【正文】 s rotating casing contains bleedoff 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 solenoidoperated hydraulic valve that controls the filling of the fluid coupling. To control the starting torque of a singledrive conveyor system, the AC motor current must be monitored to provide feedback to the solenoid control valve. Variable fill drain couplings are used in medium to highkw conveyor systems and are available in sizes up to thousands of drives can be mechanically plex and depending on the control parameters. the system can be electronically intricate. The drive system cost is medium to high,depending upon size specified. Hydrokiic scoop control drive. The scoop control fluid coupling consists of the three standard fluid coupling cmponents: a 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 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 fully retracted to the fully engaged position. This control provides reasonably smooth acceleration rates. to but the puterbased control system is very plex. Scoop control couplings are Variablefrequency control(VFC) 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 pared 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 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 voltage control. VFC drives adopt vector control or direct torquecontrol(DTC)technology, and can adopt different operating speeds according to different loads. VFC drives can make starting or stalling according to any given Scurves realizing the automatic track for starting or stalling curves. VFC drives 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 lowerpowered convey or drives, but when used at the range of mediumhigh voltage in the past. the structure of VFC controllers bees very plicated due to the limitation of voltage rating of power semiconductor devices, the bination of mediumhigh voltage drives and variable speed is often solved with lowvoltage inverters using stepup transformer at the output, or with multiple lowvoltage inverters connected in series. Threelevel voltagefed PWM converter systems are recently showing increasing popularity for multimegawatt industrial drive applications because of easy voltage sharing between the series devices and i 叩 roved 13 harmonic quality at the outputpared to twolevel converter 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. 7km long belt conveyor driving system in following the principle of threelevel inverter will be discussed in detail. 2 Neutral point clamped(NPC)threelevel inverter using IGBTs Threelevel voltagefed inverters have recently bee 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 superiorharmonic quality at the output. The availability of NVIGBTs has led to the design of a new range of mediumhigh voltage inverter using threelevel NPC topology. This kind of inverter can realize a whole range with a voltage rating from 2. 3 kV to 4. 16kV Series conn