一、外接主電路結構

變(bian)(bian)頻(pin)器(qi)的(de)(de)外接主電路(lu)如(ru)圖4所示。三相交流(liu)電源(yuan)經斷(duan)路(lu)器(qi)QF、交流(liu)接觸器(qi)KM與變(bian)(bian)頻(pin)器(qi)的(de)(de)電源(yuan)輸(shu)(shu)入端R、S、T連(lian)(lian)接；變(bian)(bian)頻(pin)器(qi)的(de)(de)輸(shu)(shu)出端U、V、W則與電動機直接相連(lian)(lian)，這時(shi)電動機的(de)(de)保護由變(bian)(bian)頻(pin)器(qi)完(wan)成。這里的(de)(de)斷(duan)路(lu)器(qi)作用(yong)有：一(yi)是(shi)變(bian)(bian)頻(pin)器(qi)停用(yong)或(huo)維修(xiu)時(shi)，可(ke)通過(guo)(guo)斷(duan)路(lu)器(qi)切(qie)斷(duan)與電源(yuan)之間的(de)(de)連(lian)(lian)接；二是(shi)斷(duan)路(lu)器(qi)具有過(guo)(guo)電流(liu)和欠(qian)電壓(ya)等保護功(gong)能，可(ke)對變(bian)(bian)頻(pin)器(qi)起一(yi)定(ding)的(de)(de)保護作用(yong)。而接觸器(qi)可(ke)通過(guo)(guo)按鈕開關方便地控(kong)制變(bian)(bian)頻(pin)器(qi)的(de)(de)通電與斷(duan)電，同時(shi)，當(dang)變(bian)(bian)頻(pin)器(qi)或(huo)相關控(kong)制電路(lu)發生(sheng)故障時(shi)可(ke)自動切(qie)斷(duan)變(bian)(bian)頻(pin)器(qi)的(de)(de)電源(yuan)。

圖4 變頻器的外(wai)接主電路

二、相關元件的選擇

變(bian)(bian)頻(pin)(pin)(pin)(pin)器(qi)(qi)(qi)(qi)輸出(chu)端與電(dian)(dian)動(dong)(dong)機(ji)之間(jian)(jian)是否(fou)需要配(pei)置交流(liu)接(jie)(jie)觸(chu)器(qi)(qi)(qi)(qi)，這要根據具(ju)體的應用(yong)環境(jing)來確(que)定(ding)。一(yi)般情況下，一(yi)臺變(bian)(bian)頻(pin)(pin)(pin)(pin)器(qi)(qi)(qi)(qi)控制(zhi)一(yi)臺電(dian)(dian)動(dong)(dong)機(ji)，且不要求與工(gong)(gong)(gong)(gong)頻(pin)(pin)(pin)(pin)進行切(qie)換時(shi)，變(bian)(bian)頻(pin)(pin)(pin)(pin)器(qi)(qi)(qi)(qi)與電(dian)(dian)動(dong)(dong)機(ji)之間(jian)(jian)不要使用(yong)接(jie)(jie)觸(chu)器(qi)(qi)(qi)(qi)，如(ru)(ru)圖4所(suo)示。而(er)一(yi)臺變(bian)(bian)頻(pin)(pin)(pin)(pin)器(qi)(qi)(qi)(qi)驅(qu)動(dong)(dong)多(duo)臺電(dian)(dian)動(dong)(dong)機(ji)時(shi)，則每(mei)臺電(dian)(dian)動(dong)(dong)機(ji)必須(xu)有(you)單獨控制(zhi)的接(jie)(jie)觸(chu)器(qi)(qi)(qi)(qi)，并選配(pei)合適的熱繼(ji)電(dian)(dian)器(qi)(qi)(qi)(qi)FR對電(dian)(dian)動(dong)(dong)機(ji)進行保護，具(ju)體電(dian)(dian)路如(ru)(ru)圖5所(suo)示。有(you)時(shi)雖然一(yi)臺變(bian)(bian)頻(pin)(pin)(pin)(pin)器(qi)(qi)(qi)(qi)僅驅(qu)動(dong)(dong)一(yi)臺電(dian)(dian)動(dong)(dong)機(ji)，但有(you)可能在(zai)變(bian)(bian)頻(pin)(pin)(pin)(pin)與工(gong)(gong)(gong)(gong)頻(pin)(pin)(pin)(pin)之間(jian)(jian)切(qie)換運行，這時(shi)也應在(zai)變(bian)(bian)頻(pin)(pin)(pin)(pin)器(qi)(qi)(qi)(qi)與電(dian)(dian)動(dong)(dong)機(ji)之間(jian)(jian)配(pei)置接(jie)(jie)觸(chu)器(qi)(qi)(qi)(qi)KM3和(he)熱繼(ji)電(dian)(dian)器(qi)(qi)(qi)(qi)FR。如(ru)(ru)圖6所(suo)示。接(jie)(jie)觸(chu)器(qi)(qi)(qi)(qi)KM3在(zai)電(dian)(dian)動(dong)(dong)機(ji)工(gong)(gong)(gong)(gong)頻(pin)(pin)(pin)(pin)運行時(shi)用(yong)于切(qie)斷(duan)變(bian)(bian)頻(pin)(pin)(pin)(pin)器(qi)(qi)(qi)(qi)輸出(chu)端與電(dian)(dian)源之間(jian)(jian)的連接(jie)(jie)；熱繼(ji)電(dian)(dian)器(qi)(qi)(qi)(qi)FR可在(zai)工(gong)(gong)(gong)(gong)頻(pin)(pin)(pin)(pin)運行時(shi)對電(dian)(dian)動(dong)(dong)機(ji)進行保護。

圖5 一(yi)臺變頻器驅動(dong)多臺電動(dong)機

圖6 變(bian)頻與工頻切換

三、變頻器與電動機之間的允許距離

變(bian)頻(pin)器的(de)輸出電(dian)(dian)(dian)(dian)壓(ya)宣稱是正弦交(jiao)流(liu)電(dian)(dian)(dian)(dian)，而實際上輸出的(de)是電(dian)(dian)(dian)(dian)壓(ya)脈沖序列，其頻(pin)率(lv)等于(yu)(yu)載波(bo)頻(pin)率(lv)，約幾(ji)kHz~20kHz，幅(fu)值等于(yu)(yu)直流(liu)回路電(dian)(dian)(dian)(dian)壓(ya)平均(jun)值，當變(bian)頻(pin)器與電(dian)(dian)(dian)(dian)動機(ji)之間的(de)連(lian)接線(xian)(xian)很長時(shi)，導線(xian)(xian)的(de)分(fen)布(bu)電(dian)(dian)(dian)(dian)感和(he)線(xian)(xian)間分(fen)布(bu)電(dian)(dian)(dian)(dian)容的(de)作(zuo)用將不(bu)可忽視，線(xian)(xian)間分(fen)布(bu)電(dian)(dian)(dian)(dian)容與電(dian)(dian)(dian)(dian)動機(ji)的(de)漏(lou)磁電(dian)(dian)(dian)(dian)感之間有可能(neng)因接近于(yu)(yu)諧振點而導致電(dian)(dian)(dian)(dian)動機(ji)的(de)輸入電(dian)(dian)(dian)(dian)壓(ya)偏高，使電(dian)(dian)(dian)(dian)動機(ji)損壞，或運(yun)行時(shi)發生(sheng)振動。因此，變(bian)頻(pin)器與電(dian)(dian)(dian)(dian)動機(ji)之間的(de)允許距離(li)（允許導線(xian)(xian)長度）受到了(le)限制。由(you)于(yu)(yu)各(ge)種變(bian)頻(pin)器內部采用了(le)不(bu)同的(de)技(ji)術(shu)方(fang)案，所以其允許距離(li)也有區別。