Belt Conveying Systems Development of driving systemAmong the methods of material conveying employed，beltconveyors play a very important part in the reliable carrying ofmaterial over long distances at competitive costConveyor systemshave become larger and more complex and drive systems have also beengoing through a process of evolution and will continue to do soNowadays，bigger belts require more power and have brought the needfor larger individual drives as well as multiple drives such as 3drives of 750 kW for one belt(this is the case for the conveyor drivesin Chengzhuang Mine)The ability to control drive accelerationtorqueiscriticaltobeltconveyors performanceAnefficientdrivesystem should be able to provide smooth，soft starts whilemaintaining belt tensions within the specified safe limitsFor loadsharing on multiple drivestorque and speed control are alsoimportant considerations in the drive systems design. Due to theadvances in conveyor drive control technology，at present many morereliableCost-effective and performance-driven conveyor drivesystems covering a wide range of power are available for customerschoices1.1 Analysis on conveyor drive technologies11 Direct drivesFull-voltage startersWith a full-voltage starter design，the conveyorhead shaft is direct-coupled to the motor through the gear driveDirectfull-voltage starters are adequate for relatively low-power,simple-profile conveyorsWith direct fu11-voltage startersno controlis provided for various conveyor loads anddepending on the ratio betweenfu11-and no-1oad power requirements，empty starting times can be threeor four times faster than full loadThe maintenance-free starting systemis simple，low-cost and very reliableHowever, they cannot controlstarting torque and maximum stall torque；thereforethey are limited tothe low-power, simple-profile conveyor belt drivesReduced-voltage startersAs conveyor power requirements increase，controlling the applied motor torque during the acceleration periodbecomes increasingly importantBecause motor torque 1s a function ofvoltage，motor voltage must be controlledThis can be achieved throughreduced-voltage starters by employing a silicon controlledrectifier(SCR)A common starting method with SCR reduced-voltagestarters is to apply low voltage initially to take up conveyor belt slackand then to apply a timed linear ramp up to full voltage and belt speedHowever, this starting method will not produce constant conveyor beltaccelerationWhen acceleration is completethe SCRs,which control theapplied voltage to the electric motor are locked in full conduction,providing fu11-line voltage to the motorMotors with higher torque andpullup torque，can provide better starting torque when combined withthe SCR starters, which are available in sizes up to 750 KWWound rotor induction motorsWound rotor induction motors areconnected directly to the drive system reducer and are a modifiedconfiguration of a standard AC induction motorBy inserting resistancein series with the motors rotor windingsthe modified motor controlsystemcontrolsmotortorqueForconveyorstarting，resistanceisplacedin series with the rotor for low initial torqueAs the conveyoraccelerates，the resistance is reduced slowly to maintain a constantaccelerationtorqueOnmultiple-drivesystemsanexternalslipresistormay be left in series with the rotor windings to aid in load sharingThe motor systems have a relatively simple designHowever, the controlsystemsforthesecanbehighlycomplex，becausetheyarebasedoncomputercontroloftheresistanceswitchingToday，themajorityofcontrolsystemsarecustomdesigned to meeta conveyorsystemsparticular specificationsWound rotor motors are appropriate for systems requiring more than 400kW DC motorDC motorsavailable from a fraction of thousands of kW ，are designed to deliver constant torque below base speed and constant kWabove base speed to the maximum allowable revolutions per minute(r/min)with the majority of conveyor drives, a DC shunt wound motor is usedWherein the motors rotating armature is connected externallyThe mostcommon technology for controlling DC drives is a SCR device which allowsfor continual variable-speed operationThe DC drive system ismechanically simple, but can include complex custom-designed electronicstomonitorandcontrolthecompletesystemThissystemoption isexpensivein comparison to other soft-start systemsbut it is a reliable,cost-effective drive in applications in which torque,1oad sharing andvariable speed are primary considerationsDC motors generally are usedwith higher-power conveyors，including complex profile conveyors withmultiple-drive systems，booster tripper systems needing belt tensioncontrol and conveyors requiring a wide variable-speed range12 Hydrokinetic couplingHydrokinetic couplings，commonly referred to as fluid couplingsare composed of three basic elements; the driven impeller, which acts asa centrifugal pump；the driving hydraulic turbine known as the runner anda casing that encloses the two power componentsHydraulic fluid is pumpedfrom the driven impeller to the driving runner, producing torque at thedriven shaftBecause circulating hydraulic fluid produces the torque andspeed，nomechanical connectionisrequiredbetweenthedrivinganddrivenshaftsThe power produced by this coupling is based on the circulatedfluids amount and density and the torque in proportion to input speedBecause the pumping action within the fluid coupling depends oncentrifugalforcestheoutputspeedislessthantheinputspeedReferredtoasslipthisnormallyisbetweenl%and3%Basichydrokineticcouplingsare available in configurations from fractional to several thousand kW Fixed-fill fluid couplingsFixed-fill fluid couplings are the mostcommonly used soft-start devices for conveyors with simpler belt profilesand limited convex/concave sectionsThey are relativelysimple,1ow-cost,reliable,maintenance free devices that provideexcellent soft starting results to the majority of belt conveyors in usetodayVariable-fill drain couplingsDrainable-fluid couplings work on thesame principle as fixed-fill couplingsThe couplings impellers aremounted on the AC motor and the runners on the driven reducer high-speedshaftHousing mounted to the drive base encloses the working circuitThe couplings rotating casing contains bleed-off orifices thatcontinually allow fluid to exit the working circuit into a separatehydraulic reservoirOil from the reservoir is pumped through a heatexchanger to a solenoid-operated hydraulic valve that controls thefilling of the fluid couplingTo control the starting torque of asingle-drive conveyor system，the AC motor current must be monitored toprovide feedback to the solenoid control valveVariable fill draincouplings are used in medium to high-kW conveyor systems and are availablein sizes up to thousands of kW The drives can be mechanically complexand depending on the control parametersthe system can be electronicallyintricateThe drive system cost is medium to high, depending upon sizespecifiedHydrokinetic scoop control driveThe scoop control fluid couplingconsists of the three standard fluid coupling components：a drivenimpeller, a driving runner and a casing that encloses the working circuitThe casing is fitted with fixed orifices that bleed a predetermined amountof fluid into a reservoirWhen the scoop tube is fully extended into thereservoir, the coupling is l00 percent filledThe scoop tube, extendingoutside the fluid coupling，is positioned using an electric actuator toengage the tube from the fully retracted to the fully engaged positionThis control provides reasonably smooth acceleration ratesto but thecomputer-based control system is very complexScoop control couplingsare applied on conveyors requiring single or multiple drives from l50 kWto 750 kW.13 Variable-frequency control(VFC)Variable frequency control is also one of the direct drive methodsThe emphasizing discussion about it here is because that it has so uniquecharacteristic and so good performance compared with other drivingmethods for belt conveyor VFC devices Provide variable frequency andvoltage to the induction motor, resulting in an excellent starting torqueandaccelerationrateforbeltconveyordrivesVFCdrivesavailablefromfractional to several thousand(kW ), are electronic controllers thatrectify AC line power to DC and，through an inverter, convert DC back toAC with frequency and voltage contro1VFC drives adopt vector controlor direct torque control(DTC)technology，and can adopt differentoperating speeds according to different loadsVFC drives can makestarting or stalling according to any given S-curvesrealizing theautomatic track for starting or stalling curvesVFC drives provideexcellent speed and torque control for starting conveyor beltsand canalso be designed to provide load sharing for multiple driveseasily VFCcontrollers are frequently installed on lower-powered conveyor drives，butwhenusedattherangeofmedium-highvoltageinthepastthestructureof VFC controllers becomes very complicated due to the limitation ofvoltage rating of power semiconductor devices，the combination ofmedium-high voltage drives and variable speed is often solved withlow-voltage inverters using step-up transformer at the output，or withmultiple low-voltage inverters connected in seriesThree-levelvoltage-fed PWM converter systems are recently showing increasingpopularity for multi-megawatt industrial drive applications because ofeasy voltage sharing between the series devices and improved harmonicquality at the output compared to two-level converter systems With simpleseries connection of devicesThis kind of VFC system with three 750 kW/23kV inverters has been successfully installed in ChengZhuang Mine forone 27-km long belt conveyor driving system in following the principleof three-level inverter will be discussed in detail2 Neutral point clamped(NPC)three-level inverter using IGBTsThree-level voltage-fed inverters have recently become more and morepopular for higher power drive applications because of their easy voltagesharing features1ower dv/dt per switching for each of the devices，and superior harmonic quality at the outputThe availability of HV-IGBTshas led to the design of a new range of medium-high voltage inverter usingthree-level NPC topologyThis kind of inverter can realize a whole rangewitha voltage rating from 23 kVto 41 6kV Series connection of HV-IGBTmodules is used in the 33 kV and 41 6 kV devicesThe 23 kV invertersneed only one HV-IGBT per switch2,3.21 Power sectionTo meet the demands for medium voltage applicationsa three-levelneutral point clamped inverter realizes the power sectionIn comparisonto a two-level inverterthe NPC inverter offers the benefit that threevoltage levels can be supplied to the output terminals，so for the sameoutput current quality，only 1/4 of the switching frequency is necessaryMoreover the voltage ratings of the switches in NPC inverter topology willbereducedto1/2andtheadditionaltransientvoltagestressonthemotorcan also be reduced to 1/2 compared to that of a two-level inverterThe switching states of a three-level inverter are summarized in Table1UV and W denote each of the three phases respectively；P N and O arethe dc bus pointsThe phase U，for example，is in state P(positive busvoltage)when the switches S1u and S2u are closed，whereas it is in stateN (negative bus voltage) when the switches S3u and S4u are closedAtneutral point clamping，the phase is in O state when either S2u or S3uconducts depending on positive or negative phase current polarity，respectivelyFor neutral point voltage balancing，the average currentinjected at O should be zero22 Line side converterFor standard applicationsa l2-pulse diode rectifier feeds thedivided DC-link capacitorThis topology introduces low harmonics on theline sideFor even higher requirements a 24-pulse diode rectifier canbe used as an input converterFor more advanced applications whereregeneration capability is necessary, an active frontend converter canreplace the diode rectifier, using the same structure as the inverter23 Inverter controlMotor Contro1Motor control of induction machines is realized byusing a rotor fluxoriented vector controllerFig2 shows the block diagram of indirect vector controlled drivethat incorporates both constant torque and high speed field-weakeningregions where the PW M modulator was usedIn this figure，the commandflux is generated as function of speedThe feedback speed is addedwith the feed forward slip command signal . the resulting frequencysignal is integrated and then the unit vector signals(cos and sin )aregeneratedThe vector rotator generates the voltage and anglecommands for the PW M as shownPWM ModulatorThe demanded voltage vector is generated using anelaborate PWM modulatorThe modulator extends the concepts ofspace-vector modulation to the three-level inverterThe operation canbe explained by starting from a regularly sampled sine-trianglecomparison from two-level inverterInstead of using one set of referencewaveforms and one triangle defining the switching frequency， thethree-level modulator uses two sets of reference waveforms Ur1 and Ur2 andjust one triangleThus, each switching transition is used in an optimalway so that several objectives are reached at the same timeVery low harmonics are generatedThe switching frequency is low andthus switching losses are minimizedAs in a two-level inverter, azero-sequence component can be added to each set of reference waveforms in order to maximize the fundamental voltage componentAs an additionaldegree of freedom，the position of the reference waveform s within thetriangle can be changedThis can be used for current balance in the twohalves of the DC-1ink3 Testing resultsAfter Successful installation of three 750 kW /23 kV three-levelinverters for one 27 km long belt conveyor driving system in ChengzhuangMineThe performance of the whole VFC