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Application of Medium-voltage VFD in Large Medium Pumps for Heavy Medium Coal Selecting Technology

I. Preface
Coal selecting with heavy medium has been applied by coal processing enterprises more and more, because it needs less investment, and has short construction period, high washing and selecting depth, stable product quality and obvious economic benefits, etc. During the production, the speed regulating of the key equipment-medium pumps and the reasonable operation condition of the cyclones are very important.
In 2002, the four medium pumps (the parameters of the pump are listed in the table 1) of Guizhou Shuicheng Mining (Group) Co., Ltd used in the technical reform of Wanjiazhai Coal Selecting Plant and Ertang Coal Selecting Plant were not equipped with speed regulating devices for reason of working deadline. To ensure the product quality, the requirement for the inlet pressure can only be satisfied with regulating the inlet valve and flow diversion. These methods have deteriorated the operating condition of the medium pump. The pump operates under critical conditions or non-working area frequently. The equipment operates with full load for long time, vibrates strongly when operating, has long starting time and serious gasping. Serious water hammer occurs when the pump stopping. In the technology design, sluice valves were not set at the pump outlet. The medium has high specific gravity and high concentration. The water pumping level is high and the working conditions are very poor, making the power supply equipment has high power losses and rather high temperature rise. The equipment wearing is serious and the maintenance load increases. Damage to the pump body sealing happened many times during operation. The bearing damaged, even the impeller of chrome alloy cracked for high temperature and the winding ending of the motor breakdown happened. The normal production was affected considerably. The operation economy cannot be improved effectively with cutting the outside diameter of impeller and modifying the impeller specification to reduce the abundant pumping level (usually the design margin is 5-10%). Therefore, the optimal technology requirements can only be satisfied with speed regulation of the medium pump.

Table 1 Equipment parameters

Medium pump Motor
Manufacturer Shijiazhuang Pump Works Manufacturer Hubei Motor Works
Model ST-AH-14/12 Model Y-500-10
Power 560KW Power 560KW
Flow 1600M3/h Voltage 6KV
Pumping level 55M Current 69A

II. Equipment type selection
There are two schemes of speed regulation for high-voltage high-power AC asynchronous motors of squirrel-cage type usually:
1. VF Speed Regulation, which include High-high and High-low method.
2. Speed Regulation with Hydraulic Couplings
The following is the technology and economy comparison of these three schemes for the convenience of equipment type selection.

Table 2 Comparison of Speed Regulation Schemes

Comparison Items High-high VF Speed Regulation High-low VF Speed Regulation Speed Regulation with Hydraulic Couplings
Initial Investment Higher Medium Lower
Working Period for Installation and Test 10 days 30 days 30 days
Maintenance Load Complicated structure, high technology level Complicated structure, high technology level High
Operating Costs Low Lower High
Economy and Technical Reasonability High Higher Poorer

Comparing with direct high-high VF speed regulation, high-low VF speed regulation has long working period, and need to replace motors and cable as well as add switching cabinet and transformer. At the same time, the electrical technology is unreasonable and need a high initial investment. Therefore, this type is suitable for motors of lower power. The low voltage VF speed regulation is selected for the medium pump (250kW, 380V) of Laoyingshan Coal Selecting Plant. The initial investment is RMB 110,000 Yuan. The VFD model is the 315kW of BT12S series made by Chengdu Senlan Co. Ltd. The voltage of the motor is 380V. Considering the operating condition for several years, it is rather reliable.
Hydraulic couplings, as the speed regulator for medium pumps, have the following features: low initial investment, capability for absorbing shock and twist vibration isolating, flexible connection, more allowable align errors between the active axle and the passive axle. However, it also has disadvantages, such as high maintenance load, limited speed regulating range, lower efficiency, small starting torque, power losses in driving, high daily operating costs, long working period and unable to perform soft starting for the motor, etc.
For medium-voltage VF speed regulation, the working period is short, operating costs is low, closed-loop automatic control can be performed, technical contents are high and fault rate is low. Two motors with one VFD can be performed and it may have bypass of industry frequency. Therefore, the operation reliability is high. It is also an important condition for the production of coal mining. In addition, it has high starting torque, satisfying the technology requirements and adapting to the environment condition with poor working conditions. However, it needs high initial investment.
After considering, it is decided to use medium-voltage VFD in the reform.
The main parameters are compared for the different manufacturers and it is listed in Table 3.

Table 3 Comparison of Technical Specifications for Different Medium-voltage VHD

Comparison Items A foreign manufacturer Beijing Leader & Harvest Electric Technologies Co. Ltd.
Starting Torque 150% 150%
Power Factor >95 >95
Efficiency ≥95 ≥98
Harmonic Contamination Lower Low
Electric Circuit Type Three levels Multilevels
Main Components IGBT IGBT
Motor adapted VF motor Conventional motor
Delivery deadline 6 month 100 days
Life 10 years 15 years
Price High Low

III. Operation of equipment
After comparison between various schemes repeatedly, it is decided to use the high-high VFD made by Beijing Leader & Harvest Electric Technologies Co., Ltd. and the model HARSVERT-A06/070 is selected.
The data for actual operation, the characteristic of pump ST-AH-14/12 and the calculated results using proportion law are listed in Table 4.

Table 4

Pumping level (M) Frequency (Hz) Power Consumed by Speed Regulator(kW) Saved Power(kW)
Before After Before After
VFD 60 50 47 12 55
50 44 23 150
Hydraulic Coupling Before After Speed(rpm)
60 52 585 500 30 40

When the input coal has rather poor quality, the VFD use 47Hz and calculate its economic benefits:
Operating time of the equipment: 20h/day, electricity price RMB 0.4yuan/kWh, 300 days operating yearly, saved electricity cost per year: 55×0.4×20×300=RMB 132,000Yuan
Wear were reduced and the equipment life extends 40,000Yuan/year
For the high reliability of the VFD, the maintenance time and the time for production affected are reduced. The product quality has been raised and the indirect benefits are more obvious. When the input coal has good quality, use frequency of 44Hz. The return period of investment is only two years.

IV. Conclusion
In 2003, Guizhou Shuicheng Mining (Group) Co. Ltd. invited biding and two medium-voltage VFDs made by Beijing Leader & Harvest Electric Technologies Co. Ltd. are selected. Since the VFD installed, it has been operated normally. The economic and technical effects are obvious. The operating condition of medium pumps has been improved considerably and the technical requirements are satisfied. The practice has proved that the project is successful.
From the existing materials, we can see that the medium-voltage VFD is the first option in Guizhou for the VF speed regulation of high-voltage, high-power asynchronous motors for medium pumps in coal mining and also the first option in the same industry in the country. Thus, it has advanced level in the country.

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