外文翻譯--- 帶式輸送機(jī)及其牽引系統(tǒng)

1、<p><b>　　中文原文:</b></p><p>　　帶式輸送機(jī)及其牽引系統(tǒng)</p><p>　　在運(yùn)送大量的物料時(shí)，帶式輸送機(jī)在長(zhǎng)距離的運(yùn)輸中起到了非常重要的競(jìng)爭(zhēng)作用。輸送系統(tǒng)將會(huì)變得更大、更復(fù)雜，而驅(qū)動(dòng)系統(tǒng)也己經(jīng)歷了一個(gè)演變過(guò)程，并將繼續(xù)這樣下去。如今，較大的輸送帶和多驅(qū)動(dòng)系統(tǒng)需耍更大的功率，比如3驅(qū)動(dòng)系統(tǒng)需耍給輸送帶750KW(成莊煤礦輸送機(jī)驅(qū)動(dòng)

2、系統(tǒng)的要求)?？刂乞?qū)動(dòng)力和加速度扭矩是輸送機(jī)的關(guān)鍵。一個(gè)高效的驅(qū)動(dòng)系統(tǒng)應(yīng)該能順利的運(yùn)行，同時(shí)保持輸送帶張緊力在指定的安全極限負(fù)荷內(nèi)。為了負(fù)載分配在多個(gè)驅(qū)動(dòng)上，扭矩和速度控制在驅(qū)動(dòng)系統(tǒng)的設(shè)計(jì)中也是很重要的因素。由于輸送機(jī)驅(qū)動(dòng)系統(tǒng)控制技術(shù)的進(jìn)步，目前更多可靠的低成本和高效驅(qū)動(dòng)的驅(qū)動(dòng)系統(tǒng)可供顧客選擇[1]</p><p><b>　　1帶式輸送機(jī)驅(qū)動(dòng)</b></p><p>

3、;　　1. 1帶式輸送機(jī)驅(qū)動(dòng)方式</p><p>　　全電壓?jiǎn)?dòng) 在全電壓?jiǎn)?dòng)設(shè)計(jì)中，帶式輸送機(jī)驅(qū)動(dòng)軸通過(guò)齒輪傳動(dòng)直接連接到電機(jī)。直接全壓驅(qū)動(dòng)沒(méi)有為變化的傳送負(fù)載提供任何控制，根據(jù)滿(mǎn)載和空載功率需求的比率，空載啟動(dòng)時(shí)比滿(mǎn)載可能快3-4倍。此種方式的優(yōu)點(diǎn)是:免維護(hù)，啟動(dòng)系統(tǒng)簡(jiǎn)單，低成本，可靠性高。但是，不能控制啟動(dòng)扭矩和最大停止扭矩。因此，這種方式只用于低功率，結(jié)構(gòu)簡(jiǎn)單的傳送驅(qū)動(dòng)中。</p>&l

4、t;p>　　降壓?jiǎn)?dòng) 隨著傳送驅(qū)動(dòng)功率的增加，在加速期間控制使用的電機(jī)扭矩變得越來(lái)越重要。由于電機(jī)扭矩是電壓的函數(shù)，電機(jī)電壓必須得到控制，一般用可控硅整流器(SCR}構(gòu)成的降壓?jiǎn)?dòng)裝置，先施加低電壓拉緊輸送帶，然后線(xiàn)性的增加供電電壓直到全電壓和最大帶速。但是，這種啟動(dòng)方式不會(huì)產(chǎn)生穩(wěn)定的加速度，當(dāng)加速完成時(shí)，控制電機(jī)電壓的SCR鎖定在全導(dǎo)通，為電機(jī)提供全壓。此種控制方式功率可達(dá)到750kW。</p><p&g

5、t;　　繞線(xiàn)轉(zhuǎn)子感應(yīng)電機(jī) 繞線(xiàn)轉(zhuǎn)子感應(yīng)電機(jī)直接連接到驅(qū)動(dòng)系統(tǒng)減速機(jī)上，通過(guò)在電機(jī)轉(zhuǎn)子繞組中串聯(lián)電阻控制電機(jī)轉(zhuǎn)矩。在傳送裝置啟動(dòng)時(shí)，把電阻串聯(lián)進(jìn)轉(zhuǎn)子產(chǎn)生較低的轉(zhuǎn)矩，當(dāng)傳送帶加速時(shí)，電阻逐漸減少保持穩(wěn)定增加轉(zhuǎn)矩。在多驅(qū)動(dòng)系統(tǒng)中，一個(gè)外加的滑差電阻可能將總是串聯(lián)在轉(zhuǎn)子繞組回路中以幫助均分負(fù)載。該方式的電機(jī)系統(tǒng)設(shè)計(jì)相對(duì)簡(jiǎn)單，但控制系統(tǒng)可能很復(fù)雜，因?yàn)樗鼈兪腔谟?jì)算機(jī)控制的電阻切換。當(dāng)今，控制系統(tǒng)的大多數(shù)是定制設(shè)計(jì)來(lái)滿(mǎn)足傳送系統(tǒng)的特殊規(guī)格繞線(xiàn)

6、轉(zhuǎn)子電機(jī)適合于需要400kVV以上的系統(tǒng)。</p><p>　　直流(DC)電機(jī) 大多數(shù)傳送驅(qū)動(dòng)使用DC并勵(lì)電機(jī)，電機(jī)的電樞在外部連接?？刂艱C驅(qū)動(dòng)技術(shù)一般應(yīng)用SCR裝置，它允許連續(xù)的變速操作。DC驅(qū)動(dòng)系統(tǒng)在機(jī)械上是簡(jiǎn)單的，但設(shè)計(jì)的電子電路，監(jiān)測(cè)和控制整個(gè)系統(tǒng)，相比于其他軟啟動(dòng)系統(tǒng)的選擇是昂貴的，但在轉(zhuǎn)矩、負(fù)載均分和變速為主要考慮的場(chǎng)合，它又是一個(gè)可靠的，節(jié)約成本的方式。DC電機(jī)一般使用在功率較大的輸送裝置上

7、，包括需耍輸送帶張力控制的多驅(qū)動(dòng)系統(tǒng)和需要寬變速范圍的輸送裝置上。</p><p><b>　　1.2液力偶合器</b></p><p>　　流體動(dòng)力偶合器通常被稱(chēng)為液力偶合器，由三個(gè)基本單元組成:充當(dāng)離心泵的葉輪，推進(jìn)水壓的渦輪和裝進(jìn)兩個(gè)動(dòng)力部件的外殼。流體從葉輪到渦輪，在從動(dòng)軸產(chǎn)生扭矩。由于循環(huán)流體產(chǎn)生扭矩和速度，在驅(qū)動(dòng)軸和從動(dòng)軸之間不需要任何機(jī)械連接。這種連接產(chǎn)

8、生的動(dòng)力決定于液力偶合器的充液量，扭矩正比于輸入速度。因在流體偶合中輸出速度小于輸入速度，其間的差值稱(chēng)為滑差，一般為1%-3%。傳遞功率可達(dá)幾千千瓦。</p><p>　　固定充液液力偶合器 固定充液液力偶合器是在結(jié)構(gòu)較簡(jiǎn)單和僅具有有限的彎曲部分的輸送裝置中最常用的軟啟動(dòng)裝置，其結(jié)構(gòu)相對(duì)比較簡(jiǎn)單，成本又低，對(duì)現(xiàn)在使用的大多數(shù)輸送機(jī)能提供優(yōu)良的軟啟動(dòng)效果。</p><p>　　可變充液液

9、力偶合器 也稱(chēng)為限矩型液力偶合器。偶合器的葉輪裝在AC電機(jī)上，渦輪裝在從動(dòng)減速器高速軸上，包含操作部件的軸箱安裝在驅(qū)動(dòng)基座。偶合器的旋轉(zhuǎn)外殼有溢出口，允許液體不斷地從工作腔中流出進(jìn)入一個(gè)分離的輔助腔，油從輔助腔通過(guò)一個(gè)熱交換器泵到控制偶合器充液量的電磁閥。為了控制單機(jī)傳動(dòng)系統(tǒng)的啟動(dòng)轉(zhuǎn)矩，必須監(jiān)測(cè)AC電機(jī)電流給電磁閥的控制提供反饋。可變充液液力偶合器可使用在中大功率輸送系統(tǒng)中，功率可達(dá)到數(shù)千千瓦口這種驅(qū)動(dòng)無(wú)論在機(jī)械，或在電氣上都是很復(fù)

10、雜的，其驅(qū)動(dòng)系統(tǒng)成本中等。</p><p>　　勺管控制液力偶合器 也稱(chēng)為調(diào)速型液力偶合器。此種液力偶合器同樣由三個(gè)標(biāo)準(zhǔn)的液力偶合單元構(gòu)成，即葉輪、渦輪和一個(gè)包含工作環(huán)路的外殼。此種液力偶合器需要在工作腔以外設(shè)置導(dǎo)管(也稱(chēng)勺管)和導(dǎo)管腔，依靠調(diào)節(jié)裝置改變勺管開(kāi)度(勺管頂端與旋轉(zhuǎn)外殼間距)人為的改變工作腔的充液量，從而實(shí)現(xiàn)對(duì)輸出轉(zhuǎn)速的調(diào)節(jié)。這種控制提供了合理的平滑加速度，但其計(jì)算機(jī)控制系統(tǒng)很復(fù)雜。勺管控制液力偶

11、合器可以應(yīng)用在單機(jī)或多機(jī)驅(qū)動(dòng)系統(tǒng)，功率范圍為150kW-750kW。</p><p>　　1. 3變頻控制(VFC)</p><p>　　變頻控制也是一種直接驅(qū)動(dòng)方式，它具有非常獨(dú)特的高性能。VFC裝置為感應(yīng)電機(jī)提供變化的頻率和電壓，產(chǎn)生優(yōu)良的啟動(dòng)轉(zhuǎn)矩和加速度。VFC設(shè)備是一個(gè)電力電子控制器，首先把AC整流成DC，然后利用逆變器，再將DC轉(zhuǎn)換成頻率、電壓可控的AC. VFC驅(qū)動(dòng)采用矢量控制

12、或直接轉(zhuǎn)矩控制(DTC)技術(shù)，能根據(jù)不同的負(fù)載采用不同的運(yùn)行速度。VFC驅(qū)動(dòng)能根據(jù)給定的S曲線(xiàn)啟動(dòng)或停車(chē)，實(shí)現(xiàn)自動(dòng)跟蹤啟動(dòng)或停車(chē)曲線(xiàn)。VFC驅(qū)動(dòng)為傳送帶啟動(dòng)提供了優(yōu)良的速度和轉(zhuǎn)矩控制，也能為多機(jī)驅(qū)動(dòng)系統(tǒng)提供負(fù)載均分。VFC控制器可以容易地裝在小功率輸送機(jī)驅(qū)動(dòng)上。過(guò)去在中高電壓使用時(shí)，VFC設(shè)備的結(jié)構(gòu)由于受電力半導(dǎo)體器件的電壓額定值限制而變得很復(fù)雜，中高電壓的變速傳動(dòng)常常使用低壓逆變器，然后在輸出端使用升壓變壓器，或使用多個(gè)低壓逆變器串聯(lián)

13、來(lái)解決。與簡(jiǎn)單的器件串聯(lián)連接的兩電平逆變器系統(tǒng)比較，由于串聯(lián)器件之間容易均壓以及輸出端可以有更好的諧波特性，三電平電壓型PWI\逆變器系統(tǒng)在數(shù)兆瓦工業(yè)傳動(dòng)中近年來(lái)獲得了越來(lái)越多的應(yīng)用。由三臺(tái)750kW/ 2.3kv的這種逆變器構(gòu)成的VFC系統(tǒng)已經(jīng)成功安裝在成莊煤礦長(zhǎng)2. 71m二的帶式輸送機(jī)驅(qū)動(dòng)系統(tǒng)中。</p><p>　　2使用IGBT的中性點(diǎn)箱位三電平逆變器</p><p>　　由于串

14、聯(lián)器件電壓均分容易，器件每次開(kāi)關(guān)的dv/dt低以及輸出端出色的諧波品質(zhì)，三電平電壓型逆變器在大功率傳動(dòng)應(yīng)用中變得越來(lái)越流行。高壓IGBT(HV-IGBT)的出現(xiàn)使得應(yīng)用三電平中性點(diǎn)箱位原理的中高壓逆變器設(shè)計(jì)有了更大的應(yīng)用范圍。這種逆變器目前可以實(shí)現(xiàn)從2. RV到4. 16kV全范圍的應(yīng)用。HV-IGBT模塊串聯(lián)可使用在3. RV和4. 16kV的設(shè)備。2. AV逆變器每個(gè)開(kāi)關(guān)只需要一個(gè)HV-IGBT[2,3]。</p>&

15、lt;p>　　2.1主功率逆變電路</p><p>　　主功率逆變電路用三電平中點(diǎn)箱位電壓型逆變器實(shí)現(xiàn)，可以滿(mǎn)足中高壓交流傳動(dòng)應(yīng)用的需要。與兩電平電壓型逆變器相比，三電平中點(diǎn)箱位電壓型逆變器提供三個(gè)電壓級(jí)別給輸出端，對(duì)于同樣的輸出電流品質(zhì)，開(kāi)關(guān)頻率可降低到原來(lái)的1/4,開(kāi)關(guān)器件的電壓額定值可減小到原來(lái)的1/2，附加到電機(jī)上的額外的瞬態(tài)電壓應(yīng)力也可能減少到原來(lái)的1/2。</p><p>

16、;　　三電平中點(diǎn)箱位電壓型逆變器的開(kāi)關(guān)狀態(tài)可歸納于表1，U，V和W分別表示三相，P, N和G是直流母線(xiàn)上的三個(gè)點(diǎn)。例如，當(dāng)開(kāi)關(guān)S1u和S2u閉合時(shí)，U相處于狀態(tài)P(正母線(xiàn)電壓)，反之，當(dāng)開(kāi)關(guān)S3u和S4U閉合時(shí)，U相處于狀態(tài)n(負(fù)母線(xiàn)電壓)。在中性點(diǎn)箱位時(shí)，該相在o狀態(tài)，這時(shí)根據(jù)相電流極性的正負(fù)，或者是S2U導(dǎo)通或者是S3U導(dǎo)通。為了保證中性點(diǎn)電壓平衡，在o點(diǎn)被注入的平均電流應(yīng)該是零。</p><p><b

17、>　　2.2輸入端變流器</b></p><p>　　為通常使用12脈沖二極管整流器給直流環(huán)節(jié)電容器充電，在輸入端引入的諧波是很小的。若對(duì)輸入諧波有更高的要求，可以使用24脈沖二極管整流器作為輸入變流器。對(duì)于需要有再生能力的更高級(jí)應(yīng)用，可以用一個(gè)有源輸入變流器取代二極管整流器，這時(shí)輸入整流器與輸出逆變器為同一結(jié)構(gòu)。</p><p><b>　　2.3逆變器控制&

18、lt;/b></p><p>　　電機(jī)控制感應(yīng)電機(jī)的控制可以使用轉(zhuǎn)子磁場(chǎng)定向矢量控制器實(shí)現(xiàn)，通過(guò)使用PWM調(diào)制器完成了恒轉(zhuǎn)矩區(qū)和高速弱磁區(qū)的控制。圖2為間接矢量控制框圖圖中指令磁通甲ψr是速度的函數(shù)，反饋速度和前饋滑差控制信號(hào)川赫目加。對(duì)相加結(jié)果的頻率信號(hào)積分，然后產(chǎn)生單位矢量(cosθ e和sinOθe )，最后通過(guò)矢量旋轉(zhuǎn)器產(chǎn)生電壓V’角θ控制PWM調(diào)制器。</p><p>　　

19、PWM調(diào)制器 該調(diào)制器實(shí)際上是把空間矢量調(diào)制概念擴(kuò)展到三電平逆變器。其基本原理是三電平PWM調(diào)制器使用兩個(gè)參考波認(rèn)Ur1和Ur2，但只使用一個(gè)三角波。它以一種優(yōu)化方式確定每一次開(kāi)關(guān)時(shí)刻。</p><p>　　產(chǎn)生的諧波盡可能的小，使用盡可能低的開(kāi)關(guān)頻率以最小化開(kāi)關(guān)損耗;可將零序成分加到每一個(gè)參考波里以便最大化基波電壓。作為一個(gè)附加的自由度，參考波與三角波的相對(duì)位置可改變，這可以用于直流環(huán)節(jié)中點(diǎn)的電流平衡。&l

20、t;/p><p><b>　　3 測(cè)試結(jié)果</b></p><p>　　三個(gè)750kW/ 2. 3V三電平逆變器在成莊煤礦2. 7km.長(zhǎng)帶式輸送機(jī)驅(qū)動(dòng)系統(tǒng)成功安裝之后，對(duì)整個(gè)變頻傳動(dòng)系統(tǒng)(VFC)的性能進(jìn)行了測(cè)試，測(cè)試結(jié)果顯示出使用VFC控制系統(tǒng)的帶式輸送機(jī)的優(yōu)良特性。圖3為測(cè)試結(jié)果波形。由圖看出，曲線(xiàn)1顯示受控帶速，帶速呈S形曲線(xiàn)形狀，曲線(xiàn)2、3分別表示電流和扭矩，曲

21、線(xiàn)4顯示帶張力。從圖中可以發(fā)現(xiàn)，帶張力的波動(dòng)范圍很小，所有檢測(cè)結(jié)果顯示出帶式輸送機(jī)驅(qū)動(dòng)系統(tǒng)令人滿(mǎn)意的特性。</p><p><b>　　4結(jié)論</b></p><p>　　近年來(lái)輸送機(jī)驅(qū)動(dòng)控制技術(shù)的進(jìn)步已更為可靠，符合低成本效益和高效驅(qū)動(dòng)的驅(qū)動(dòng)系統(tǒng)為用戶(hù)提供了選擇。在這些選擇中，可變頻率控制(VFC)的方法顯現(xiàn)出在將來(lái)長(zhǎng)距離輸送中帶式輸送機(jī)扮演了重要的角色。使用高壓工

22、GBT的中點(diǎn)嵌位三電平逆變器本身可以提供電機(jī)終端所需的供電中高壓，使變頻控制的應(yīng)用更為簡(jiǎn)單。通過(guò)成莊煤礦2. 7km長(zhǎng)帶式輸送機(jī)中采用的中點(diǎn)嵌位三電平逆變器變頻調(diào)速(VFC)控制系統(tǒng)的測(cè)試結(jié)果表明，采用BV-IGBT的中點(diǎn)嵌位三電平逆變器以及使用轉(zhuǎn)子磁場(chǎng)矢量控制策略的感應(yīng)電機(jī)變頻傳動(dòng)，使帶式輸送機(jī)驅(qū)動(dòng)系統(tǒng)具有非常優(yōu)秀的性能，顯示出良好的應(yīng)用前景。</p><p><b>　　英文譯文</b>

23、</p><p>　　Belt Conveying Systems Development of driving system</p><p>　　Among the methods of material conveying emploved, belt conveyors play a very imporient part in the reliable carrying of ma

24、terial over long distances at competitive cost. Conveyor systems have become larger and more complex and drive systems have a l so been going through a process of evolution and will continue to do so. Nowadays, bigger be

25、lts require more power and have brought the need for larger individual drives as well as multiple drives such as 3 drives of 750 kW for one belt(this is the c</p><p>　　1 Analysis on conveyor drive technologi

26、es</p><p>　　1. 1 Direct drives</p><p>　　Full-voltage starters. With a full-voltage starter design, the conveyor</p><p>　　head shaft is direct-coupled to the motor through the gear d

27、rive. Direct</p><p>　　full-voltage starters are adequate for relatively low-power, simple- Profile conveyors. With direct full-voltage starters. no control is provided for various conveyor loads and. dependi

28、ng on the ratio between full- and no-load power requirements, empty starting times can be three or four times faster than full load. The maintenance-free starting system is simple, low-cost and very reliable. However, th

29、ey cannot control starting torque and maximum stall torque; therefore. they are limited to the </p><p>　　Reduced-voltage starters. As conveyor power requirements increase,controlling the applied motor torque

30、 during the acceleration period becomes increasingly important. Because motor torque is a function of voltage, motor voltage must be controlled. This can be achieved through reduced-voltage starters by employing a silico

31、n controlled rectifier (SCR). A common starting method with SCR reduced-voltage starters is to apply low voltage initially to take up conveyor belt slack. and then to apply a tim</p><p>　　Wound rotor inducti

32、on motors. Wound rotor induction motors are</p><p>　　connected directly to the drive system reducer and are a modified configuration of a standard AC induction motor. By inserting resistance in series with t

33、he motor's rotor windings. the modified motor control System controls motor torque. For conveyor starting, resistance is placed in series with the rotor for low initial torque. As the conveyor accelerates,the resista

34、nce is reduced slowly to maintain a constant acceleration torque. On multiple-drive systems. an external slip resistor may be left</p><p>　　DC motor. DC motors. available from a fraction of thousands of KW，a

35、re designed to deliver constant torque below base speed and constant KW above base speed to the maximum allowable revolutions per minute (r/min). with the majority of conveyor drives, a .DC shunt wound motor is used. Whe

36、rein the motor's rotating armature is connected externally. The most common technology for controlling DC drives is a SCR device. which allows for continual variable-speed operation. The DC drive system is mechani<

37、;/p><p>　　1. 2 Hydrokinetic coupling</p><p>　　Hydrokinetic couplings, commonly referred to as fluid couplings. are composed of three basic elements; the driven impeller, which acts as a</p>

38、<p>　　centrifugal pump; the driving hydraulic turbine known as the runner and</p><p>　　a casing that encloses the two power components. Hydraulic fluid is pumped from the driven impeller to the driving

39、runner, producing torque at the driven shaft. Because circulating hydraulic fluid produces the torque and speed, no mechanical connection is required between the driving and driver shafts.The power produced by this coupl

40、ing is based on the circulated fluid's amount and density and the torque in proportion to input speed. Because the pumping action within the fluid coupling depends o</p><p>　　Fixed-fill fluid couplings.

41、Fixed-fill fluid couplings are the most commonly used soft-start devices for conveyors with simpler belt profiles and limited convex/concave sections. They are relatively simple,low-cost,reliable,maintenance free devices

42、 that provide excellent soft starting results to the majority of belt conveyors in use today.</p><p>　　Variable-fill drain couplings. Drainable-fluid couplings work on the same principle as fixed-fill coupli

43、ngs. The coupling's impellers are mounted on the AC motor and the runners on the driven reducer high-speed shaft. Housing mounted to the drive base encloses the working circuit. The coupling's rotating casing con

44、tains bleed-off orifices that continually allow fluid to exit the working circuit into a separate hydraulic reservoir. Oil from the reservoir is pumped through a heat exchanger to a so</p><p>　　Hydrokinetic

45、scoop control drive. The scoop control fluid coupling consists of the three standard fluid coupling cmponents: a driven impeller, a driving runner and a casing that encloses the working circuit. The casing is fitted with

46、 fixed orifices that bleed a predetermined amount of fluid into a reservoir. When the scoop tube is fully extended into the reservoir, the coupling is 100 percent filled. The scoop tube, extending outside the fluid coupl

47、ing, is positioned using an electric actuator t</p><p>　　1. 3 Variable-frequency control(VFC)</p><p>　　Variable frequency control is also one of the direct drive methods. the emphasizing discuss

48、ion about it here is because that it has so unique characteristic and so good performance compared with other driving methods for belt conveyor. VFC devices Provide variable frequency and voltage to the induction motor,

49、resulting in an excellent starting torque and acceleration rate for belt conveyor drives. VFC drives. available from fractional to several thousand (kW)，are electronic controllers that rect</p><p>　　2 Neutra

50、l point clamped(NPC)three-level inverter using IGBTs</p><p>　　Three-level voltage-fed inverters have recently become more and more popular for higher power drive applications because of their easy voltage sh

51、aring features. lower dv/dt per switching for each of the devices, and superiorharmonic quality at the output. The availability of NV-IGBTs </p><p>　　has led to the design of a new range of medium-high volt

52、age inverter using three-level NPC topology. This kind of inverter can realize a whole range with a voltage rating from 2. 3 kV to 4. 1 6kV Series connection of IIV-IGBT modules is used in the 3. 3 kV and 4. 1 6kV device

53、s. The 2. 3 kV inverters need only one HV-IGBT per switch[2,3].</p><p>　　2. 1 Power section</p><p>　　To meet the demands for medium voltage applications. a three-level</p><p>　　neut

54、ral point clamped inverter realizes the power section. In comparison</p><p>　　to a two-level inverter. the NPC inverter offers the benefit that three voltage levels can be supplied to the output terminals, s

55、o for the same output current quality, only 1/4 of the switching frequency is necessary. Moreover the voltage ratings of the switches in NPC inverter topology will be reduced to 1/2. and the additional transient voltage

56、stress on the motor can also be reduced to 1/2 compared to that of a two-level inverter. </p><p>　　The switching states of a three-level inverter are summarized in Table 1. U. V and W denote each

57、of the three phases respectively; P N and 0 are the do bus points. The phase U, for example, is in state P (positive bus voltage)when the switches S1uand S2u are closed, whereas it is in state N (negative bus voltage) wh

58、en the switches S3u and S4u, are closed. At neutral point clamping, the phase is in 0 state when either S2u.or S3u, conducts depending on positive or negative phase current polarity, </p><p>　　2. 2 Line side

59、 converter</p><p>　　For standard applications. a 12-pulse diode rectifier feeds the divided DC-link capacitor. This topology introduces low harmonics on. the line side. For even higher requirements a 24-puls

60、e diode rectifier can be used as an input converter. For more advanced applications where regeneration. capability is necessary, an active front. end converter can replace the diode rectifier, using the same structure as

61、 the inverter.</p><p>　　2. 3 Inverter control</p><p>　　Motor Control. Motor control of induction machines is realized by</p><p>　　using a rotor flux. oriented vector controller.<

62、/p><p>　　Fig. 2 shows the block diagram of indirect vector controlled drive that incorporates both constant torque and high speed field-weakening regions where the PW M modulator was used. In this figure, the c

63、ommand fluxψ.is generated as function of speed. The feedback speed is added with the feed forward slip command signalψ,the resulting frequency signal is integrated and then the unit vector signals(cosθe and sinθ e)are ge

64、nerated. The vector rotator generates the voltage Vs and Angle θe commands for </p><p>　　PWM Modulator. The demanded voltage vector is generated using an elaborate PWM modulator. The modulator extends the co

65、ncepts of space-vector modulation to the three-level inverter. The operation can be</p><p>　　explained by starting from a regularly sampled sine-triangle comparison</p><p>　　from two-level inver

66、ter. Instead of using one set of reference waveforms</p><p>　　and one triangle defining the switching frequency, three-level Modulator uses two sets of reference waveforms Uand U and just one triangle. Thus,

67、 each switching transition is used in an optimal way so that several objectives are reached at the same time.</p><p>　　Very low harmonics are generated. The switching frequency is low and thus switching loss

68、es are minimized. As in a two-level inverter, a zero-sequence component can be added to each set of reference waveform s in order to maximize the fundamental voltage component. As an additional degree of freedom, the pos

69、ition of the reference waveform s within the triangle can be changed. This can be used for current balance in the two halves of the DC-link.</p><p>　　3 Testing results</p><p>　　After Successful

70、installation of three 750 kW /2. 3 kV three-level</p><p>　　inverters for one 2. 7 km long belt conveyor driving system in Cheng </p><p>　　zhuang Mine. The performance of the whole VFC system was

71、 tested. Fig. 3 is taken from the test, which shows the excellent characteristic of the belt conveyor driving system with VFC controller.</p><p>　　Fig. 3 includes four curves. The curve 1 shows the belt tens

72、ion . From the curve it can be find that the fluctuation range of the belt tension is very small. Curve 2 and curve 3 indicate current and torque separately. Curve 4 shows the velocity of the controlled belt. The belt ve

73、locity have the "s" shape characteristic. All the results of the test show a very satisfied characteristic for belt driving system.</p><p>　　4 Conclusions</p><p>　　Advances in conveyor

74、 drive control technology in recent years have</p><p>　　resulted in many more reliable. Cost-effective and performance-driven conveyor drive system choices for users.Among these choices,theVariable frequency

75、 control (VFC) method shows promising use in the future for long distance belt conveyor drives due to its excellent performances. The NPC three-level inverter using high voltage TGBTs make the Variable frequency control