1、机械专业外文翻译中英文翻译外文翻译英文原文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 dr
2、ive systems have also 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 Che
3、ngzhuang Mine)The ability 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 a
4、re also important considerations 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
5、drive technologies11 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 fu11-voltage startersno contr
6、ol is provided for various 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
7、and maximum stall torque;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
8、 of voltage,motor voltage 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 linea
9、r ramp up to full voltage 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
10、higher torque and pullup 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
11、AC induction motorBy 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
12、a constant acceleration 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 contro
13、l of the resistance switchingToday,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
14、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 usedWherein the motors rotating armature is connected externallyThe most common technology for controlling DC drives
15、 is a SCR device which 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,
16、 cost-effective drive 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
17、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 as a centrifugal pump;the driving hydraulic turbine known as the runner and a casing that encloses the two
18、power componentsHydraulic 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 couplin
19、g is based on the circulated 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 cou
20、plings 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 sectionsThey are relatively simple,1ow-cost,reliable,ma
21、intenance free devices 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
22、 reducer high-speed shaftHousing 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
23、solenoid-operated hydraulic 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 convey
24、or systems and are available 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
25、fluid coupling consists 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
26、 reservoir, the coupling 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 con
27、trol system is very complexScoop 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
28、 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 drivesVFC drivesavailable from fractional
29、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 contro1VFC drives adopt vector control or direct torque control(DTC)technology,and can adopt different operating speeds according to differe
30、nt loadsVFC drives can 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
31、 controllers are frequently 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
32、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 seriesThree-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 devicesThis kind of VFC system with three 750 kW /23kV inverters has been
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