Yaskawa inverter - introduction to the basic principle of high-voltage inverter

High-voltage and high-power variable frequency speed control devices are widely used in various fans, pumps, compressors, rolling mills, etc. in large-scale mining production plants, petrochemical industry, municipal water supply, metallurgical steel, power and energy industries.

Pump loads, which are widely used in metallurgy, chemical industry, electric power, municipal water supply and mining, account for about 40% of the energy consumption of the entire electrical equipment, and the electricity bill even accounts for 50% of the cost of water production in the waterworks. This is because: on the one hand, the equipment is usually designed with a certain margin; On the other hand, due to changes in operating conditions, the pump needs to output different flow rates. With the development of market economy and automation, the improvement of intelligence, the use of high-voltage inverter to control the speed of pump load, not only to improve the process, improve product quality is beneficial, but also the requirements of energy saving and economic operation of equipment, is the inevitable trend of sustainable development. The benefits of speed control of pump loads are numerous. From the application examples, most of them have achieved good results (some energy saving as high as 30%-40%), greatly reducing the cost of water production in the waterworks, improving the degree of automation, and is conducive to the pressure reduction operation of pumps and pipe networks, reducing leakage and pipe bursts, and prolonging the service life of equipment.

Adjustment method

Flow regulation method and principle of pump load

Pump loads are usually controlled by the flow of liquid to be conveyed, for which valve control and speed control are often used.

Valve control

This method regulates the flow rate by changing the size of the opening of the outlet valve. It is a mechanical method that has been around for a long time. The essence of valve control is to change the flow rate by changing the magnitude of the fluid resistance in the pipeline. Because the speed of the pump remains the same, the head characteristic curve H-Q remains the same.

When the valve is fully open, the pipe resistance characteristic curve R1-Q and the head characteristic curve H-Q intersect at point A, the flow rate is Qa, and the pump outlet head is Ha. If the small valve is closed, the pipe resistance characteristic curve becomes R2-Q, and the intersection point of it and the head characteristic curve H-Q moves to point B, and the flow rate is Qb, and the pump outlet pressure head is raised to Hb. Then the elevation of the indenter is: ΔHb=Hb-Ha. As a result, the energy loss shown in the black candle section is generated: ΔPb=ΔHb×Qb.

Rotational speed control

The flow rate is regulated by changing the speed of the pump, which is an advanced electronic control method. The essence of speed control is to change the flow rate by changing the energy of the liquid being conveyed. Because only the speed changes, the opening of the valve remains unchanged, and the pipe resistance characteristic curve R1-Q remains unchanged. The head characteristic curve Ha-Q and the pipe resistance characteristic curve at the rated speed intersect at the point A, the flow rate is Qa, and the outlet head is Ha.

When the rotational speed decreases, the head characteristic curve becomes Hc-Q, and the intersection point of it with the pipe resistance characteristic curve R1-Q will move down to C, and the flow will change to Qc. In this case, assuming that the flow Qc is controlled to the flow rate Qb in the valve control mode, the outlet head of the pump will be lowered to Hc. As a result, the head is reduced compared to the valve control method: ΔHc = Ha-Hc. The energy savings are as follows: ΔPc = ΔHc×Qb. Compared with the valve control mode, the energy saved is: P=ΔPb+ΔPc=(ΔHb——ΔHc)×Qb.

Comparing the two methods, it can be seen that under the same flow rate, the speed control avoids the energy loss caused by the increase of the pressure head and the increase of pipe resistance under the control of the valve. When the flow rate is reduced, the speed control reduces the pressure head significantly, so it only needs a much smaller power loss than the valve control to make full use of it.

Efficiency analysis

Analysis of the efficiency of a pump at variable speeds

As the speed decreases, the high-efficiency section of the pump moves to the left. This shows that the speed control mode can still make the pump run efficiently at low speed and low flow.

Research on water supply mode in the state of frequency conversion

In the water supply system composed of multi-point and multi-pumping stations, the pressure head at the outlet of the pumping station needs to be controlled in order to adapt to the pipe network system and achieve better system performance indicators, which can be divided into constant pressure water supply, variable pressure water supply and time-based variable pressure water supply.

Constant pressure water supply

Keeping the outlet head of the pumping station unchanged is the goal of the system's control. The given outlet indenter is Hg.

When the flow rate Q changes, the head characteristic H1-Q moves up and down due to the change of speed, and the working point of the pump will move horizontally (A, B, C, D) on the H=Hg line. Although this satisfies the flow requirements, the energy is wasted because the pipe resistance characteristic R becomes steeper.

The implementation of constant pressure water supply system is more convenient, easy to coordinate with the medium and large pipe network system of water supply of multiple pumping stations, has certain versatility, and practicability, so some waterworks equipped with speed regulating pumps are willing to adopt this method, under the constant pressure control mode, because the pressure head at the outlet of the pumping station remains unchanged, there is a certain gap between the pump parallel characteristics and the actual characteristics of the load, and the energy-saving effect is not as good as the variable pressure water supply system.

Variable pressure water supply method

In order to save energy, the outlet pressure head should be reduced as much as possible with the decrease of flow (at least not increased), and the "variable pressure water supply" mode at the outlet end of the pumping station can be used at this time. Because the head characteristic moves down when the speed decreases, it intersects with the pipe resistance characteristic R1-Q at point C, and the flow rate decreases from Qa to Qc (let the flow rate Qc be equal to the QB when the constant pressure is controlled). The transformer control results in a large differential pressure H=Hac, which saves the energy shown in the female section. Due to the reduction of the outlet pressure head, the variable pressure water supply inhibits the loss that is favored by the change of pipe resistance characteristics and the additional loss of the pump, and the energy saving effect is remarkable.

summary

Through analysis, the inverter can be optimized for water supply in the process of speed regulation of pump loads, and has achieved better power-saving effects.

Yaskawa inverter - introduction to the basic principle of high-voltage inverter

Yaskawa inverter - introduction to the basic principle of high-voltage inverter

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