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MV VFD application in Xinyuliangshan Ore Mill
Xinyuliangshan Ore Mill locates in Jiangxi Province. It supplies ore to Xinyu Steel Plant. Ore mill added an ore dressing equipment. There were 3 same model pumps with isolation which were driven by high voltage motors. In normal operation, 2 pumps worked and the third was spare. If motor started in direct on line, big current would impact on grid. Furthermore, flow of pumps was controlled by tails remaining.
In order to lower production cost and heighten precision and efficiency, mill decided to adopt variable frequency technology. After research and comparing, mill chose HARSVERT-A06/060 VFD.

II. Ore mill introduction
1. Work procedure
Put tailing slurry (density 8%) to ore separating cabinet first. Add liquid medicine to accelerate separation of ore and water. Then dispatch ore slurry to 1# and 2# concentrating tank to deposit. After depositing, water is spilled. Ore slurry flows to bottom pump station. The ore slurry (density 28%) is discharged to slag detaching cabinet. Grainess ore is left and slurry is transported to mix room. Volumetric pump which is driven by VFD discharges ore slurry to tailing ore cabinet. In the case of variable frequency pump broken, accident cabinet beside tailing ore cabinet can discharge tailing slurry left in pipe to ore separating cabinet.
Fall gap of volumetric pump and tailing ore cabinet is about 160m. pipe diameter is 4000m. pump model is YBJ-210/6.3, rated pressure is 6.3Mpa, actural pressure is 3.8Mpa~4.2Mpa.
Ore is separated into refine ore and tailing ore by ball mill. Refine ore is used to smelt in steel plant. Tailing ore which is not pured must return to tailing ore cabinet. Compressor feeds tailing ore in pump and then transports it to tailing ore cabinet 4km away. Tailing ore is all but slurry, as water with 30% impurity.
Motor drives double-cylinder compressor. Compressor is special load. When stopcock beats slurry, current is big. When stopcock returns, current is down suddenly. So VFD current waveform is in sawtooth. Plant installs balance ring in motor side to keep current smooth.
2. VFD room layout

Personnel control VFD in three modes: local control, upper PC cnotrol, and operation box. Motor and VFD data:

Motor parameters VFD parameters
Rated power: 450kW Model: HARSVERT-A06/060
Rated voltage; 6kV Rated capacity: 570kVA
Rated current: 55.44A Input voltage: 6kV
Rated speed: 998 r/min Output voltage: 6kV

III. Circuit
In order to keep reliability, we install bypass cabinet for VFD. When VFD is broken, motor is switched to direct on line. Bypass circuit is composed of three high voltage breakers QS1, QS2 and QS3. QS2 and QS3 are mechanical interlock and can not close together. In variable frequency state, QS1 and QS2 close, QS3 open. In direct on line state, QS3 close, QS1 and QS2 open.
In order to keep safety, VFD interlocks with QF. Once VFD is broken, QF trips off. So it needs user to change circuit. In direct on line state, VFD cancels QF trip-off signal and allows QF to close.

Because concentration of tailing ore is more than that of water, load is heavy when VFD starts up. User can change " torque boost" to 2 or 3 and then start VFD in order to keep VFD normal start-up.

IV. Concluding remarks
VFD operation in Xinyuliangshan Ore Mill indicates that VFD not only saves energy but also brings following advantages:
1. Motor can realize soft start with VFD. Start-up current is smaller than rated current, which makes start-up smooth.
2. Heighten system efficiency and eliminate throttle loss.
3. Work performance is changed and equipment lifetime is prolonged.
4. Because of cell-cascade technology of VFD, 41 down harmonic can be deleted. Total harmonic can be controlled in 2% and current harmonic is less than 4% for manufacturer process limitation.
5. VFD adopts technology of PWM control. Output voltage waveform is nearly sin. Total harmonic is less than 1%. Above indexes all meet IEEE-519 requirement.
6. VFD is voltage source instructure. Power factor is 0.95.
7. Plant noise lowers.

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