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當三(san)相(xiang)負載的(de)性(xing)質(電(dian)阻(zu)性(xing)、電(dian)感性(xing)或(huo)電(dian)容性(xing))和大小都(dou)相(xiang)同，即為三(san)相(xiang)對稱負載或(huo)三(san)相(xiang)平(ping)衡負載時(shi)：

星形連接電源的線電壓是負載相電壓的√3倍，即：U線=√3U相，即380V=1.732*220V，相位上，線電壓超前其對應的相電壓30度。線電流等于相電流，即I線=I相。任何時刻流經三相的電流矢量和等于零。

三角形連接負載的相電壓等于電源的線電壓，即U相=U線，電源的線電流等于負載相電流的√3倍，即I線=√3I相，且線電流相位滯后于相電流相位30度。

電(dian)機的星形和(he)三角形接法

電(dian)機星形(xing)(xing)接(jie)(jie)(jie)法(fa)(fa)和三(san)(san)角形(xing)(xing)接(jie)(jie)(jie)法(fa)(fa)都是指電(dian)機本身(shen)的繞組(zu)接(jie)(jie)(jie)法(fa)(fa)，星形(xing)(xing)接(jie)(jie)(jie)法(fa)(fa)指將(jiang)電(dian)機繞組(zu)三(san)(san)相末端(duan)接(jie)(jie)(jie)在一(yi)起(qi)，三(san)(san)相首端(duan)為電(dian)源端(duan)；三(san)(san)角形(xing)(xing)接(jie)(jie)(jie)法(fa)(fa)指將(jiang)三(san)(san)相繞組(zu)首尾(wei)互相連(lian)接(jie)(jie)(jie)，三(san)(san)個端(duan)點為電(dian)源端(duan)。接(jie)(jie)(jie)線(xian)方法(fa)(fa)如下圖所示：

從(cong)上(shang)圖可以(yi)看出(chu)，上(shang)下樁依次聯結是三角形，上(shang)面三根樁接(jie)一(yi)起是星(xing)形，根據前文(wen)可知電(dian)機三角形接(jie)法時線電(dian)壓等(deng)(deng)于(yu)相(xiang)電(dian)壓，線電(dian)流(liu)等(deng)(deng)于(yu)相(xiang)電(dian)流(liu)的(de)約1.73倍；電(dian)機星(xing)形接(jie)法時線電(dian)壓等(deng)(deng)于(yu)相(xiang)電(dian)壓的(de)約1.73倍，線電(dian)流(liu)等(deng)(deng)于(yu)相(xiang)電(dian)流(liu)。

Y-△降壓啟動(dong)電(dian)路

對(dui)于正(zheng)常運(yun)行(xing)時定子繞組(zu)(zu)接(jie)成(cheng)三角形的(de)三相異步電動(dong)(dong)(dong)機，可采用Y-△減壓啟(qi)(qi)動(dong)(dong)(dong)。啟(qi)(qi)動(dong)(dong)(dong)時先將定子繞組(zu)(zu)接(jie)成(cheng)星(xing)形，使(shi)得每相繞組(zu)(zu)電壓為正(zheng)常運(yun)行(xing)時相電壓的(de)1/√3，啟(qi)(qi)動(dong)(dong)(dong)完畢再恢復成(cheng)三角形聯結(jie)，電動(dong)(dong)(dong)機便進入(ru)全(quan)壓下正(zheng)常運(yun)行(xing)。其優點是啟(qi)(qi)動(dong)(dong)(dong)設備成(cheng)本低、方法簡單(dan)易操作。雖然此法的(de)啟(qi)(qi)動(dong)(dong)(dong)電流降至全(quan)壓啟(qi)(qi)動(dong)(dong)(dong)時的(de)1/3，但啟(qi)(qi)動(dong)(dong)(dong)轉(zhuan)矩只有額定轉(zhuan)矩的(de)1/3，故這種方法多用于輕(qing)載(zai)或(huo)空載(zai)啟(qi)(qi)動(dong)(dong)(dong)。

減(jian)壓啟動控制(zhi)電(dian)路(lu)圖如下圖所(suo)示：

電(dian)(dian)(dian)路工作(zuo)原理：合上電(dian)(dian)(dian)源隔(ge)離開(kai)(kai)(kai)關QS，按(an)下起動(dong)按(an)鈕SB2，KM1得(de)電(dian)(dian)(dian)并自鎖(suo)，KM3通(tong)(tong)電(dian)(dian)(dian)，KM3主觸(chu)(chu)頭(tou)閉(bi)(bi)合，電(dian)(dian)(dian)動(dong)機(ji)(ji)(ji)(ji)接成(cheng)Y形聯結，接入三相電(dian)(dian)(dian)源進行(xing)減壓起動(dong)，同(tong)時(shi)(shi)時(shi)(shi)間(jian)繼電(dian)(dian)(dian)器KT通(tong)(tong)電(dian)(dian)(dian)并開(kai)(kai)(kai)始計時(shi)(shi)，經一段時(shi)(shi)間(jian)延(yan)時(shi)(shi)后(hou)，KT的延(yan)時(shi)(shi)常閉(bi)(bi)觸(chu)(chu)頭(tou)斷開(kai)(kai)(kai)，KM3失電(dian)(dian)(dian)釋放，電(dian)(dian)(dian)動(dong)機(ji)(ji)(ji)(ji)Y中性點斷開(kai)(kai)(kai)，同(tong)時(shi)(shi)KT的延(yan)時(shi)(shi)常開(kai)(kai)(kai)觸(chu)(chu)頭(tou)閉(bi)(bi)合，KM2通(tong)(tong)電(dian)(dian)(dian)自鎖(suo)，電(dian)(dian)(dian)動(dong)機(ji)(ji)(ji)(ji)接成(cheng)△聯結運行(xing)，同(tong)時(shi)(shi)用KM2斷開(kai)(kai)(kai)KT線圈，另外輔助觸(chu)(chu)頭(tou)KM2和KM3為互鎖(suo)觸(chu)(chu)頭(tou)，防止KM2和KM3同(tong)時(shi)(shi)帶電(dian)(dian)(dian)。至此，電(dian)(dian)(dian)動(dong)機(ji)(ji)(ji)(ji)Y-△減壓起動(dong)過程結束，電(dian)(dian)(dian)動(dong)機(ji)(ji)(ji)(ji)投(tou)入正常運行(xing)。停(ting)車時(shi)(shi)，按(an)下SB1即可(ke)。

關于時(shi)間繼電器KT的延時(shi)時(shi)間可(ke)按估算公式：功率開方后乘(cheng)以2再加4s即可(ke)。例如(ru)電動(dong)機功率為16kW，則延時(shi)時(shi)間t=2*√16+4=12s。