電器的動合觸點(diǎn)閉合,電動機(jī)停轉(zhuǎn)時,動合觸點(diǎn)斷開。在電氣原理圖中,用虛線將速度繼電器與電動機(jī)相連表示二者同軸連接。
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