1、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 costConveyor systems have become larger and more complex and drive systems have also
2、 been going through a process of evolution and will continue to do soNowadays，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 abi
3、lity to control drive acceleration torque is critical to belt conveyors performanceAn efficient drive system should be able to provide smooth，soft starts while maintaining belt tensions within the specified safe limitsFor load sharing on multiple drivestorque and speed control are also important con
4、siderations in the drive systems design. Due to the advances in conveyor drive control technology，at present many more reliableCost-effective and performance-driven conveyor drive systems covering a wide range of power are available for customers choices1.1 Analysis on conveyor drive technologies11
5、Direct drivesFull-voltage startersWith a full-voltage starter design，the conveyor head shaft is direct-coupled to the motor through the gear driveDirect full-voltage starters are adequate for relatively low-power, simple-profile conveyorsWith direct full-voltage startersno control is provided for va
6、rious conveyor loads anddepending on the ratio between fu11- and no-1oad power requirements，empty starting times can be three or four times faster than full loadThe maintenance-free starting system is simple，low-cost and very reliableHowever, they cannot control starting torque and maximum stall tor
7、que；thereforethey are limited to the low-power, simple-profile conveyor belt drivesReduced-voltage startersAs conveyor power requirements increase，controlling the applied motor torque during the acceleration period becomes increasingly importantBecause motor torque 1s a function of voltage，motor vol
8、tage must be controlledThis 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 slackand then to apply a timed linear ramp up to full vol
9、tage and belt speedHowever, this starting method will not produce constant conveyor belt accelerationWhen acceleration is completethe SCRs, which control the applied voltage to the electric motor are locked in full conduction, providing fu11-line voltage to the motorMotors with higher torque and pul
10、lup torque，can provide better starting torque when combined with the SCR starters, which are available in sizes up to 750 KWWound rotor induction motorsWound rotor induction motors are connected directly to the drive system reducer and are a modified configuration of a standard AC induction motorBy
11、inserting resistance in series with the motors rotor windingsthe modified motor control system controls motor torqueFor conveyor starting，resistance is placed in series with the rotor for low initial torqueAs the conveyor accelerates，the resistance is reduced slowly to maintain a constant accelerati
12、on torqueOn multiple-drive systemsan external slip resistor may be left in series with the rotor windings to aid in load sharingThe motor systems have a relatively simple designHowever, the control systems for these can be highly complex，because they are based on computer control of the resistance s
13、witchingToday，the majority of control systems are custom designed to meet a conveyor systems particular specificationsWound rotor motors are appropriate for systems requiring more than 400 kW DC motorDC motorsavailable from a fraction of thousands of kW ，are designed to deliver constant torque below
14、 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 usedWherein the motors rotating armature is connected externallyThe most common technology for controlling DC drives is a SCR device whic
15、h allows for continual variable-speed operationThe DC drive system is mechanically simple, but can include complex custom-designed electronics to monitor and control the complete systemThis system option is expensive in comparison to other soft-start systemsbut it is a reliable, cost-effective drive
16、 in applications in which torque,1oad sharing and variable speed are primary considerationsDC 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 vari
17、able-speed range12 Hydrokinetic couplingHydrokinetic couplings，commonly referred to as fluid couplingsare 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 componentsHydra
18、ulic fluid is pumped from the driven impeller to the driving runner, producing torque at the driven shaftBecause circulating hydraulic fluid produces the torque and speed，no mechanical connection is required between the driving and driven shaftsThe power produced by this coupling is based on the cir
19、culated fluids amount and density and the torque in proportion to input speedBecause the pumping action within the fluid coupling depends on centrifugal forcesthe output speed is less than the input speedReferred to as slipthis normally is between l% and 3%Basic hydrokinetic couplings are available
20、in configurations from fractional to several thousand kW Fixed-fill fluid couplingsFixed-fill fluid couplings are the most commonly used soft-start devices for conveyors with simpler belt profiles and limited convex/concave sectionsThey are relatively simple,1ow-cost,reliable,maintenance free device
21、s that provide excellent soft starting results to the majority of belt conveyors in use todayVariable-fill drain couplingsDrainable-fluid couplings work on the same principle as fixed-fill couplingsThe couplings impellers are mounted on the AC motor and the runners on the driven reducer high-speed s
22、haftHousing mounted to the drive base encloses the working circuitThe couplings rotating casing contains bleed-off orifices that continually allow fluid to exit the working circuit into a separate hydraulic reservoirOil from the reservoir is pumped through a heat exchanger to a solenoid-operated hyd
23、raulic valve that controls the filling of the fluid couplingTo control the starting torque of a single-drive conveyor system，the AC motor current must be monitored to provide feedback to the solenoid control valveVariable fill drain couplings are used in medium to high-kW conveyor systems and are av
24、ailable in sizes up to thousands of kW The drives can be mechanically complex and depending on the control parametersthe system can be electronically intricateThe drive system cost is medium to high, depending upon size specifiedHydrokinetic scoop control driveThe scoop control fluid coupling consis
25、ts of the three standard fluid coupling components：a driven impeller, a driving runner and a casing that encloses the working circuitThe casing is fitted with fixed orifices that bleed a predetermined amount of fluid into a reservoirWhen the scoop tube is fully extended into the reservoir, the coupl
26、ing is l00 percent filledThe 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 positionThis control provides reasonably smooth acceleration ratesto but the computer-based control system is very c
27、omplexScoop control couplings are applied on conveyors requiring single or multiple drives from l50 kW to 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 unique characteristi
28、c 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 drivesVFC drivesavailable from fractional to several thousand(k
29、W ), are electronic controllers that rectify AC line power to DC and，through an inverter, convert DC back to AC with frequency and voltage contro1VFC drives adopt vector control or direct torque control(DTC)technology，and can adopt different operating speeds according to different loadsVFC drives ca
30、n make starting or stalling according to any given S-curvesrealizing the automatic track for starting or stalling curvesVFC drives provide excellent speed and torque control for starting conveyor beltsand can also be designed to provide load sharing for multiple driveseasily VFC controllers are freq
31、uently installed on lower-powered conveyor drives，but when used at the range of medium-high voltage in the pastthe 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 s
32、peed is often solved with low-voltage inverters using step-up transformer at the output，or with multiple low-voltage inverters connected in seriesThree-level voltage-fed PWM converter systems are recently showing increasing popularity for multi-megawatt industrial drive applications because of easy
33、voltage sharing between the series devices and improved harmonic quality at the output compared to two-level converter systems With simple series connection of devicesThis kind of VFC system with three 750 kW /23kV inverters has been successfully installed in ChengZhuang Mine for one 27-km long belt
34、 conveyor driving system in following the principle of 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 more popular for higher power drive applications because of their easy vo
35、ltage sharing features1ower dv/dt per switching for each of the devices，and superior harmonic quality at the outputThe availability of HV-IGBTs has led to the design of a new range of medium-high voltage inverter using three-level NPC topologyThis kind of inverter can realize a whole range with a vo
36、ltage rating from 23 kV to 41 6 kV Series connection of HV-IGBT modules is used in the 33 kV and 41 6 kV devicesThe 23 kV inverters need only one HV-IGBT per switch2,3.21 Power sectionTo meet the demands for medium voltage applicationsa three-level neutral point clamped inverter realizes the power s
37、ectionIn comparison to a two-level inverterthe 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 necessaryMoreover the voltage ratings of the switches in NPC inverter topolog
38、y will be reduced to 1/2and 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 1UV and W denote each of the three phases respectively；P N and O are the dc bus
39、pointsThe 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 closedAt neutral point clamping，the phase is in O state when either S2u or S3u conducts depending on positive or
40、 negative phase current polarity，respectivelyFor neutral point voltage balancing，the average current injected at O should be zero22 Line side converterFor standard applicationsa l2-pulse diode rectifier feeds the divided DC-link capacitorThis topology introduces low harmonics on the line sideFor eve
41、n higher requirements a 24-pulse diode rectifier can be used as an input converterFor more advanced applications where regeneration capability is necessary, an active frontend converter can replace the diode rectifier, using the same structure as the inverter23 Inverter controlMotor Contro1Motor con
42、trol of induction machines is realized by using a rotor fluxoriented vector controllerFig2 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 usedIn this figure，the command flux is gen
43、erated as function of speedThe 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 generatedThe vector rotator generates the voltage and angle commands for the PW M as shownPWM ModulatorThe
44、 demanded voltage vector is generated using an elaborate PWM modulatorThe modulator extends the concepts of space-vector modulation to the three-level inverterThe operation can be explained by starting from a regularly sampled sine-triangle comparison from two-level inverterInstead of using one set
45、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 triangleThus, each switching transition is used in an optimal way so that several objectives are reached at the same time Very low harmoni
46、cs are generatedThe switching frequency is low and thus switching losses are minimizedAs 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 componentAs an additional degree of freedom，the position of the re
47、ference waveform s within the triangle can be changedThis can be used for current balance in the two halves of the DC-1ink3 Testing resultsAfter Successful installation of three 750 kW /23 kV three-level inverters for one 27 km long belt conveyor driving system in Chengzhuang MineThe performance of
48、the whole VFC system was testedFig3 is taken from the test，which shows the excellent characteristic of the belt conveyor driving system with VFC controllerFig3 includes four curvesThe curve 1 shows the belt tensionFrom the curve it can be find that the fluctuation range of the belt tension is very s
49、mal1Curve 2 and curve 3 indicate current and torque separatelyCurve 4 shows the velocity of the controlled beltThe belt velocity have the“s”shape characteristicA1l the results of the test show a very satisfied characteristic for belt driving system4 ConclusionsAdvances in conveyor drive control technology in recent years have resulted in many
