【正文】 e current injected at 0 should be zero.2. 2 Line side converter For standard applications. a 12pulse diode rectifier feeds the divided DClink capacitor. This topology introduces low harmonics on. the line side. For even higher requirements a 24pulse diode rectifier 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 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 fieldweakening regions where the PW M modulator was used. In this figure, the mand generated as function of speed. The feedback speed is added with the feed forward slip mand signalψ,the resulting frequency signal is integrated and then the unit vector signals(cosθe and sinθ e)are generated. The vector rotator generates the voltage Vs and Angle θe mands for the PW M as shown. PWM Modulator. The demanded voltage vector is generated using an elaborate PWM modulator. The modulator extends the concepts of spacevector modulation to the threelevel inverter. The operation can beexplained by starting from a regularly sampled sinetriangle parisonfrom twolevel inverter. Instead of using one set of reference waveformsand one triangle defining the switching frequency, threelevel Modulator uses two sets of reference waveforms Uand 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. The switching frequency is low and thus switching losses are minimized. As in a twolevel inverter, a zerosequence ponent can be added to each set of reference waveform s in order to maximize the fundamental voltage ponent. As an additional degree of freedom, the position of the reference waveform s within the triangle can be changed. This can be used for current balance in the two halves of the DClink.3 Testing results After Successful installation of three 750 kW /2. 3 kV threelevelinverters for one 2. 7 km long belt conveyor driving system in Cheng zhuang Mine. The performance of the 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. The belt velocity have the s shape characteristic. All the results of the test show a very satisfied characteristic for belt driving system.4 Conclusions Advances in conveyor drive control technology in recent years haveresulted in many more reliable. Costeffective and performancedriven conveyor drive system choices for these choices,theVariable frequency control (VFC) method shows promising use in the future for long distance belt conveyor drives due to its excellent performances. The NPC threelevel inverter using high voltage TGBTs make the Variable frequency control in medium voltage applications bee much more simple because the inverter itself can provide the medium voltage needed at the motor terminals, thus eliminating the stepup transformer in most applications in the past. The testing results taken from the VFC control system with NTC three. level inverters used in a 2. 7 km long belt conveyor drives in Chengzhuang Mine indicates that the performance of NPC threelevel inverter using HVTGBTs together with the control strategy of rotor fieldoriented vector control for induction motor drive is excellent for belt conveyor driving system.17