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0.9
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φ0.9x3m四筒平衡节能球磨机设计,0.9,x3m,平衡,节能,球磨机,设计
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外文资料翻译Belt Conveying Systems Development of driving systemAmongthemethodsofmaterialconveyingemployed，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 Nowadaysbigger belts require more power and have brought theneed for larger individual drives, the ability to control drive acceleration torque is critical to belt conveyorsperformance An efficient drive system should be able to provide smoothsoft 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 systems design. Due to the advances in conveyor drive control technologyat present many more reliable Cost-effective and performance-drivenconveyor drive systems covering a wide range of power are available for choices.1Analysis on conveyor drive technologies11DirectdrivesFull-voltage startersWith a full-voltage starter designthe conveyor head shaft is direct-coupled tothe motor through the gear driveDirect full-voltage starters are adequate for relatively low-power, simple-profile conveyorsWithdirectfu11-voltagestarters no control is provided for various conveyor loads and depending on the ratio between fu11- and no-1oad power requirementsempty starting times can be three or four times faster than full load The maintenance-free starting system is simplelow-costand very reliable However,they cannot control starting torque and maximum stall torquetherefore they are limited to the low-power, simple-profileconveyor belt drivesReduced-voltage startersAs conveyor power requirements increasecontrolling the applied motor torqueduring the acceleration period becomes increasingly importantBecause motor torque 1s a function of voltagemotor 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 motorare locked in full conduction, providing fu11-linevoltage to the motor Motors with higher torque and pullup torquecan provide betterstarting torque when combined with the SCR starters, which are available in sizes upto 750 KWWoundrotorinductionmotorsWound 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 motors rotor windings the modified motor control system controls motor torque For conveyor startingresistance is placed in series with the rotor for low initial torqueAs the conveyor acceleratesthe resistance is reduced slowly tomaintain 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 motorsystems have a relatively simple design However, the control systems for these can be highly complexbecause they are based on computer control of the resistance switching T odaythe majority of control systems are custom designed to meet a conveyor systems particular specificationsWound rotor motors are appropriate for systems requiring more than 400kWDC motorDC motors available from a fraction of thousands of kW aredesigned 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, aDC shunt wound motor is used Wherein the motors rotating armature is connected externally The most common technology for controlling DC drives is a SCR devicewhich allows for continual variable-speed operationThe DC drive system is mechanically simple, but can include complex custom-designed electronics to monitor andcontrol the complete system This system option is expensive in comparison to othersoft-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 generallyareusedwithhigher-powerconveyors，including complex profile conveyors with multiple-drive systemsbooster tripper systems needing belt tension control and conveyors requiring a wide variable-speed range12 Hydrokinetic couplingHydrokinetic couplingscommonly referred to as fluid couplingsare composedof three basic elements; the driven impeller, which acts as a centrifugal pumpthe driving hydraulic turbine known as the runner and a casing that encloses the two powercomponents 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 speedno mechanical connection is required between the driving and driven shaftsThe power produced by this coupling is based on the circulated fluids amount and density and the torque in proportion to input speed Because the pumpingaction 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 couplingsFixed-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 softstarting results to the majority of belt conveyors in use today Variable-filldraincouplings Drainable-fluid couplings work on the same principle as fixed-fill couplings Thecouplings 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 couplings 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 ontrol the starting torque of a single-drive conveyor systemthe AC motor current must be monitored to provide feedback to the solenoid control valveVariablefilldrain 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 Thedrive system cost is medium to high, depending upon size specifiedHydrokineticscoop control driveThe scoop control fluid coupling consists of the three standard fluid coupling componentsa 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 reservoirWhen the scoop tube is fully extended into the reservoir, the coupling is l00 percent filled The scoop tube, extending outside the fluid couplingis 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 complexScoop control couplings are applied on conveyors requiring single or multipledrivesfroml50kWto750kW.13 Variable-frequency control(VFC)Variablefrequency control is also one of thedirect drive methods The emphasizingdiscussion about it here is because that it has so unique characteristic and so good performance compared with other driving methods for belt conveyorVFC devices Provide variable frequency and voltage to the induction motor, resulting in an excellentstarting torque and acceleration rate for belt conveyor drives VFC drives availablefrom fractional to several thousand(kW ), are electronic controllers that rectify AC linepowertoDCand，throughan inverter,convertDCbacktoACwith frequency and voltage contro1VFCdrives adopt vector control or direct torque control(DTC)technologyand can adopt different operating speeds according to different loads VFC drives can make starting orstalling according to any given S-curves realizing the automatic track for starting orstalling curves VFC 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 frequently installed on lower-powered conveyor drivesbutwhen 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 devicesthe combination of medium-high voltage drives and variable speed is often solved with low-voltage inverters using step-up transformer at the outputor with multiple low-voltage inverters connected in seriesThree-level voltage-fedPWM converter systems are recently showing increasing popularity for multi-megawatt industrial drive applications because of easy voltage sharing between the seriesdevices 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 forone 27-km long belt 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 popularfor higher power drive applications because of their easy voltage sharing features1ower dv/dt per switching for each of the devicesand superior harmonic quality at theoutput TheavailabilityofHV-IGBTshasledtothedesignofanewrange of medium-high voltage inverter using three-level NPC topologyThis kind of inverter can realizea whole range with a voltage rating from 2 3kVto4 16kVSeries connectionofHV-IGBTmodules isusedinthe3 3kVand4 1 6kVdevices The 2 3 kVinverters needonlyoneHV-IGBTperswitch2,3.21 Power sectionTo meet the demands for medium voltage applicationsa three-level neutral pointclamped 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 outputterminalssoforthesameoutputcurrentquality，only 1/4 of the switching frequency is necessary Moreover the voltage ratings of theswitches in NPC inverter topology will be reduced to 1/2 and the additional transientvoltage stress on the motor can also be reduced to 1/2 compared to that of a two-levelinverter The switching states of a three-level inverter are summarized in Table 1 U V andWdenoteeachofthethreephasesrespectively；PNandOarethedcbuspointsThephaseU，for exampleis in state P(positive bus voltage)when the switches S1u and S2u are closedwhereas it is in state N (negative bus voltage) when the switches S3u and S4u are closedAt neutral point clampingthe phase is in O state when either S2u or S3u conductsdependingonpositiveornegativephasecurrentpolarity，respectivelyForneutralpointvoltagebalancing，the average current injected at O should be zero22 Line side converterFor standard applicationsa 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 converterFormore advanced applications where regeneration capability is necessary, an active front end converter can replace the diode rectifier, using the same structure as the inverter23InvertercontrolMotorContro1Motor control of induction machines is realized by using a rotor fluxorientedvector controller Fig 2 shows the block diagram of indirect vector controlled drive that incorporates both constant torque and high speed field-weakening regions wherethe PW M modulator was usedIn this figurethe 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 andsin )are generated The vector rotator generates the voltage and angle commands forthe PW M as shown PWM Modulator The demanded voltage vector is generatedusing an elaboraePWM modulatorThe modulator extends the concepts of space-vector modulion to the three-level inverter The operation can be explained by startingfrom a regularly sampled sine-triangle comparison from two-level inverter Instead of using one set ofreference waveforms and one triangle defining the switching frequencythe 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 thatseveral objectives are reached at the same time Verylowharmonicsaregenerated Theswitching 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 component As an additional degree of freedomthe position of the reference 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 /2 3 kV three-level inverters for one2 7 km long belt conveyor driving system in Chengzhuang Mine The performanceof the whole VFC system was tested Fig
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