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Among the methods of material conveying employed，belt conveyors play a very important part in the reliable carrying of material over long distances at competitive cost．Conveyor systems have become larger and more complex and drive systems have also been going through a process of evolution and will continue to do so．Nowadays，bigger belts 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 case for the conveyor drives in Chengzhuang Mine)．The ability to control drive acceleration torque is critical to belt conveyors’ performance．An efficient drive system should be able to provide smooth，soft starts while maintaining belt tensions within the specified safe limits．For load sharing on multiple drives．torque and speed control are also important considerations in the drive system’s design. Due to the advances in conveyor drive control technology，at present many more reliable． 1 Analysis on conveyor drive technologies 1．1 Direct drives Full-voltage starters.With a full-voltage starter design ， the conveyor head shaft is direct-coupled to the motor through the gear drive．Direct full-voltage starters are adequate for relatively low-power, simple-profile conveyors．With direct fu11-voltage starters．no control is provided for various conveyor loads and．depending on the ratio between fu11- and no-1oad 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, they cannot control starting torque and maximum stall torque；therefore．they are limited to the low-power, simple-profile conveyor belt drives． Reduced-voltage starters．As conveyor power requirements increase，controlling the applied motor torque during the acceleration period becomes increasingly important．Because motor torque 1s a function of voltage，motor voltage must be controlled．This can be achieved through reduced-voltage starters by employing a silicon 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 timed linear ramp up to full voltage and belt speed．However, this starting method will not produce constant conveyor belt acceleration．When acceleration is complete．the SCRs, which control the applied voltage to the electric motor． are locked in full conduction, providing fu11-line voltage to the motor．Motors with higher torque and pull—up torque，can provide better starting torque when combined with the SCR starters, which are available in sizes up to 750 KW． Wound rotor induction motors．Wound rotor induction motors are connected directly to the drive system reducer and are a modified configuration of a standard AC induction motor．By inserting resistance in series with the 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 resistance is reduced slowly to maintain a constant acceleration torque．On multiple-drive systems．an external slip resistor may be left in series with the rotor windings to aid in load sharing．The motor systems have a relatively simple design．However, 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 systems are custom designed to meet a conveyor system’s particular specifications．Wound rotor motors are appropriate for systems requiring more than 400 kW ． DC motor．DC motors．available from a fraction of thousands of kW ，are 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．Wherein 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 mechanically simple, but can include complex custom-designed electronics to monitor and control the complete system．This system option is expensive in comparison to other soft-start systems．but it is a reliable, cost-effective drive in applications in which torque,1oad sharing and variable speed are primary considerations．DC motors generally are used with higher-power conveyors，including complex profile conveyors with multiple-drive systems，booster tripper systems needing belt tension control and conveyors requiring a wide variable-speed range． 1．2 Hydrokinetic coupling Hydrokinetic 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．Hydraulic 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 driven shafts．The power produced by this coupling 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 on centrifugal forces．the output speed is less than the input speed．Referred to as slip．this normally is between l% and 3%．Basic hydrokinetic couplings are available in configurations from fractional to several thousand kW. Fixed-fill fluid couplings．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,1ow-cost,reliable,maintenance free devices that provide excellent soft starting results to the majority of belt conveyors in use today． Variable-fill drain couplings．Drainable-fluid couplings work on the same principle as fixed-fill couplings．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 contains 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 solenoid-operated 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 control valve．Variable fill drain couplings are 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 system can be electronically intricate．The drive system cost is medium to high, depending upon size specified． Hydrokinetic scoop control drive．The scoop control fluid coupling consists of the three standard fluid coupling components：a driven impeller, a driving runner and a casing that encloses the working circuit．The casing is fitted with 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 l00 percent filled．The scoop tube, extending outside the fluid coupling，is positioned using an electric actuator to engage the tube from the fully retracted to the fully engaged position．This control provides reasonably smooth acceleration rates．to but the computer-based control system is very complex．Scoop control couplings are applied on conveyors requiring single or multiple drives from l50 kW to 750 kW. 1．3 Variable-frequency control(VFC) Variable frequency control is also one of the direct drive methods．The emphasizing discussion 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, 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 rectify AC line power to DC and，through an inverter, convert DC back to AC with frequency and voltage contro1．VFC drives adopt vector control or direct torque control(DTC)technology，and can adopt different operating speeds according to different loads．VFC drives can make starting or stalling according to any given S-curves．realizing the automatic track for starting or stalling curves．VFC drives provide excellent speed and torque control for starting conveyor belts．and can also be designed to provide load sharing for multiple drives．easily VFC controllers are frequently installed on lower-powered conveyor drives，but when used at the range of medium-high voltage in the past．the structure of VFC controllers becomes very complicated due to the limitation of voltage rating of power semiconductor devices，the combination of medium-high voltage drives and variable speed is often solved with low-voltage inverters using step-up transformer at the output， or with multiple low-voltage inverters connected in series．Three-level voltage-fed PWM converter systems are recently showing increasing popularity for multi-megawatt industrial drive applications because of easy voltage sharing between the series devices and improved harmonic quality at the output compared to two-level converter systems With simple series connection of devices．This kind of VFC system with three 750 kW /2．3kV inverters has been successfully installed 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 point clamped(NPC)three-level inverter using IGBTs Three-level voltage-fed inverters have recently become more and more popular for higher power drive applications because of their easy voltage sharing features．1ower dv/dt per switching for each of the devices，and superior harmonic quality at the output．The availability of HV-IGBTs has led to the design of a new range of medium-high voltage 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 6 kV Series connection of HV-IGBT modules is used in the 3．3 kV and 4．1 6 kV devices．The 2．3 kV inverters need only one HV-IGBT per switch. 2．1 Power section To meet the demands for medium voltage applications．a three-level neutral point clamped inverter 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 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． The switching states of a three-level inverter are summarized in Table 1．U．V and W denote each of the three phases respectively；P N and O are the dc bus points．The phase U，for example，is in state P(positive bus voltage)when the switches S1u and S2u are closed，whereas it is in state N (negative bus voltage) when the switches S3u and S4u are closed．At neutral point clamping，the phase is in O 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 O should be zero． 2．2 Line side converter For standard applications ． a l2-pulse 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 capability is necessary, an active front．end converter can replace the diode rectifier, using the same structure as the inverter． 2．3 Inverter control Motor Contro1 ． Motor control of induction machines is realized by using a rotor flux．oriented vector controller． 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 command flux is generated as function of speed．The feedback speed is added with the feed forward slip command signal . the resulting frequency signal is integrated and then the unit vector signals(cos and sin )are generated．The vector rotator generates the voltage and angle commands for the PW M as shown． PWM Modulator．The demanded voltage vector is generated using an elaborate PWM modulator．The modulator extends the concepts of space-vector modulation to the three-level inverter．The operation 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， the three-level modulator uses two sets of reference waveforms Ur1 and Ur2 and just one triangle．Thus, each switching transition is used in an optimal way so that several objectives are reached at the same time． Very low harmonics are generated．The switching frequency is low and thus switching losses 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 position 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-1ink． 3 Testing results After Successful installation of three 750 kW /2．3 kV three-level inverters for one 2．7 km long belt conveyor driving system in Chengzhuang Mine．The performance of the whole VFC system was tested．Fig．3 is taken from the test，which shows the excellent characteristic of the belt conveyor driving system with VFC controller． Fig．3 includes four curves．The curve 1 shows the belt tension．From the curve it can be find that the fluctuation range of the belt tension is very smal1．Curve 2 and curve 3 indicate current and torque separately．Curve 4 shows the velocity of the controlled belt．The belt velocity have the“s”shape characteristic．A1l the results of the test show a very satisfied characteristic for belt driving system． 4 Conclusions Advances in conveyor drive control technology in recent years have resulted in many more reliable．Cost-effective and performance-driven conveyor drive system choices for users．Among these choices，the Variable frequency 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 IGBTs make the Variable frequency control in medium voltage applications become much more simple because the inverter itself can provide the medium voltage needed at the motor terminals，thus eliminating the step-up transformer in most applications in the past． The testing results taken from the VFC control system with NPC three．1evel inverters used in a 2．7 km long belt conveyor drives in Chengzhuang Mine indicates that the performance of NPC three-level inverter using HV-IGBTs together with the control strategy of rotor field-oriented vector control for induction motor drive is excellent for belt conveyor driving system． http://www.bisheziliao.com/ 在運送大量的物料時，帶式輸送機在長距離的運輸中起到了非常重要的競爭作用。輸送系統(tǒng)將會變得更大、更復雜，而驅動系統(tǒng)也已經(jīng)歷了一個演變過程， 并將繼續(xù)這樣下去。如今，較大的輸送帶和多驅動系統(tǒng)需要更大的功率，比如 3 驅動系統(tǒng)需要給輸送帶 750KW (成莊煤礦輸送機驅動系統(tǒng)的要求)?？刂乞寗恿图铀俣扰ぞ厥禽斔蜋C的關鍵。一個高效的驅動系統(tǒng)應該能順利的運行，同時保持輸送帶張緊力在指定的安全極限負荷內。為了負載分配在多個驅動上，扭矩和速度控制在驅動系統(tǒng)的設計中也是很重要的因素。 1 帶式輸送機驅動 1.1 帶式輸送機驅動方式 全電壓啟動 在全電壓啟動設計中，帶式輸送機驅動軸通過齒輪傳動直接連接到電機。直接全壓驅動沒有為變化的傳送負載提供任何控制，根據(jù)滿載和空載功率需求的比率，空載啟動時比滿載可能快 3～4 倍。此種方式的優(yōu)點是：免維護，啟動系統(tǒng)簡單，低成本，可靠性高。但是，不能控制啟動扭矩和最大停止扭矩。因此，這種方式只用于低功率，結構簡單的傳送驅動中。 降壓啟動 隨著傳送驅動功率的增加,在加速期間控制使用的電機扭矩變得越來越重要。由于電機扭矩是電壓的函數(shù)，電機電壓必須得到控制，一般用可控硅整流器(SCR) 構成的降壓啟動裝置，先施加低電壓拉緊輸送帶,然后線性的增加供電電壓直到全電壓和最大帶速。但是，這種啟動方式不會產生穩(wěn)定的加速度， 當加速完成時，控制電機電壓的 SCR 鎖定在全導通，為電機提供全壓。此種控制方式功率可達到 750kW。 繞線轉子感應電機 繞線轉子感應電機直接連接到驅動系統(tǒng)減速機上，通過在電機轉子繞組中串聯(lián)電阻控制電機轉矩。在傳送裝置啟動時，把電阻串聯(lián)進轉子產生較低的轉矩，當傳送帶加速時，電阻逐漸減少保持穩(wěn)定增加轉矩。在多驅動系統(tǒng)中，一個外加的滑差電阻可能將總是串聯(lián)在轉子繞組回路中以幫助均分負載。該方式的電機系統(tǒng)設計相對簡單，但控制系統(tǒng)可能很復雜，因為它們是基于計算機控制的電阻切換。當今，控制系統(tǒng)的大多數(shù)是定制設計來滿足傳送系統(tǒng)的特殊規(guī)格。繞線轉子電機適合于需要 400kW 以上的系統(tǒng)。 直流(DC)電機 大多數(shù)傳送驅動使用DC 并勵電機，電機的電樞在外部連接?？刂?DC 驅動技術一般應用 SCR 裝置，它允許連續(xù)的變速操作。DC 驅動系統(tǒng)在機械上是簡單的,但設計的電子電路，監(jiān)測和控制整個系統(tǒng)，相比于其他軟啟動系統(tǒng)的選擇是昂貴的，但在轉矩、負載均分和變速為主要考慮的場合,它又是一個可靠的，節(jié)約成本的方式。DC 電機一般使用在功率較大的輸送裝置上,包括需要輸送帶張力控制的多驅動系統(tǒng)和需要寬變速范圍的輸送裝置上。 1.2 液力偶合器 流體動力偶合器通常被稱為液力偶合器，由三個基本單元組成：充當離心泵的葉輪，推進水壓的渦輪和裝進兩個動力部件的外殼。流體從葉輪到渦輪，在從動軸產生扭矩。由于循環(huán)流體產生扭矩和速度，在驅動軸和從動軸之間不需要任何機械連接。這種連接產生的動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構較簡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裝置，其結構相對比較簡單，成本又低， 對現(xiàn)在使用的大多數(shù)輸送機能提供優(yōu)良的軟啟動效果。 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 電機上,渦輪裝在從動減速器高速軸上，包含操作部件的軸箱安裝在驅動基座。偶合器的旋轉外殼有溢出口，允許液體不斷地從工作腔中流出進入一個分離的輔助腔，油從輔助腔通過一個熱交換器泵到控制偶合器充液量的電磁閥。為了控制單機傳動系統(tǒng)的啟動轉矩,必須監(jiān)測 AC 電機電流,給電磁閥的控制提供反饋?？勺兂湟阂毫ε己掀骺墒褂迷谥写蠊β瘦斔拖到y(tǒng)中，功率可達到數(shù)千千瓦。這種驅動無論在機械,或在電氣上都是很復雜的，其驅動系統(tǒng)成本中等。 勺管控制液力偶合器 也稱為調速型液力偶合器。此種液力偶合器同樣由三個標準的液力偶合單元構成，即葉輪、渦輪和一個包含工作環(huán)路的外殼。此種液力偶合器需要在工作腔以外設置導管(也稱勺管) 和導管腔，依靠調節(jié)裝置改變勺管開度(勺管頂端與旋轉外殼間距) 人為的改變工作腔的充液量，從而實現(xiàn)對輸出轉速的調節(jié)。這種控制提供了合理的平滑加速度，但其計算機控制系統(tǒng)很復雜。勺管控制液力偶合器可以應用在單機或多機驅動系統(tǒng)，功率范圍為150kW～ 750kW。 1.3 變頻控制(VFC) 變頻控制也是一種直接驅動方式，它具有非常獨特的高性能。VFC 裝置為感應電機提供變化的頻率和電壓，產生優(yōu)良的啟動轉矩和加速度。VFC 設備是一個電力電子控制器，首先把 AC 整流成 DC ，然后利用逆變器，再將 DC 轉換成頻率、電壓可控的 AC。VFC 驅動采用矢量控制或直接轉矩控制(DTC) 技術，能根據(jù)不同的負載采用不同的運行速度。VFC 驅動能根據(jù)給定的 S 曲線啟動或停車，實現(xiàn)自動跟蹤啟動或停車曲線。VFC 驅動為傳送帶啟動提供了優(yōu)良的速度和轉矩控制，也能為多機驅動系統(tǒng)提供負載均分。VFC 控制器可以容易地裝在小功率輸送機驅動上。過去在中高電壓使用時，VFC 設備的結構由于受電力半導體器件的電壓額定值限制而變得很復雜，中高電壓的變速傳動常常使用低壓逆變器，然后在輸出端使用升壓變壓器，或使用多個低壓逆變器串聯(lián)來解決。與簡單的器件串聯(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 PWM 逆 變器系統(tǒng)在數(shù)兆瓦工業(yè)傳動中近年來獲得了越來越多的應用。由三臺 750kW/ 2. 3kV 的這種逆變器構成的 VFC 系統(tǒng)已經(jīng)成功安裝在成莊煤礦長 2. 7km 的帶式輸送機驅動系統(tǒng)中。 2 使用 IGBT 的中性點箝位三電平逆變器 由于串聯(lián)器件電壓均分容易,器件每次開關的 d v/ d t 低以及輸出端出色的諧波品質，三電平電壓型逆變器在大功率傳動應用中變得越來越流行。高壓IGBT(HV-IGBT) 的出現(xiàn)使得應用三電平中性點箝位原理的中高壓逆變器設計有了更大的應用范圍。這種逆變器目前可以實現(xiàn)從2. 3kV 到4. 16kV 全范圍的應用。HV-IGBT 模塊串聯(lián)可使用在 3. 3kV 和 4. 16kV 的設備。2. 3kV 逆變器每個開關只需要一個 HV-IGBT[2,3]。 2．1 主功率逆變電路 主功率逆變電路用三電平中點箝位電壓型逆變器實現(xiàn),可以滿足中高壓交流傳動應用的需要。與兩電平電壓型逆變器相比，三電平中點箝位電壓型逆變器提供三個電壓級別給輸出端，對于同樣的輸出電流品質，開關頻率可降低到原來的1/ 4，開關器件的電壓額定值可減小到原來的 1/ 2 ，附加到電機上的額外的瞬態(tài)電壓應力也可能減少到原來的 1/ 2 。 三電平中點箝位電壓型逆變器的開關狀態(tài)可歸納于表 1 ，U ，V 和 W 分別表示三相， P，N 和 O 是直流母線上的三個點。例如，當開關 S1U 和 S2U 閉合時，U 相處于狀態(tài) P(正母線電壓) ，反之，當開關 S3U 和 S4U 閉合時，U 相處于狀態(tài) N (負母線電壓) 。在中性點箝位時，該相在 O 狀態(tài)，這時根據(jù)相電流極性的正負,或者是 S2U 導通或者是 S3U 導通。為了保證中性點電壓平衡，在 O 點被注入的平均電流應該是零。 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。對于需要有再生能力的更高級應用，可以用一個有源輸入變流器取代二極管整流器，這時輸入整流器與輸出逆變器為同一結構。 2.3 逆變器控制 電機控制 感應電機的控制可以使用轉子磁場定向矢量控制器實現(xiàn),通過使用 PWM 調制器完成了恒轉矩區(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 Ψr 是速度的函數(shù),反饋速度和前饋滑差控制信號 ωsl 相加。對相加結果的頻率信號積分，然后產生單位矢量(cosθe 和 sinθe ) ，最后通過矢量旋轉器產生電壓角控制 PWM 調制器。 PWM 調制器 該調制器實際上是把空間矢量調制概念擴展到三電平逆變器。其基本原理是三電平 PWM 調制器使用兩個參考波 Ur1 和 Ur2，但只使用一個三角波。它以一種優(yōu)化方式確定每一次開關時刻。 產生的諧波盡可能的小，使用盡可能低的開關頻率以最小化開關損耗；可將零序成分加到每一個參考波里以便最大化基波電壓。作為一個附加的自由度,參考波與三角波的相對位置可改變，這可以用于直流環(huán)節(jié)中點的電流平衡。 3 測試結果 三個 750kW/ 2. 3kV 三電平逆變器在成莊煤礦 2. 7km 長帶式輸送機驅動系統(tǒng)成功安裝之后,對整個變頻傳動系統(tǒng)(VFC) 的性能進行了測試,測試結果顯示出使用 VFC 控制系統(tǒng)的帶式輸送機的優(yōu)良特性。圖 3 為測試結果波形。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),帶張力的波動范圍很小，所有檢測結果顯示出帶式輸送機驅動系統(tǒng)令人滿意的特性。 4 結論 近年來輸送機驅動控制技術的進步已更為可靠，符合低成本效益和高效驅動的驅動系統(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)出在將來長距離輸送中帶式輸送機扮演了重要的角色。使用高壓 IGBT 的中點嵌位三電平逆變器本身可以提供電機終端所需的供電中高壓,使變頻控制的應用更為簡單。通過成莊煤礦 2. 7km 長帶式輸送機中采用的中點嵌位三電平逆變器變頻調速(VFC)控制系統(tǒng)的測試結果表明,采用 HV-IGBT 的中點嵌位三電平逆變器以及使用轉子磁場矢量控制策略的感應電機變頻傳動,使帶式輸送機驅動系統(tǒng)具有非常優(yōu)秀的性能,顯示出良好的應用前景。