【文章內(nèi)容簡(jiǎn)介】 .1．3 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．a(chǎn)vailable 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 contro1．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 Scurves．realizing the automatic track for starting or stalling curves．VFC drives provide excellent speed and torque control for starting conveyor belts．a(chǎn)nd can also be designed to provide load sharing for multiple drives．easily VFC controllers are frequently installed on lowerpowered conveyor 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 improved harmonic quality at the output pared to twolevel 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 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 IGBTsThreelevel voltagefed inverters have recently bee more and more popular for higher power drive applications because of their easy voltage sharing features．1ower dv/dt per switching for each of the devices，and superior harmonic quality at the output．The availability of HVIGBTs 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 4．1 6 kV Series connection of HVIGBT modules is used in the 3．3 kV and 4．1 6 kV devices．The 2．3 kV inverters need only one HVIGBT per switch[2,3].2．1 Power sectionTo meet the demands for medium voltage applications．a(chǎn) threelevel neutral point clamped inverter realizes the power section．In parison to a twolevel inverter．the NPC inverter offers the benefit that three 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．a(chǎn)nd the additional transient voltage stress on the motor can also be reduced to 1/2 pared to that of a twolevel inverter． The switching states of a threelevel inverter are summarized in Table 1．U．V and W denote each of the three phases respectively；P N and O are the dc bus points．The phase U，for example，is in state P(positive bus voltage)when the switches S1u and 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 O state when either S2u or S3u conducts depending on positive or negative phase current polarity，respectively．For neutral point voltage balancing，the average current injected at O should be zero．2．2 Line side converterFor standard applicati