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1、英文译文 tape transport Among the methods of material conveying quantity, 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 a l so been going through a proces
2、s 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 a
3、cceleration torque is critical to belt conveyors performance. A 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 considerations in the drive sy
4、stems 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 choices1.1 tape transport on conveyor drive technologies1. 1 The belt transm
5、ission modeFull-voltage starters. With a full-voltage starter design, the conveyorhead shaft is direct-coupled to the motor through the gear drive. Directfull-voltage starters are adequate for relatively low-power, simple- Profile conveyors. With direct full-voltage starters. no control is provided
6、for various conveyor loads and. depending on the ratio between full- and no-load 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 max
7、imum stall torque; therefore. they are limited to the low-power, simple-profile conveyor belt drives. Reduced-voltage starters. As conveyor power requirements increase,controlling the applied motor torque during the acceleration period becomes increasingly important. Because motor torque is a functi
8、on 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 t
9、imed 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 full-line voltage to the
10、motor. Motors with higher torque and pull -vp 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 areconnected directly to the drive system reducer and are a modified con
11、figuration 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 starting, resistance is placed in series with the rotor for low initial torque. As the conveyor accelerates,the resista
12、nce is reduced slowly to maintain 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 motor systems have a relatively simple a design.However,the control systems for these can be highly complex
13、, because they are based on computer control of the resistance switching. Today, the majority of control systems are custom designed to meet a conveyor systems particular specifications. Wound rotor motors are appropriate for systems requiring more than 400KW. DC motor. DC motors. available from a f
14、raction 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 motors rotating armature is connected exter
15、nally. 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 expensiv
16、e in comparison to other soft-start systems. but it is a reliable, cost-effective drive in applications in which torque,load sharing and variable speed are primary considerations. DC motors generally are used with higher-power conveyors, including complex profile conveyors with multiple-drive system
17、s, 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 acentrifugal pump; the driv
18、ing hydraulic turbine known as the runner anda 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
19、 required between the driving and driver shafts.The power produced by this coupling is based on the circulated fluids 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
20、input speed. Referred to as slip. this normally is between 1% 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 b
21、elt profiles and limited convex/concave sections. They are relatively simple,low-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
22、fixed-fill couplings. The couplings 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
23、 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
24、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. Th
25、e 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: a driven impeller, a driving runner and a casing that encloses the working circuit. The casing is fitted w
26、ith 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 100 percent filled. The scoop tube, extending outside the fluid coupling, is positioned using an electric actuator to engage the tube from the ful
27、ly retracted to the fully engaged position. This control provides reasonably smooth acceleration rates. to but the computer-based control system is very complex. Scoop control couplings are applied on conveyors requiring single or multiple drives from 150KWto 750KW.1. 3 Variable-frequency control(VF
28、C) 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 ind
29、uction 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 voltag
30、e control. 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 drive
31、s 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 convey- or drives, but when used at the range of medium-high voltage in the past. the struc
32、ture 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 i
33、nverters 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 i叩roved harmonic quality at the output compared to two-level converte
34、r systems With simple series connection of devices. This kind of VFC system with three 750 kW /2. AV 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.2 Neutra
35、l point clamped(NPC)three-level inverter using IGBTThree-level voltage-fed inverters have recently become more and more popular for higher power drive applications because of their easy voltage sharing features. lower dv / dt per switching for each of the devices, and super or harmonic quality at th
36、e output. The availability of NV-IGBT 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 6kV Series connection of IIV-IGBT modules is used in the 3. 3 kV and 4
37、. 1 6kV devices. The 2. 3 kV inverters need only one HV-IGBT per switch2,3.2. 1 Power section To meet the demands for medium voltage applications. a three-levelneutral point clamped inverter realizes the power section. In comparisonto a two-level inverter. the NPC inverter offers the benefit that th
38、ree 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
39、 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 0 are the do bus points. The phase U, for example, is in state P (positive bus voltage)when
40、 the switches S1uand 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 0 state when either S2u.or S3u, conducts depending on positive or negative phase current polarity, respectively. For neutral poin
41、t voltage balancing, the average current injected at 0 should be zero.2. 2 Line side converter For standard applications. a 12-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
42、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 Control. Motor control of induction machines is realized
43、 byusing 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. T
44、he feedback speed is added with the feed forward slip command signal,the resulting frequency signal is integrated and then the unit vector signals(cose and sin e)are generated. The vector rotator generates the voltage Vs and Angle e commands for the PW M as shown. PWM Modulator. The demanded voltage
45、 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 beexplained by starting from a regularly sampled sine-triangle comparisonfrom two-level inverter. Instead of using one set of reference
46、waveformsand one triangle defining the switching frequency, three-level Modulator uses two sets of reference waveforms U and U 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. T
47、he 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 wave
48、form s within the triangle can be changed. This can be used for current balance in the two halves of the DC-link.3 Testing results After Successful installation of three 750 kW /2. 3 kV three-levelinverters for one 2. 7 km long belt conveyor driving system in Cheng zhuang Mine. The performance of th
49、e 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 small. Curve 2 and curve 3 indicate current and torque separately. Curve 4 shows the velocity of the controlled belt.
