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The application of VFD mated with the dust collecting fan in the Grinding Plant
Abstract : This article is mainly concerning production technology with respect to cement as well as the application of medium voltage variable frequency drive manufactured by Beijing Leader & Harvest Electric Technologies Co., Ltd. mated with the dust collecting fan in the Pingyin Grinding Plant of Jinan Shanshui Cement Factory. Also there elaborate economic analysis and other relevant innovations in this article. Consequently, installation of medium voltage variable frequency drive from Leader & Harvest for energy-saving innovation features economic investment, good efficiency and stable running.
Keywords: Medium voltage VFD Central grinding plant Energy saving

I Preface
There is a cement production line of annual 1000000t in Jinan Shanshui Group. At first, dust collecting fan was constant speed running. Its air flow was controlled by damper. Because of variety of output and product kinds, it needs different air flow to satisfy process requirement. If controlled by damper, it is not only bad in adjusting precision but also wasting energy. In order to improve process technology and reduce power consumption, Shanshui Group decided to innovate with technology of variable frequency.
After lots of inspections and demonstrations, the plant chose HARSVERT-A06/055 VFD produced by Beijing Leader &Harvest Electric Technologies Co., Ltd.

1. Motor drives millstones to rotate via reducer. Materials fall in center of millstones from feed opening and move to edge by centrifugal force. After grinding, comminuted materials leave millstones and are brought to isolator by high-speed air flow. After isolating, coarse powder returns millstones and is milled again. Thin powder leaves millstones following air flow and is collected by dust collecting system.
Speed of fan is decided by process technology (output and fineness).
2. Main equipments of grinding system

Name Model Capacity Motor power(kW)
Roll squeezer RP120-80 Diameter 1200mm, Effective width 899mm 2×500 kW
Tubing mill ∮4.2×11.5 120t/h 2800 kW
Powder concentrator XWS40 150 t/h 220 kW
Bag type dust collector LPM2X70 157000m3/h
Exhaust fan R6-2X40No15.57 160000 m3/h, 7300Pa 450 kW

Technics process

II Control scheme of dust collection fan
1. Main circuit

Principle: VFD drives a motor. Input of VFD connects to 6kV power source and output connects to motor. QF1 is high voltage breaker in site. In order to protect VFD, VFD interlocks with QF1. only when VFD control system is normal, can QF1 close.
2. Control circuit
According to user requirement, VFD can adjust air pressure as per pressure signal and also interlink with air gate.
There are local control and remote control in controller cabinet of VFD. User can choose any one. VFD supports protocol of MODBUS, PROFIBUS, TCP/IP and hard wire to implement remote control. In this project, VFD connects to DCS through hard wire. Namely, DCS gives command of start, stop, given frequency to VFD, VFD feeds back DCS digital signals of "ready", "running", "alarm", "fault"and analogue signal of "motor speed"and "motor curret".

3. VFD data

NO Item Unit Parameter Remark
1 Standard Q/CP BLH003-2004
2 Model HARSVERT-A06/055
3 Manufacturer Leader&Harvest
4 Technology Cell-cascade, high-high
5 Motor Asynchronous squirrel cage motor
6 Input voltage range kV 6kV±15%
7 Input voltage kV 6kV
8 Input frequency range 50Hz±10%
9 Voltage fluctuation +15%~-35%
10 VFD efficiency >0.98
11 Harmonic Input current<4%,output current<2%
12 Free fault time H 20000h
13 Power factor >0.95(>20%load)
14 Control mode Multi level sin PWM
15 Control power 220V±10%AC,3kVA, 220V±10%DC,3kVA
16 UPS Last for 30min
17 Rectifier 42 pulse
18 Inverter IGBT inverter birdge series connection
19 Optical fiber HFBR-EUS100
20 Noise ≤75dB
21 Cooling Air cooling
22 Over load capacity 120% 1min,150% 3s(<10μs) 200% immediately
23 Control connection Hard wire to DCS
24 Analogue signal (input) 4~20mA or 0~10V,4 routes Can be expanded according to user
25 Analogue signal (output) 4~20mA or 0~10V,2 routes
26 Digital signal (input) Relay passive node,6points
27 Digital signal (output) Relay passive node,5points
28 IP IP20
29 Dimension 3956×1200×2574mm
30 Weight 5000Kg

III. Energy saving analysis
1. VFD parameters
Speed n, flow Q, pressure H, shaft power P
There are relations: Q∝n ,H∝n2,P∝n3
According to fan performance: when system adopts damper control originally, work point of fan moves from A to B. when system adopts VFD, speed is adjusted n' from n0. work point lowers to C from B. Energy saving is shown in following:

According to technology requirement, when fan speed reduces to 80% of rated speed, power of adusting system(VFD+motor) reduces to 51% of that in rated speed, namely 0.83×100%=51%. Consequently, energy saving is considerable when fan speed lowers.
At present, the production target is very heavy. Production line always works in full load, even over load. The output is usually 120-130t/h. In max load, VFD frequency is 37.5Hz,input current is 28A. compared to the other production line without VFD, current is 44A. From the data, we can see evergy saving is obvious.

IV Other benefits
(1) System can implement soft start via VFD. Start current is rated current. Start-up time is prolonged and no big impact on grid and transformer, which reduces mechanical damage and prolongs motor lifetime.
(2) In damper control mode, air flow is adjusted repeatedly. With VFD, user change motor speed in DCS, which reduces labour a lot, heightens efficiency.
(3) Because of cell-cascade technology in VFD, 41 harmonic down is eliminated. Total harmonic is controlled in 2% and current harmonic is less than 4%.
(4) PWM control makes output voltage waveform nearly sine, harmonic less than 1%. Above indexes all satisfy IEEE-519 standard.
(5) Voltage source structure, power factor 0.95.
(6) Noise lowers a lot.

V Concluding remarks
VFD has been running stably since it was installed in May 2005 in production line of 100t/h. It not only saves lots of power, but also gains considerable economic benefits. It also lowers maintenance labour. So Changle Grinding plant and Binzhou Grinding plant all adopt Leader&Harvest VFD. Variable frequency technology will be widely used in more and more cement plants.

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