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Belt Conveying Systems Development of driving system
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.Cost-effective and performance-driven conveyor drive systems covering a wide range of power are available for customers’ choices[1]. 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 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. conveyor power requirements increase, As 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 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. motors generally are used with higher-power conveyors, DC 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. coupling’s rotating casing contains The


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: driven impeller, a driving runner a 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 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. Variablecontrol(VFC) 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. clamped(NPC)three2 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[2,3]. 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. converter 2.2 Line side converter For standard applications. l2-pulse diode rectifier feeds the divided DC-link a 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 and sin )are generated.The vector rotator generates the voltage

signals(cos 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.


中文译文: 中文译文: 带式输送机及其牵引系统

在运送大量的物料时,带式输送机在长距离的运输中起到了非常重要的竞争作用。 输送系统将会变得更大、更复杂,而驱动系统也已经历了一个演变过程,并将继续这样下 去。如今,较大的输送带和多驱动系统需要更大的功率,比如 3 驱动系统需要给输送带 750KW (成庄煤矿输送机驱动系统的要求)。控制驱动力和加速度扭矩是输送机的关键。一 个高效的驱动系统应该能顺利的运行,同时保持输送带张紧力在指定的安全极限负荷内。 为 了负载分配在多个驱动上,扭矩和速度控制在驱动系统的设计中也是很重要的因素。由于 输送机驱动系统控制技术的进步,目前更多可靠的低成本和高效驱动的驱动系统可供顾客 选择[1]。 1 带式输送机驱动 1.1 带式输送机驱动方式 . 全电压启动 在全电压启动设计中,带式输送机驱动轴通过齿轮传动直接连接到电

机。直接全压驱动没有为变化的传送负载提供任何控制,根据满载和空载功率需求的比率, 空载启动时比满载可能快3~4倍。此种方式的优点是:免维护,启动系统简单,低成本,可 靠性高。但是,不能控制启动扭矩和最大停止扭矩。因此,这种方式只用于低功率,结构简 单的传送驱动中。 降压启动 随着传送驱动功率的增加,在加速期间控制使用的电机扭矩变得越来越

重要。由于电机扭矩是电压的函数,电机电压必须得到控制,一般用可控硅整流器(SCR) 构 成的降压启动装置, 先施加低电压拉紧输送带,然后线性的增加供电电压直到全电压和最大 带速。 但是, 这种启动方式不会产生稳定的加速度, 当加速完成时,控制电机电压的SCR 锁 定在全导通,为电机提供全压。此种控制方式功率可达到750kW。 绕线转子感应电机 绕线转子感应电机直接连接到驱动系统减速机上,通过在电机

转子绕组中串联电阻控制电机转矩。在传送装置启动时,把电阻串联进转子产生较低的转 矩,当传送带加速时,电阻逐渐减少保持稳定增加转矩。在多驱动系统中,一个外加的滑 差电阻可能将总是串联在转子绕组回路中以帮助均分负载。该方式的电机系统设计相对简 单,但控制系统可能很复杂,因为它们是基于计算机控制的电阻切换。当今,控制系统的 大多数是定制设计来满足传送系统的特殊规格。绕线转子电机适合于需要400kW以上的系 统。

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大多数传送驱动使用DC 并励电机,电机的电枢在外部连接。控制

DC 驱动技术一般应用SCR装置,它允许连续的变速操作。DC 驱动系统在机械上是简单 的,但设计的电子电路,监测和控制整个系统,相比于其他软启动系统的选择是昂贵的,但 在转矩、负载均分和变速为主要考虑的场合,它又是一个可靠的,节约成本的方式。DC 电 机一般使用在功率较大的输送装置上,包括需要输送带张力控制的多驱动系统和需要宽变 速范围的输送装置上。 1.2 液力偶合器 . 流体动力偶合器通常被称为液力偶合器,由三个基本单元组成:充当离心泵的叶轮, 推进水压的涡轮和装进两个动力部件的外壳。流体从叶轮到涡轮,在从动轴产生扭矩。由 于循环流体产生扭矩和速度,在驱动轴和从动轴之间不需要任何机械连接。这种连接产生 的动力决定于液力偶合器的充液量,扭矩正比于输入速度。因在流体偶合中输出速度小于 输入速度,其间的差值称为滑差,一般为1 %~3 %。传递功率可达几千千瓦。 固定充液液力偶合器 固定充液液力偶合器是在结构较简单和仅具有有限的弯曲

部分的输送装置中最常用的软启动装置,其结构相对比较简单,成本又低,对现在使用的 大多数输送机能提供优良的软启动效果。 可变充液液力偶合器 也称为限矩型液力偶合器。偶合器的叶轮装在AC 电机上,

涡轮装在从动减速器高速轴上,包含操作部件的轴箱安装在驱动基座。偶合器的旋转外壳 有溢出口,允许液体不断地从工作腔中流出进入一个分离的辅助腔, 油从辅助腔通过一个热 交换器泵到控制偶合器充液量的电磁阀。为了控制单机传动系统的启动转矩,必须监测AC 电机电流,给电磁阀的控制提供反馈。可变充液液力偶合器可使用在中大功率输送系统中, 功率可达到数千千瓦。这种驱动无论在机械,或在电气上都是很复杂的,其驱动系统成本中 等。 勺管控制液力偶合器 也称为调速型液力偶合器。此种液力偶合器同样由三个标准

的液力偶合单元构成,即叶轮、涡轮和一个包含工作环路的外壳。此种液力偶合器需要在 工作腔以外设置导管(也称勺管) 和导管腔,依靠调节装置改变勺管开度(勺管顶端与旋转 外壳间距) 人为的改变工作腔的充液量,从而实现对输出转速的调节。这种控制提供了合理 的平滑加速度,但其计算机控制系统很复杂。勺管控制液力偶合器可以应用在单机或多机 驱动系统, 功率范围为150kW~750kW。 1.3 变频控制(VFC) 变频控制 VFC 装置为感应电机提 变频控制也是一种直接驱动方式, 它具有非常独特的高性能。 供变化的频率和电压,产生优良的启动转矩和加速度。 VFC设备是一个电力电子控制器,首先

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把AC 整流成DC ,然后利用逆变器,再将DC 转换成频率、电压可控的AC。VFC 驱动 采用矢量控制或直接转矩控制(DTC) 技术,能根据不同的负载采用不同的运行速度。VFC 驱动能根据给定的S 曲线启动或停车,实现自动跟踪启动或停车曲线。VFC 驱动为传送 带启动提供了优良的速度和转矩控制, 也能为多机驱动系统提供负载均分。 VFC 控制器可 以容易地装在小功率输送机驱动上。 过去在中高电压使用时, VFC 设备的结构由于受电力 半导体器件的电压额定值限制而变得很复杂,中高电压的变速传动常常使用低压逆变器, 然后在输出端使用升压变压器,或使用多个低压逆变器串联来解决。与简单的器件串联连 接的两电平逆变器系统比较,由于串联器件之间容易均压以及输出端可以有更好的谐波特 性,三电平电压型PWM 逆 变器系统在数兆瓦工业传动中近年来获得了越来越多的应用。由三台750kW/ 2. 3kV 的这 种逆变器构成的VFC 系统已经成功安装在成庄煤矿长2. 7km的带式输送机驱动系统中。 2 使用IGBT的中性点箝位三电平逆变器 的中性点箝位三电平逆变器 使用 由于串联器件电压均分容易,器件每次开关的d v/ d t 低以及输出端出色的谐波品质, 三电平电压型逆变器在大功率传动应用中变得越来越流行。高压IGBT(HV-IGBT) 的出现 使得应用三电平中性点箝位原理的中高压逆变器设计有了更大的应用范围。这种逆变器目 前可以实现从2. 3kV到4. 16kV全范围的应用。HV-IGBT 模块串联可使用在3. 3kV和4. 16kV的设备。2. 3kV逆变器每个开关只需要一个HV-IGBT[2,3]。 2.1 主功率逆变电路 主功率逆变电路用三电平中点箝位电压型逆变器实现,可以满足中高压交流传动应用 的需要。与两电平电压型逆变器相比,三电平中点箝位电压型逆变器提供三个电压级别给 输出端,对于同样的输出电流品质,开关频率可降低到原来的1/ 4,开关器件的电压额定 值可减小到原来的1/ 2 ,附加到电机上的额外的瞬态电压应力也可能减少到原来的1/ 2 。 三电平中点箝位电压型逆变器的开关状态可归纳于表1 , , 和W 分别表示三相, U V P,N 和O 是直流母线上的三个点。例如,当开关S1U和S2U闭合时,U 相处于状态P(正母 线电压) ,反之,当开关S3U和S4U闭合时,U 相处于状态N (负母线电压) 。在中性点箝位 时,该相在O 状态, 这时根据相电流极性的正负,或者是S2U导通或者是S3U导通。 为了保证中 性点电压平衡,在O 点被注入的平均电流应该是零。 2.2 输入端变流器 为通常使用12 脉冲二极管整流器给直流环节电容器充电,在输入端引入的谐波是很小 的。若对输入谐波有更高的要求,可以使用24 脉冲二极管整流器作为输入变流器。对于 需要有再生能力的更高级应用,可以用一个有源输入变流器取代二极管整流器,这时输入

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整流器与输出逆变器为同一结构。 2.3 逆变器控制 电机控制 感应电机的控制可以使用转子磁场定向矢量控制器实现,通过使用PWM

调制器完成了恒转矩区和高速弱磁区的控制。图2 为间接矢量控制框图。图中指令磁通Ψ r 是速度的函数,反馈速度和前馈滑差控制信号ωsl相加。对相加结果的频率信号积分,然 后产生单位矢量(cosθe 和sinθe ) ,最后通过矢量旋转器产生电压 制器。 PWM调制器 该调制器实际上是把空间矢量调制概念扩展到三电平逆变器。 其基本 角 控制PWM 调

原理是三电平PWM 调制器使用两个参考波Ur1 和Ur2,但只使用一个三角波。它以一种优 化方式确定每一次开关时刻。 产生的谐波尽可能的小,使用尽可能低的开关频率以最小化开关损耗;可将零序成分 加到每一个参考波里以便最大化基波电压。 作为一个附加的自由度,参考波与三角波的相对 位置可改变,这可以用于直流环节中点的电流平衡。 3 测试结果 三个750kW/ 2. 3kV 三电平逆变器在成庄煤矿2. 7km 长带式输送机驱动系统成功安装 之后,对整个变频传动系统(VFC) 的性能进行了测试,测试结果显示出使用VFC 控制系统 的带式输送机的优良特性。图3为测试结果波形。由图看出,曲线1 显示受控带速,带速呈S 形曲线形状,曲线2 、3 分别表示电流和扭矩,曲线4 显示带张力。从图中可以发现,带张力 的波动范围很小,所有检测结果显示出带式输送机驱动系统令人满意的特性。 4 结论 近年来输送机驱动控制技术的进步已更为可靠, 符合低成本效益和高效驱动的驱动系 统为用户提供了选择。在这些选择中,可变频率控制(VFC)的方法显现出在将来长距离输 送中带式输送机扮演了重要的角色。使用高压IGBT 的中点嵌位三电平逆变器本身可以提 供电机终端所需的供电中高压,使变频控制的应用更为简单。通过成庄煤矿2. 7km长带式输 送机中采用的中点嵌位三电平逆变器变频调速(VFC)控制系统的测试结果表明,采用 HV-IGBT 的中点嵌位三电平逆变器以及使用转子磁场矢量控制策略的感应电机变频传动, 使带式输送机驱动系统具有非常优秀的性能,显示出良好的应用前景。



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