In recent years, although China has made rapid progress in energy development, it still cannot keep up with the growth in demand, and energy conservation remains a prominent issue. Frequency converters are a concrete manifestation of power electronics science; they are electronic circuit devices that utilize the switching action of power electronic semiconductor devices to achieve high-power conversion and control of electrical energy, that is, the electronicization of power circuits.
Foreword
While most people are familiar with variable frequency drive (VFD) technology, they generally associate it with energy conservation. In recent years, despite rapid progress in energy development in my country, it still lags behind the growth in demand, and energy conservation remains a prominent issue. Xuan Steel Plant, after numerous theoretical verifications and applications in its No. 5 and No. 6 converters, has adopted a large number of VFDs in its 150T converter dust collectors, water pumps, and vibrating feeders, making a significant contribution to my country's energy conservation efforts.
1. Working principle of frequency converter
A frequency converter mainly consists of modules, a CPU control board, and a power drive board assembly.
We know that the synchronous speed expression of an AC motor is: n = 60f(1-s)/p (1), where n is the speed of the asynchronous motor; f is the frequency of the asynchronous motor; s is the slip of the motor; and p is the number of pole pairs of the motor. From equation (1), it can be seen that the speed n is directly proportional to the frequency f. As long as the frequency f is changed, the speed of the motor can be changed. When the frequency f changes within the range of 0 to 50 Hz, the speed adjustment range of the motor is very wide. The frequency converter achieves speed regulation by changing the power supply frequency of the motor. It is an ideal high-efficiency and high-performance speed regulation method. It would be a stretch to assume that the motor can save energy as long as a frequency converter is used.
Frequency converters are a concrete manifestation of power electronics science. They are electronic circuit devices that utilize the switching action of power electronic semiconductor devices to achieve high-power conversion and control of electrical energy. In other words, they digitize power circuits, allowing for intuitive control and display. Due to this advantage, frequency converters have increasingly wider applications, and the technologies employed are constantly expanding. Simultaneously, in pursuit of miniaturization, researchers have continuously strived to reduce the heat generated by components. Because the new generation of IGBTs employs a drain-control electrode technology, the collector-emitter saturation voltage (Ucesat) is significantly reduced. Therefore, using this new device results in lower losses and effectively reduces heat generation, thus eliminating energy consumption.
380V small-capacity general-purpose frequency converters are currently widely used, but they consume a relatively large amount of electricity. High-voltage, large-capacity frequency converters offer the most significant energy-saving performance. With the development of frequency conversion speed control technology, high-voltage frequency conversion speed control technology, as a large-capacity transmission method, has been widely applied, covering almost all major industries in my country, such as power, metallurgy, petroleum, chemical, and papermaking. Product voltage levels include 3kV, 6kV, and 10kV, as well as 1,600V-2,400V products for oilfield submersible pumps, which can basically drive various loads such as fans, pumps, and compressors.
High-voltage motors can achieve stepless speed regulation using high-voltage frequency converters, which not only meets the speed control requirements of production processes but also significantly saves energy and reduces production costs. With the development of self-turn-off devices such as GTOs, IGBTs, and IGCTs, research on various main circuit topologies suitable for high-voltage frequency converters has deepened. The rapid popularization of intelligent control chips, represented by DSPs, has also laid a solid foundation for the application research of high-voltage variable frequency speed control technology. As a resource-saving product, high-voltage frequency converters will show enormous technological development prospects and market demand as power electronics technology continues to develop, products are continuously improved, concepts deepen, and the market expands. Correspondingly, the energy-saving requirements for frequency converters are also increasing.
2. Energy-saving principle of frequency converters
2.1 Variable Frequency Energy Saving
The energy-saving effect of frequency converters is mainly reflected in the application of fans and water pumps. Xuan Steel's steelmaking plant has a large number of fan and water pump loads. According to fluid mechanics, for these loads, p (power) = Q (flow rate) × H (pressure). Flow rate Q is proportional to the first power of speed N, pressure H is proportional to the square of speed N, and power P is proportional to the cube of speed N. If the efficiency of the water pump is constant, when the required flow rate is reduced, the speed N can be reduced proportionally, and the shaft output power P decreases cubically. That is, the power consumption of the water pump motor is approximately cubically proportional to the speed. Therefore, when the required flow rate Q decreases, the output frequency of the frequency converter can be adjusted to reduce the motor speed n proportionally. Speed control can still allow the pump to operate efficiently at low speeds and low flow rates. At this time, the motor power p will decrease significantly according to a cubic relationship, saving 40% to 50% more energy than adjusting baffles and valves, thus achieving the purpose of energy saving.
To ensure production reliability, all production machinery is designed with a certain margin of safety in its power drive system. When the motor cannot operate at full load, the excess torque beyond meeting the power drive requirements increases active power consumption, resulting in energy waste. Traditional speed control methods for equipment such as fans and pumps adjust the airflow and water flow by regulating the opening of inlet or outlet baffles and valves. This method has high input power, and a significant amount of energy is consumed in the flow throttling process of the baffles and valves. When using variable frequency speed control, if the flow requirement decreases, the requirement can be met by reducing the speed of the pump or fan. For example, a centrifugal pump motor with a power of 55 kW consumes 28.16 kW of electricity when the speed is reduced to 4/5 of its original speed, saving 48.8% of electricity; when the speed is reduced to 1/2 of its original speed, the electricity consumption is 6.875 kW, saving 87.5% of electricity.
2.2 Power Factor Compensation Energy Saving
Reactive power not only increases line losses and equipment heating, but more importantly, the reduction in power factor leads to a decrease in the active power of the power grid. A large amount of reactive power is consumed in the lines, resulting in low equipment efficiency and serious waste. After using a variable frequency speed control device, the reactive power loss is reduced and the active power of the power grid is increased due to the effect of the filter capacitor inside the frequency converter.
2.3 Soft start energy saving
Hard starting of motors causes severe impacts on the power grid and places excessive demands on grid capacity. The large current and vibration generated during startup cause significant damage to baffles and valves, severely shortening the lifespan of equipment and pipelines. However, using a variable frequency drive (VFD) energy-saving device utilizes the VFD's soft-start function to ensure the starting current starts from zero, with a maximum value not exceeding the rated current. This reduces the impact on the power grid and the demand on power supply capacity, extending the lifespan of equipment and valves and saving on equipment maintenance costs.
3. Conclusion
The energy-saving effect of variable frequency speed control lies primarily in saving the electrical energy wasted during full-speed operation. Especially in closed-loop speed control systems, such as constant pressure water supply systems, on-demand drive is achieved, almost completely eliminating waste during the operation of the drive system. This achieves energy saving on a large scale, but it doesn't mean that the energy-saving potential has been fully tapped. In fact, in many situations, there is still the phenomenon of oversized motors pulling small loads, indicating significant potential in this area. It's an undeniable fact that frequency converters can save electricity, sometimes by more than 40%, but in other situations, it can be more wasteful than not using a frequency converter at all! Frequency converters achieve energy savings by reducing voltage under light loads. For loads with low torque, since the speed doesn't change much, even with voltage reduction, the energy saving is minimal. However, the situation is different in fan environments. When a smaller airflow is needed, the motor speed decreases. We know that fan energy consumption is proportional to the 1.7th power of the speed, so the motor torque drops sharply, resulting in significant energy savings.
Of course, if environmental requirements necessitate speed regulation, the energy-saving effect of frequency converters is quite significant. In situations where speed regulation is not required, frequency converters will not save electricity, but only improve the power factor. In fact, the specific effects achieved by frequency converters and even the power electronics industry are as follows: 1) Further improving power conversion efficiency and reducing standby losses. 2) Avoiding electrical pollution, minimizing current harmonics, and improving the power factor. 3) Improving the electromagnetic compatibility of power supply devices and systems. 4) Reducing electrical noise. 5) Achieving high-performance controllability. The dust removal fans, water systems, and feeding systems of Xuan Steel's steelmaking plant have also adapted to these characteristics of frequency converters. After multiple theoretical verifications, frequency converters were adopted, making a certain contribution to my country's energy conservation efforts.
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