不銹鋼(gang)管穿(chuan)過磁(ci)(ci)化線圈時(shi)會發生如(ru)下兩(liang)種電磁(ci)(ci)感應現象(xiang)：
1. 鋼管在橫向漏磁檢測磁化器內運動時，不銹鋼(gang)管切割磁力線而在其內部形成感生渦流。
2. 不銹(xiu)鋼管(guan)(guan)磁(ci)介(jie)質在管(guan)(guan)頭進(jin)入磁(ci)化(hua)(hua)(hua)線(xian)(xian)(xian)圈和(he)管(guan)(guan)尾離開磁(ci)化(hua)(hua)(hua)線(xian)(xian)(xian)圈時，由(you)于磁(ci)化(hua)(hua)(hua)線(xian)(xian)(xian)圈的(de)磁(ci)通總量發生(sheng)急劇變(bian)化(hua)(hua)(hua)，線(xian)(xian)(xian)圈中會產生(sheng)感生(sheng)電流。
一、鋼管內(nei)產(chan)生的感(gan)生渦流
不銹鋼管(guan)橫向缺(que)陷(xian)漏磁檢測方法采用(yong)穿(chuan)過式(shi)線(xian)(xian)圈(quan)(quan)產(chan)生(sheng)軸向磁化(hua)場，并在磁化(hua)線(xian)(xian)圈(quan)(quan)內(nei)布置檢測傳(chuan)感器。當鋼管(guan)沿著(zhu)軸向移(yi)動時，處于磁化(hua)線(xian)(xian)圈(quan)(quan)內(nei)的不銹鋼管(guan)段(duan)被(bei)(bei)磁化(hua)至近飽和狀態(tai)，如(ru)存在缺(que)陷(xian)將在鋼管(guan)表面產(chan)生(sheng)泄漏磁場，然后被(bei)(bei)磁敏感元件拾取并依次轉(zhuan)換(huan)為模擬信號(hao)和數字(zi)信號(hao)，最終由計(ji)算機(ji)信號(hao)處理系統實施(shi)報(bao)警和分類。
如圖5-1所(suo)示，以鋼(gang)管(guan)軸線(xian)為(wei)(wei)中(zhong)心(xin)建立(li)圓柱坐標(biao)系。沿著鋼(gang)管(guan)運(yun)動方向，以磁(ci)(ci)化(hua)線(xian)圈為(wei)(wei)中(zhong)心(xin)將鋼(gang)管(guan)劃(hua)分為(wei)(wei)進入區(qu)和離開區(qu)，在磁(ci)(ci)化(hua)線(xian)圈中(zhong)施(shi)加如圖所(suo)示的磁(ci)(ci)化(hua)電流，磁(ci)(ci)力線(xian)分布(bu)特征為(wei)(wei)：在進入區(qu)磁(ci)(ci)力線(xian)從空氣中(zhong)進入鋼(gang)管(guan)，并在磁(ci)(ci)化(hua)線(xian)圈中(zhong)部匯聚，然后(hou)在離開區(qu)折射(she)入空氣中(zhong)。
如圖5-1所示，將磁感應強(qiang)度(du)矢量B(r,z)分解為軸向分量和徑向分量，即 B(r,z)=B2(r,z)+B,(r,z)
從圖5-1中可(ke)以看出，軸向(xiang)分(fen)量B2(r,z)在進入區(qu)(qu)(qu)和離開區(qu)(qu)(qu)方(fang)向(xiang)一致，沿(yan)著鋼管(guan)前進方(fang)向(xiang)，其強(qiang)度在進入區(qu)(qu)(qu)逐漸增(zeng)大(da)(da)，并在磁(ci)(ci)化線圈(quan)中部(bu)(bu)達到極大(da)(da)值，之后(hou)在離開區(qu)(qu)(qu)逐漸減小(xiao)。徑向(xiang)分(fen)量B,(r,z)在進入區(qu)(qu)(qu)方(fang)向(xiang)指(zhi)向(xiang)鋼管(guan)內部(bu)(bu)，并在磁(ci)(ci)化線圈(quan)中部(bu)(bu)發生轉變(bian)，在離開區(qu)(qu)(qu)方(fang)向(xiang)指(zhi)向(xiang)鋼管(guan)外部(bu)(bu)。
為(wei)(wei)了研究與鋼管同軸(zhou)圓環(huan)l(ro ,zo)的渦流分(fen)布，設圓環(huan)半徑為(wei)(wei)ro，軸(zhou)向位(wei)置為(wei)(wei)200根據楞次定律，當(dang)圓環(huan)移動時(shi)，軸(zhou)向分(fen)量B,(r,z)的強度變(bian)(bian)化導(dao)致圓環(huan)磁通(tong)量也發(fa)生改變(bian)(bian)，從而在圓環(huan)中產生感生電動勢。因磁化場為(wei)(wei)軸(zhou)對稱，建立圓環(huan)感應(ying)電動勢方程(cheng)為(wei)(wei)
根(gen)據(ju)式(shi)（5-3），沿(yan)鋼管(guan)前進方(fang)向，在(zai)(zai)進入區，軸(zhou)向分量(liang)強(qiang)度逐漸增(zeng)強(qiang)，感(gan)(gan)生(sheng)渦流方(fang)向與(yu)(yu)原(yuan)磁化(hua)(hua)電(dian)(dian)流方(fang)向相反；在(zai)(zai)磁化(hua)(hua)線圈中間位(wei)置，由于(yu)軸(zhou)向分量(liang)變化(hua)(hua)率為(wei)零(ling)，故此部位(wei)無感(gan)(gan)生(sheng)流產(chan)生(sheng)；在(zai)(zai)離(li)開區，軸(zhou)向分量(liang)強(qiang)度由中間最大值逐漸減小，于(yu)是形(xing)成與(yu)(yu)原(yuan)磁化(hua)(hua)電(dian)(dian)流方(fang)向相同的感(gan)(gan)生(sheng)渦流，最終鋼管(guan)中感(gan)(gan)生(sheng)渦流分布(bu)如圖(tu)(tu)5-2a所示。如果改變磁化(hua)(hua)電(dian)(dian)流方(fang)向，根(gen)據(ju)式(shi)（5-3），同樣(yang)可(ke)得出鋼管(guan)內感(gan)(gan)生(sheng)渦流分布(bu)，如圖(tu)(tu)5-2b所示。
從(cong)(cong)圖(tu)5-2中(zhong)(zhong)可以看(kan)出，鋼管中(zhong)(zhong)感(gan)生(sheng)(sheng)(sheng)電(dian)(dian)流分布方(fang)向(xiang)(xiang)由磁(ci)化(hua)電(dian)(dian)流方(fang)向(xiang)(xiang)和(he)鋼管運(yun)動方(fang)向(xiang)(xiang)共同決定。在(zai)進入區(qu)(qu)，鋼管中(zhong)(zhong)的感(gan)生(sheng)(sheng)(sheng)渦流J1與(yu)磁(ci)化(hua)電(dian)(dian)流方(fang)向(xiang)(xiang)相反；在(zai)磁(ci)化(hua)線圈(quan)中(zhong)(zhong)間位置無感(gan)生(sheng)(sheng)(sheng)渦流產(chan)生(sheng)(sheng)(sheng)；在(zai)離開區(qu)(qu)，感(gan)生(sheng)(sheng)(sheng)渦流J2與(yu)磁(ci)化(hua)電(dian)(dian)流方(fang)向(xiang)(xiang)相同。從(cong)(cong)而(er)，在(zai)感(gan)生(sheng)(sheng)(sheng)渦流產(chan)生(sheng)(sheng)(sheng)的磁(ci)場(chang)作(zuo)用下，鋼管的磁(ci)化(hua)狀態將(jiang)發生(sheng)(sheng)(sheng)變(bian)化(hua)。
建立如(ru)圖5-3所示(shi)的仿真模型。鋼(gang)(gang)管直徑為400mm、壁厚為15mm、長(chang)度(du)為3000mm，材(cai)質(zhi)為25鋼(gang)(gang)（電(dian)導率為）。磁化(hua)線圈內徑為440mm、外徑為750mm、厚度(du)為160mm，磁化(hua)電(dian)流密(mi)度(du)iA／㎡，電(dian)流方向(xiang)如(ru)圖5-3所示(shi)。
對不銹(xiu)鋼(gang)(gang)(gang)(gang)管(guan)(guan)中(zhong)的(de)感(gan)(gan)生(sheng)渦(wo)(wo)流(liu)(liu)(liu)(liu)(liu)(liu)分(fen)布(bu)(bu)進(jin)(jin)行(xing)仿真研究。磁化線(xian)圈(quan)固定不動，鋼(gang)(gang)(gang)(gang)管(guan)(guan)運(yun)行(xing)速度(du)設置為(wei)1m／s，不銹(xiu)鋼(gang)(gang)(gang)(gang)管(guan)(guan)從左(zuo)端(duan)進(jin)(jin)人并向(xiang)(xiang)右端(duan)移動，當鋼(gang)(gang)(gang)(gang)管(guan)(guan)中(zhong)心(xin)與(yu)磁化線(xian)圈(quan)中(zhong)心(xin)重合時(shi)獲取(qu)感(gan)(gan)生(sheng)渦(wo)(wo)流(liu)(liu)(liu)(liu)(liu)(liu)分(fen)布(bu)(bu)云圖，如圖5-4所示。從圖中(zhong)可以看(kan)出，進(jin)(jin)入區的(de)感(gan)(gan)生(sheng)渦(wo)(wo)流(liu)(liu)(liu)(liu)(liu)(liu)方(fang)向(xiang)(xiang)與(yu)磁化電(dian)流(liu)(liu)(liu)(liu)(liu)(liu)方(fang)向(xiang)(xiang)相反(fan)，離開區的(de)感(gan)(gan)生(sheng)渦(wo)(wo)流(liu)(liu)(liu)(liu)(liu)(liu)方(fang)向(xiang)(xiang)與(yu)磁化電(dian)流(liu)(liu)(liu)(liu)(liu)(liu)方(fang)向(xiang)(xiang)相同，在(zai)線(xian)圈(quan)中(zhong)部感(gan)(gan)生(sheng)渦(wo)(wo)流(liu)(liu)(liu)(liu)(liu)(liu)幾(ji)乎為(wei)零。進(jin)(jin)入區和(he)離開區的(de)渦(wo)(wo)流(liu)(liu)(liu)(liu)(liu)(liu)分(fen)布(bu)(bu)相對于線(xian)圈(quan)呈對稱分(fen)布(bu)(bu)，方(fang)向(xiang)(xiang)相反(fan)，強度(du)基本相同，仿真結(jie)果與(yu)圖5-2所示的(de)渦(wo)(wo)流(liu)(liu)(liu)(liu)(liu)(liu)分(fen)布(bu)(bu)理(li)論分(fen)析結(jie)論相同，其中(zhong)感(gan)(gan)生(sheng)渦(wo)(wo)流(liu)(liu)(liu)(liu)(liu)(liu)最大值為(wei)1.4×105A/㎡。
為(wei)了研究感(gan)(gan)生(sheng)渦流與(yu)不(bu)(bu)銹鋼管(guan)運(yun)(yun)行速(su)度(du)(du)(du)的關系(xi)(xi)，分別取(qu)速(su)度(du)(du)(du)0.1m／s、1m/s、2m/s、5m/s、8m／s、10m／s、20m／s、30m／s、40m／s和(he)(he)50m／s進行仿(fang)真。當鋼管(guan)中部(bu)與(yu)磁化線圈重(zhong)合時(shi)(shi)提取(qu)渦流密度(du)(du)(du)最大值(zhi)(zhi)和(he)(he)最小值(zhi)(zhi)，繪制成如圖5-5所示的渦流密度(du)(du)(du)與(yu)運(yun)(yun)行速(su)度(du)(du)(du)關系(xi)(xi)曲線。從圖中可以(yi)看(kan)出，感(gan)(gan)生(sheng)渦流與(yu)不(bu)(bu)銹鋼管(guan)運(yun)(yun)行速(su)度(du)(du)(du)成近(jin)似正比關系(xi)(xi)。鋼管(guan)低速(su)運(yun)(yun)動時(shi)(shi)感(gan)(gan)生(sheng)渦流很小，可忽略不(bu)(bu)計；當運(yun)(yun)行速(su)度(du)(du)(du)增至(zhi)50m／s時(shi)(shi)，渦流密度(du)(du)(du)為(wei)2。此時(shi)(shi)，感(gan)(gan)生(sheng)渦流已接(jie)近(jin)傳導(dao)電(dian)流密度(du)(du)(du)。因此，高速(su)運(yun)(yun)動時(shi)(shi)，感(gan)(gan)生(sheng)渦流對鋼管(guan)漏(lou)磁檢測(ce)的影(ying)響不(bu)(bu)可忽視。
二、磁化線圈中產生的感(gan)生電流
當不(bu)銹鋼管端部進(jin)入和離(li)開磁(ci)(ci)(ci)化(hua)(hua)線(xian)(xian)(xian)圈(quan)(quan)(quan)時，線(xian)(xian)(xian)圈(quan)(quan)(quan)中(zhong)的磁(ci)(ci)(ci)通(tong)(tong)量發(fa)生(sheng)變化(hua)(hua)而產生(sheng)感生(sheng)電(dian)(dian)(dian)(dian)流。設磁(ci)(ci)(ci)化(hua)(hua)電(dian)(dian)(dian)(dian)源提(ti)供的電(dian)(dian)(dian)(dian)壓(ya)為(wei)(wei)Uo，磁(ci)(ci)(ci)化(hua)(hua)線(xian)(xian)(xian)圈(quan)(quan)(quan)電(dian)(dian)(dian)(dian)阻為(wei)(wei)R，則(ze)磁(ci)(ci)(ci)化(hua)(hua)電(dian)(dian)(dian)(dian)源在線(xian)(xian)(xian)圈(quan)(quan)(quan)中(zhong)產生(sheng)的初(chu)始傳導(dao)(dao)電(dian)(dian)(dian)(dian)流為(wei)(wei)Io=Uo/RR。磁(ci)(ci)(ci)化(hua)(hua)線(xian)(xian)(xian)圈(quan)(quan)(quan)通(tong)(tong)過的磁(ci)(ci)(ci)通(tong)(tong)總(zong)(zong)量為(wei)(wei)Φ，當磁(ci)(ci)(ci)化(hua)(hua)線(xian)(xian)(xian)圈(quan)(quan)(quan)中(zhong)磁(ci)(ci)(ci)通(tong)(tong)總(zong)(zong)量發(fa)生(sheng)變化(hua)(hua)時，根據楞次定律，線(xian)(xian)(xian)圈(quan)(quan)(quan)中(zhong)將產生(sheng)感生(sheng)電(dian)(dian)(dian)(dian)動勢，對應的感生(sheng)電(dian)(dian)(dian)(dian)流磁(ci)(ci)(ci)化(hua)(hua)線(xian)(xian)(xian)圈(quan)(quan)(quan)中(zhong)通(tong)(tong)過的電(dian)(dian)(dian)(dian)流I為(wei)(wei)初(chu)始傳導(dao)(dao)電(dian)(dian)(dian)(dian)流和感生(sheng)電(dian)(dian)(dian)(dian)流之(zhi)和，即
當線圈中沒有鋼(gang)管時，磁(ci)化線圈磁(ci)通(tong)(tong)總量(liang)(liang)為線圈自身(shen)產生的靜態磁(ci)通(tong)(tong)量(liang)(liang)，其與磁(ci)化電(dian)流強度(du)成正(zheng)比，當磁(ci)化電(dian)流不變時，線圈磁(ci)通(tong)(tong)總量(liang)(liang)也(ye)不發生變化。此時線圈中通(tong)(tong)過的電(dian)流為磁(ci)化電(dian)源產生的初始磁(ci)化傳導電(dian)流
當管頭進入磁化(hua)線(xian)(xian)圈(quan)(quan)(quan)時，具有高磁導率的(de)(de)鋼管磁介質進入磁化(hua)線(xian)(xian)圈(quan)(quan)(quan)內部，使得線(xian)(xian)圈(quan)(quan)(quan)內部的(de)(de)磁通總量增大(da)。根據式（5-4），磁化(hua)線(xian)(xian)圈(quan)(quan)(quan)中會(hui)產(chan)生(sheng)與(yu)初(chu)始(shi)磁化(hua)傳導電流方向相反(fan)的(de)(de)感(gan)生(sheng)電流，此時線(xian)(xian)圈(quan)(quan)(quan)中通過的(de)(de)電流為，如圖5-6a所示。
當管體通過磁(ci)(ci)化(hua)(hua)(hua)線(xian)圈(quan)時，線(xian)圈(quan)內部(bu)磁(ci)(ci)介(jie)質(zhi)總(zong)量及分布特性基(ji)本(ben)不(bu)變，從而線(xian)圈(quan)內部(bu)的磁(ci)(ci)通總(zong)量也保(bao)持恒定(ding)。根據式（5-4），磁(ci)(ci)化(hua)(hua)(hua)線(xian)圈(quan)基(ji)本(ben)無感生電流(liu)產生，此時，磁(ci)(ci)化(hua)(hua)(hua)線(xian)圈(quan)中通過的電流(liu)與無鋼管時相(xiang)同，為磁(ci)(ci)化(hua)(hua)(hua)電源產生的初始磁(ci)(ci)化(hua)(hua)(hua)傳導電流(liu)1＝，如圖(tu)5-6b所(suo)示(shi)。
當管尾離開磁(ci)化線(xian)圈(quan)(quan)時，由(you)于線(xian)圈(quan)(quan)內部的(de)高(gao)磁(ci)導(dao)率磁(ci)介質(zhi)不斷減(jian)少，導(dao)致磁(ci)化線(xian)圈(quan)(quan)的(de)磁(ci)通總量也(ye)不斷減(jian)少。根(gen)據式（5-4），磁(ci)化線(xian)圈(quan)(quan)中會產生(sheng)與初(chu)始磁(ci)化傳導(dao)電(dian)流(liu)方向相同的(de)感生(sheng)電(dian)流(liu)，此時線(xian)圈(quan)(quan)中通過的(de)電(dian)流(liu)為，如圖5-6c所示。
不(bu)銹(xiu)鋼(gang)管內(nei)的(de)磁(ci)(ci)場包括：磁(ci)(ci)化線(xian)圈(quan)通(tong)過(guo)電流(liu)I產生(sheng)(sheng)的(de)磁(ci)(ci)場和鋼(gang)管中(zhong)感(gan)生(sheng)(sheng)渦(wo)流(liu)J形成的(de)磁(ci)(ci)場。磁(ci)(ci)化線(xian)圈(quan)的(de)磁(ci)(ci)通(tong)總量(liang)包含(han)了由感(gan)生(sheng)(sheng)渦(wo)流(liu)J產生(sheng)(sheng)的(de)部分磁(ci)(ci)通(tong)量(liang)，因此(ci)鋼(gang)管中(zhong)的(de)渦(wo)流(liu)效應會對磁(ci)(ci)化線(xian)圈(quan)中(zhong)的(de)感(gan)生(sheng)(sheng)電流(liu)產生(sheng)(sheng)一定影響。
采(cai)用如(ru)圖5-3所示(shi)模型(xing)，進(jin)一(yi)步(bu)研(yan)究磁(ci)化(hua)(hua)(hua)線(xian)圈(quan)中(zhong)產生的(de)感(gan)(gan)生電(dian)(dian)流(liu)(liu)變化(hua)(hua)(hua)規律。其中(zhong)，線(xian)圈(quan)匝數為600匝，磁(ci)化(hua)(hua)(hua)電(dian)(dian)流(liu)(liu)為5A。當鋼(gang)管(guan)(guan)管(guan)(guan)頭、管(guan)(guan)體和(he)管(guan)(guan)尾分(fen)(fen)別(bie)與(yu)磁(ci)化(hua)(hua)(hua)線(xian)圈(quan)耦合時提取磁(ci)化(hua)(hua)(hua)線(xian)圈(quan)內部產生的(de)感(gan)(gan)生電(dian)(dian)流(liu)(liu)，如(ru)圖5-7所示(shi)。仿真分(fen)(fen)兩種：一(yi)是考慮(lv)鋼(gang)管(guan)(guan)渦流(liu)(liu)效應(ying)時分(fen)(fen)析線(xian)圈(quan)感(gan)(gan)生電(dian)(dian)流(liu)(liu)與(yu)運動(dong)速度的(de)關系(xi)，二是忽略鋼(gang)管(guan)(guan)渦流(liu)(liu)效應(ying)而單獨分(fen)(fen)析線(xian)圈(quan)感(gan)(gan)生電(dian)(dian)流(liu)(liu)與(yu)鋼(gang)管(guan)(guan)運動(dong)速度關系(xi)。分(fen)(fen)別(bie)取速度0.1m／s、1m／s、2m／s、5m／s、8m／s、10m／s、20m／s、30m／s、40m／和(he)50m／s進(jin)行仿真，獲得如(ru)圖5-8所示(shi)的(de)磁(ci)化(hua)(hua)(hua)線(xian)圈(quan)感(gan)(gan)生電(dian)(dian)流(liu)(liu)與(yu)運動(dong)速度關系(xi)曲線(xian)。其中(zhong)I1csI2cs和(he)I3cs分(fen)(fen)別(bie)為考慮(lv)鋼(gang)管(guan)(guan)渦流(liu)(liu)效應(ying)時在管(guan)(guan)頭、管(guan)(guan)體和(he)管(guan)(guan)尾處線(xian)圈(quan)中(zhong)產生的(de)感(gan)(gan)生電(dian)(dian)流(liu)(liu)，11cI2c和(he)I3c分(fen)(fen)別(bie)為忽略不銹(xiu)鋼(gang)管(guan)(guan)渦流(liu)(liu)效應(ying)時磁(ci)化(hua)(hua)(hua)線(xian)圈(quan)中(zhong)產生的(de)感(gan)(gan)生電(dian)(dian)流(liu)(liu)。
圖(tu)5-8所(suo)(suo)示的(de)(de)仿真結果(guo)與(yu)圖(tu)5-6所(suo)(suo)示的(de)(de)理論(lun)(lun)分析結論(lun)(lun)相同：當(dang)管頭進(jin)入(ru)(ru)磁(ci)(ci)(ci)化線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)時(shi)，線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)中(zhong)產(chan)生(sheng)(sheng)(sheng)的(de)(de)感(gan)(gan)(gan)生(sheng)(sheng)(sheng)電(dian)(dian)流(liu)(liu)幅值(zhi)為負(fu)，即與(yu)磁(ci)(ci)(ci)化電(dian)(dian)流(liu)(liu)方(fang)向相反；當(dang)管體通(tong)(tong)過磁(ci)(ci)(ci)化線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)時(shi)，線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)中(zhong)基(ji)本(ben)無感(gan)(gan)(gan)生(sheng)(sheng)(sheng)電(dian)(dian)流(liu)(liu)產(chan)生(sheng)(sheng)(sheng)；隨著管尾離開磁(ci)(ci)(ci)化線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)，此時(shi)線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)中(zhong)產(chan)生(sheng)(sheng)(sheng)與(yu)磁(ci)(ci)(ci)化電(dian)(dian)流(liu)(liu)方(fang)向相同的(de)(de)感(gan)(gan)(gan)生(sheng)(sheng)(sheng)電(dian)(dian)流(liu)(liu)。根據楞(leng)次定律，線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)中(zhong)產(chan)生(sheng)(sheng)(sheng)的(de)(de)感(gan)(gan)(gan)生(sheng)(sheng)(sheng)電(dian)(dian)流(liu)(liu)會(hui)阻礙線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)磁(ci)(ci)(ci)通(tong)(tong)量(liang)(liang)的(de)(de)變化：當(dang)管頭進(jin)入(ru)(ru)磁(ci)(ci)(ci)化線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)時(shi)，線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)中(zhong)會(hui)產(chan)生(sheng)(sheng)(sheng)反向感(gan)(gan)(gan)生(sheng)(sheng)(sheng)電(dian)(dian)流(liu)(liu)來阻礙磁(ci)(ci)(ci)通(tong)(tong)量(liang)(liang)的(de)(de)增大(da)；當(dang)管體與(yu)磁(ci)(ci)(ci)化線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)耦合時(shi)，由于線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)磁(ci)(ci)(ci)通(tong)(tong)量(liang)(liang)基(ji)本(ben)不變而無感(gan)(gan)(gan)生(sheng)(sheng)(sheng)電(dian)(dian)流(liu)(liu)產(chan)生(sheng)(sheng)(sheng)；當(dang)管尾離開磁(ci)(ci)(ci)化線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)時(shi)，線(xian)(xian)圈(quan)(quan)(quan)(quan)(quan)中(zhong)會(hui)產(chan)生(sheng)(sheng)(sheng)同向感(gan)(gan)(gan)生(sheng)(sheng)(sheng)電(dian)(dian)流(liu)(liu)來阻礙磁(ci)(ci)(ci)通(tong)(tong)量(liang)(liang)的(de)(de)減小。
另外，從(cong)圖5-8中(zhong)可(ke)以看出，在運(yun)行速(su)度(du)較低時，磁(ci)(ci)化(hua)線(xian)圈中(zhong)感生(sheng)電流(liu)隨著速(su)度(du)的增加而(er)快速(su)上升；當(dang)速(su)度(du)達到一(yi)定幅值時，磁(ci)(ci)化(hua)線(xian)圈中(zhong)的感生(sheng)電流(liu)基(ji)本保持不變(bian)。因為感生(sheng)電流(liu)只能減緩磁(ci)(ci)化(hua)線(xian)圈磁(ci)(ci)通(tong)量的變(bian)化(hua)速(su)度(du)，而(er)不能改變(bian)磁(ci)(ci)通(tong)量的變(bian)化(hua)趨勢(shi)。
從圖(tu)5-8中還可以看出(chu)，鋼管中的渦流(liu)會削(xue)弱磁化線圈中產生(sheng)的感生(sheng)電(dian)流(liu)，根據(ju)楞次定律(lv)，鋼(gang)(gang)管(guan)(guan)中的(de)(de)渦(wo)流同樣會阻(zu)礙(ai)鋼(gang)(gang)管(guan)(guan)中磁(ci)(ci)(ci)通(tong)(tong)量(liang)(liang)的(de)(de)變(bian)化。當不(bu)(bu)銹鋼(gang)(gang)管(guan)(guan)進入和(he)離開磁(ci)(ci)(ci)化線(xian)(xian)圈(quan)時，鋼(gang)(gang)管(guan)(guan)中的(de)(de)磁(ci)(ci)(ci)通(tong)(tong)量(liang)(liang)變(bian)化規律(lv)同樣先增大后減小。由于磁(ci)(ci)(ci)化線(xian)(xian)圈(quan)磁(ci)(ci)(ci)通(tong)(tong)總量(liang)(liang)包含(han)了(le)不(bu)(bu)銹鋼(gang)(gang)管(guan)(guan)磁(ci)(ci)(ci)通(tong)(tong)量(liang)(liang)，所以，感生渦(wo)流在阻(zu)礙(ai)鋼(gang)(gang)管(guan)(guan)磁(ci)(ci)(ci)通(tong)(tong)量(liang)(liang)變(bian)化的(de)(de)同時也阻(zu)礙(ai)了(le)線(xian)(xian)圈(quan)磁(ci)(ci)(ci)通(tong)(tong)量(liang)(liang)的(de)(de)變(bian)化速率(lv)，最終(zhong)削弱(ruo)了(le)線(xian)(xian)圈(quan)感生電流的(de)(de)強度。
