外文翻譯---帶式輸送機(jī)及其牽引系統(tǒng)

1、外文翻譯一 其牽引系統(tǒng)帶式輸送機(jī)及帶式輸送機(jī)及其牽引系統(tǒng)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)系統(tǒng)的要求）o控制驅(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)

2、的驅(qū)動(dòng)系統(tǒng)可供顧客選擇11帶式輸送機(jī)驅(qū)動(dòng)仁1帶式輸送機(jī)驅(qū)動(dòng)方式全電壓?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)滿載和空載功率需求的比率，空載啟動(dòng)時(shí)比滿載可能快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。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

3、帶，然后線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750kWo繞線轉(zhuǎn)子感應(yīng)電機(jī) 繞線轉(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)

4、計(jì)來(lái)滿足傳送系統(tǒng)的特殊規(guī)格繞線轉(zhuǎn) 子電機(jī)適合于需要400kW以上的系統(tǒng)。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，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。12液力偶合器流體動(dòng)力偶合器通常被稱為液力偶合器，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

5、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)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，其間的差值稱為滑差，一般為1%-3%。傳遞功率可達(dá)幾千千 瓦。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。可變充液液力偶合器也稱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

6、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?？勺兂湟阂毫ε己掀骺墒褂迷谥写?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)雜的，其驅(qū)動(dòng)系統(tǒng)成本中等。勺管控制液力偶合器也稱為調(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

7、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-750kWo1. 3變頻控制(VFC)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

8、啟動(dòng)或停車，實(shí)現(xiàn)自動(dòng)跟蹤啟動(dòng)或停車曲線。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)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VF

9、C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。2使用IGBT的中性點(diǎn)箱位三電平逆變器由于串聯(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 髙壓逆變器設(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-IGBTE2, 3 »2.1主功率逆變電路主功率逆變電路用三電平中點(diǎn)箱位

10、電壓型逆變器實(shí)現(xiàn)，可以滿足中 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l/2o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是直流母線上的三個(gè)點(diǎn)。例如，當(dāng)開(kāi)關(guān)Siu和 S2u閉合時(shí)，U相處于狀態(tài)P （正母線電壓），反乙 當(dāng)開(kāi)關(guān)S3u和S4U閉 合時(shí)，U相處于狀態(tài)n （負(fù)母線電壓）。在中性點(diǎn)箱位時(shí)，該相在。狀態(tài), 這時(shí)根據(jù)相電流極性的正負(fù)，

11、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。點(diǎn)被注入的平均電流應(yīng)該是零。2. 2輸入端變流器為通常使用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)。2. 3逆變器控制電機(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是速度的函數(shù)，反饋速度和前饋滑差 控制信號(hào)川赫目

12、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（cos 0 e和sinO Oe ）,最后通過(guò)矢量旋轉(zhuǎn)器產(chǎn)生電壓V角0控制PWM調(diào) 制器。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)Url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。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。3測(cè)試結(jié)果三個(gè)750kW/23V三電平逆變器在成莊煤礦27km.長(zhǎng)帶式

13、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1顯示受控帶速，帶速呈S形曲線形狀，曲 線2、3分別表示電流和扭矩，曲線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 意的特性。4結(jié)論近年來(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GBT的中點(diǎn)嵌位三電平逆變器本身可以提供

14、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27km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，釆用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。英文譯文Belt Conveying Systems Development of drivingsystemAmong the methods of material conveying employed, belt conveyors play a very imporient p

15、art in the reliable carrying of ma/terial over long distances at competitive cost Conveyor systems have become larger and more complex and drive systems have a 1 so been going through a process of evolution and will continue to do so. Nowadays, bigger belts require more power and have brought the ne

16、ed for larger individual drives as well as multi pie drives such as 3 drives of 750 kW for one belt(this is the case for the conveyor drives in Chengzhuang Mine) The ability to control drive acceleration torque is critical to beltconveyors' performance. A efficierrt drive system should be able t

17、o provide smooth, soft starts while maintaining belt tensions within the specified safe limits. For load sharing on multipie drives torque and speed control are also considerations in the drive system's design. Due to the advances in conveyor drive control technology, at present many more reliab

18、leCost-effective and performance- driven conveyor drive systems covering a wide range of power are available for customers" choices11 Analysis on conveyor drive technologies1. Direct drivesFull-voltage full-voltage conveyorstarters<With astarter design, thehead shaft is direct-coupled to the

19、 motor through the gear drive Direct full-volt age start ers are adequa te for relatively low-power, simple- Profile conveyors With direct full-voltage starters no control is provided for various conveyor loads and. depending on the ratio bet ween full- and no-load power requirements,empty startingt

20、i mes can be t hree or four ti mes fas ter than full load. The maintenance-free starting system is simple, low-cost and very reliable However, they cannot control starting torque and maximum stall torque; therefore they are limited to the low-power, simple-profile conveyor belt drivesReduced-vo I ta

21、ge starters As conveyor power requirements increase,controlling the applied motor torque during the acceleration period becomes increasingly important. Because motor to rque is a func tion of volt age, motor volt age must be cont rolled This can be achieved through reduced-voltagestarters by employi

22、ng a siliconcontrolled rectifier(SCR). Acommonstartingmethodwith SCRreduced-voltage starters is to apply low voltage initially to take up conveyor belt slack and then to apply a timed linear ramp up to full voltage and belt speed However, this starting method will not produce constant conveyor bel t

23、 acceleration. When acceleration is complete the SCRs, which con trol the applied volt age to the electrie motor, are locked in full12conduction,providing full-linevolt age to the motor. Motors with higher torque and pull -vp torque, can provide better starting torque when combined with the SCR star

24、ters, which are available in sizes up to 750 KW.Wound rotor i nduct i on motors. Wound rotor induetion motors are connected directly to the drive system reducer and are a modified configuration of a standard AC induetion motor. By inserting resistance in series with the motor's rotor windings th

25、e modified motor control System controls motor torque For conveyor starting, resistance is placed in series with the rotor for low initial torqueAs the conveyoraccelerates, the resistance is reduced iislowly to main tain a cons taiit accelera tion to rque On multi pie-drive sys tems. an ext ernal sl

26、ip resis tor maybe lef t in series with the rotor windings to aid in load sharing the motor systems have a relatively simple a designHowever, the control systems for these can be highly complex, because they are based on computer control of the resistance switching Today, the majority of control sys

27、tems are custom designed to meet a conveyor system's particular specifications. Wound rotor motors are appropriate for systems requiring more than 400KW.DC motor. DC motors, available from a fraction of thousands of KW , are designed to deliver constant torque below base speed and constant KW ab

28、ovebase speed to the maximum allowable revolutions per minute (r/min) with the majority of conveyor drives, a DC shunt wound motor is used. Wherein themotor's rotating armature is connected for cont rolling DC drives is a SCR device which allows for continual variable-speed operation. The DC dri

29、ve system is mechanically simple, but can include complex custom-designed electronics to monitor and control the complete system. this system option is expensive in comparison to ot her soft-start systems. but it is a reliable, cost-effective drive in applications in which torque, load sharing and v

30、ariable speed are primary considerations DC motors generally are used withext email y> The most coiHillon technologyhigher-power conveyors, including complex profile conveyors with multipie-drivesystems, boostertripper systems needing belt tension control and conveyors requiring a wide variable-s

31、peed range1. 2 Hydrokinetic coup Ii ngHydrokinetic couplings, commonly referred to as fluid couplings are composed of three basic elements; the driven impeller, which acts as a centrifugal pump; the driving hydraulic turbine known as the runner and a casing that encloses the two power components Hyd

32、raulic fluid is pumped from the driven impeller to the driving 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 coupling is based on t

33、he circulated fluid's amount and density and the torque in proportion to input speed Because the pumping action within the fluid coupling depends on centrifugal forces the output speed is less than the input speed Referred to as slip t his normally is bet ween 1% and 3%. Basic hydrokinetic coupl

34、ings are available in configurations from fractional to several thousand KWF i xed-f i I I fluid coup I i ngs. Fixed 一 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

35、, reliable, maintenance free devices that provide excellent soft starting resuIts to the majority of belt conveyors in use today.Var iabIe-f i11draincoup IingsDrainable-fluid couplings work on the same principle as fixed-fill couplings The coupling's impellers are mounted on the AC motor and the

36、 runners on the driven reducer high-speed shaft Housing mounted to the drive base encloses the working circuit. The coupling, s rotating casing contains bleed-off orifices that continually allow fluid to exit the working circuit into a separate hydraulic reservoir Oil from the reservoir is pumped th

37、rough a hea t exchanger to a solenoid-opera ted hydraulic valve that controls the filling of the fluid coupling To control the starting torque of a single-drive conveyor system, the AC motor current must be monitored to provide feedback to the solenoid con trol valve Variable fill drain couplings ar

38、e used in medium to high-kw conveyor systems and are available in sizes up to thousands of kw The drives can be mechanically complex and depending on the control parameters the sys tem can be elec tronically intricate The drive system cost is medium to high, depending upon size specifiedHydrokinetic

39、 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 fixed orifices that bleed a predetennined amount of fluid into a reservoir When

40、 the scoop tube is fully extended into the reservoir, the coupling is 100 percent filled The scoop t ube, extending outside the fluid coupling, is positioned using an electrie actuator to engage the tube from the fully retracted to the fully engaged position. This control provides reasonably smooth

41、acceleration rates, to but the computer-based control system is very complex. Scoop control couplings are applied on conveyors requiring single or multipie drives from 150KWto 750KW1. 3 Var iabIe-frequency control (VFC)Variable frequency control is also one of the direc t drive met hods the emphasiz

42、ing discussion about it here is b ecausethat it has so uniquecharacteristic and so good performance compared with ot her driving met hods for belt conveyor VFC devices Provide variable frequency and voltage to the induetion motor,resulting in anexcellent starting torque and acceleration rate for bel

43、t conveyor drives VFC drives available from fractional to several thousand (kW), are electronic controllers that rectify AC line power to DC and, through an inver ter, conver t DC back to AC with frequency and voltage control. VFC drives adopt vector control or direct torque control (DTC) technology

44、, and can adop t different operating speeds according to different loads VFC drives can make starting or st ailing according to any given S-curves realizing the automatic track for starting or st ailing curves VFC drives provide excellent speed and torque control for starting conveyor belts, and can

45、 also be designed to provide load sharing for multi pie drives, easily VFC con trollers are frequen tly ins tailed on lower-powered convey- or drives, but when used at the range of medium-high voltage in the past the strueture of VFC controIlers becomes very complicated due to the limitation of volt

46、age rating of power semiconductor devices, the combination of medium-high voltage drives and variable speed is often solved w ith low-volt age inver ters using step-up transformer at the output, or with multipie low-voltage inverters connected in series Three-level voltage-fed PWM converter systems

47、are recently showing increasing popularity for multi-megawatt industrial drive applications because of easy voltage sharing between the series devices and i 叩 roved harmonic quality at the output compared to two-level converter systems With simple series connection of devices This kind of VFC system

48、 with three 750 kW /2 AV inverters has been successfully instailed in ChengZhuang Mine for one 2 7-km long belt conveyor driving system in following the principle of three-level inverter will be discussed in detail.2 Neutral po i nt cI amped(NPC) three-1 eve I inverter using IGBTsThree-level voltage

49、-fed inverters have recently become more and more popular for higher power drive applications because of their easy voltage sharing features lower dv/dt per switching for each of the devices, and superiorharmonic quality at the output. The availability of NV-IGBTs has led to the design of a new rang

50、e of medium-high voltage inverter using three-level NPC topology This kind of inver ter can realize a whole range with a volt age 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 devices The 2 3 kV inverters need onlyone HV-IGBT per switch2, 32.

51、1 Power sect i onTo meet the demands for medium voltage applications. a three一level neu tral poin t clamped in ver ter realizes the power section. In comparison to a two-level inverter. the NPC inverter offers the benefit that three voltage levels can be supplied to the output terminals, so for the

52、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 stress on the motor can also be reduced to 1/2 compared to that of a two-level inverter.Thes

53、witchingstates of a three-level inverter are summarized in Table 1 U V and W denote each 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 Sluand S2u are closed, whereas it is in state N (negative bus vol

54、tage) when 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, respectively For neutral point voltage balancing, the average current injected at 0 should be zer o2. 2 Line s

55、ide converterFor standardapplicationsa12pulse diode rectifier feeds the divided DC-link capacitor. This topology introduces low harmonics on. the line side For even higher requirements a 24-pulse diode rectifier can be used as an input converter. For more advanced applications where regeneration. ca

56、pability is necessary, an active front end converter can replace the diode rectifier, using the same strueture as the inverter.2. 3 Inverter controlMotor Control Motor control of induetion machines is realized by using a :rotor flux oriented vector controllerFig 2 shows the block diagram of indirect

57、 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 command flux ® is generated as function of speed The feedback speed is added with the feed forward slip command signal 2 , the resulting f

58、requency signal is integrated and then the unit vector signals (cos 0 e and sin 0 e)are genera ted. The vec tor rotator genera tes the voltage Vs and Angle 0 e commands for the PW M as shown.PWM Modulator. The demanded volt age vector is generated using an elaborate PWM modulator. The modulator exte

59、nds the concepts of space-vector modulation to the t hree-level inver ter.Theoperation can be explained by starting from a regularly sampled sine-triangle comparison from two -level inverter. Instead of using one set of reference waveforms and one triangle defining the switching frequency, three-level Modulator uses two set