Frequency inverter for heavy duty application (mining)

Coal selecting with heavy medium pump in mining industry 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-heavy medium pumps and the reasonable operation condition of the cyclones are very important.

In 2002, the four heavy medium pumps (the parameters of the pump are listed in the table 1) of a Mining company used in the technical reform of 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 heavy 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 heavy medium pump 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 heavy medium pump.

Table 1 Equipment parameters

Heavy medium pump		Motor
Power	560KW	Power	560KW
Flow	1600M3/h	Voltage	6KV
Pumping level	55M	Current	69A

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 heavy medium pump (250kW, 380V) of the Coal Selecting Plant. The initial investment is RMB 110,000 Yuan. The frequency inverter is 315kW. 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 heavy 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 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 frequency inverter 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 frequency inverter 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 frequency inverters

Comparison Items	A foreign manufacturer	Shenzhen Gozuk
Starting Torque	150%	150%
Power Factor	>95	>95
Efficiency	≥95	≥98
Harmonic Contamination	Lower	Low
Electric Circuit Type	Three levels	Multi-levels
Main Components	IGBT	IGBT
Motor adapted	VF motor	Conventional motor
Delivery deadline	6 month	100 days
Life	10 years	15 years
Price	High	Low

Operation of equipment
After comparison between various schemes repeatedly, it is decided to use the high-end frequency inverter.
The data for actual operation, the characteristic of pump 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
frequency inverter	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 frequency inverter 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 frequency inverter, 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.

Conclusion
In 2003, a Mining company invited biding and two frequency inverters are selected. Since the frequency inverter installed, it has been operated normally. The economic and technical effects are obvious. The operating condition of heavy 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 frequency inverter is the first option for the VF speed regulation of high-voltage, high-power asynchronous motors for heavy 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.