Abstract: Frequency converters are a concrete manifestation of power electronics science. They are electronic circuit devices that use the switching action of power electronic semiconductor devices to realize the high-power conversion and control of electrical energy. This article briefly describes the working principle and energy-saving principle of frequency converters applied to electric motors.
Keywords: frequency converter, electric motor, energy-saving principle
0 Introduction
While most people are familiar with variable frequency drive (VFD) technology, they often 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, making energy conservation a persistently 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
The frequency converter mainly consists of modules, a CPU control board, and a power drive board, as shown in the figure above.
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. The frequency converter is a concrete manifestation of power electronics science. It is an electronic circuit device that uses the switching action of power electronic semiconductor devices to realize the high-power conversion and control of electrical energy. That is, the power circuit is electronic, and it can be directly controlled and displayed. Due to the superior performance of frequency converters, their application fields are becoming increasingly wider, and the technologies employed are constantly expanding. At the same time, in pursuit of miniaturization of frequency converters, people are continuously striving to reduce the heat generated by components. Because the new generation of IGBTs adopts a new drain-controller technology, the collector-emitter saturation voltage (Ucesat) is greatly reduced. Therefore, using this new device results in low losses and effectively reduces heat generation and eliminates losses.
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 effect. With the development of frequency conversion speed regulation technology, high-voltage frequency conversion speed regulation 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 used in 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, meeting the speed control requirements of production processes while significantly saving energy and reducing 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 conversion has been further deepened. The rapid popularization of intelligent control chips, represented by DSPs, has also laid a solid foundation for the application research of high-voltage frequency conversion speed regulation technology. As a resource-saving product, high-voltage frequency converters will show tremendous technological development prospects and market demand as power electronics technology continues to develop, products are constantly improved, concepts become more sophisticated, 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 at this time, 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, various production machinery is designed with a certain margin of safety when using power drives. When a 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 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 during 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 pump or fan speed. For example, a centrifugal pump motor with a power of 55 kW consumes 28.16 kW of electricity when its speed is reduced to 4/5 of its original speed, saving 48.8%; when the speed is reduced to 1/2 of its original speed, its electricity consumption is 6.875 kW, saving 87.5%.
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 internal filter capacitor of 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 service life 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 service life 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 energy waste during operation. This achieves energy saving on a large scale, but it doesn't mean the energy-saving potential has been fully realized. In fact, in many situations, there is still the phenomenon of oversized motors pulling small loads, indicating significant untapped potential. It's an undeniable fact that frequency converters can save electricity, sometimes exceeding 40%, but in other situations, it can be more wasteful than not using a frequency converter at all! Frequency converters achieve energy savings through voltage reduction 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 the environment requires speed adjustment, the energy-saving effect of frequency converters is still quite noticeable. In situations where speed regulation is not used, frequency converters do not save electricity; they 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 power 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 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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