Can energy be saved just by using an inverter?

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This post was last edited by yunrun on 2015-12-17 at 14:44. The working principle of an inverter is as follows: the power input circuit rectifies the incoming alternating current into direct current. The power output circuit, based on instructions from the control unit, monitors, compares, and processes relevant operating data, issues control commands, and converts the rectified direct current into alternating current of a certain frequency for supply to the motor. Reasons for energy savings with frequency converters: The output frequency of a frequency converter can vary between 0–50 Hz or even higher frequencies. As can be learned from the technical article “How to Understand the U_f Curve of Inverters,” a decrease in the power supply frequency leads to a corresponding drop in the power supply voltage, which reduces the motor’s instantaneous power and thus lowers power consumption. Under such circumstances, variable frequency drives can help save energy, but this energy-saving effect is conditional and depends on the application and the characteristics of the load driven by the motor. Energy savings through speed regulation are feasible in the drive of equipment such as pumps, fans, and compressors. The load torque of these devices is proportional to the square of the speed, while power is proportional to the cube of the speed; when the speed is reduced by 1/2, the load torque becomes only 1/4, and the power drops to just 1/8. Such equipment is designed to allow for production and further development; the capacity of its drive motors is usually selected based on the maximum flow rate and pressure. Flow rate or pressure is typically regulated through valves or dampers, which results in energy waste. By using an inverter to reduce the motor speed, it is possible to regulate the flow rate or pressure, thereby achieving energy savings. According to statistical data, an average energy savings of 1/3 can be achieved; due to its significant effects, it has been widely adopted. Inverters used for fans and pump loads can help save energy, but due to design or equipment limitations, if the fans or pumps operate at full load all the time, the output frequency of the inverter remains at 50Hz. In such a case, it is impossible to talk about energy savings, as there is no difference from operating at the standard frequency. Energy savings can only be achieved when the actual flow rate of air or water is less than 100% of the rated flow rate. Engineers from Changhui Instruments once compared the power consumption of two 15kW fans when operating at the mains frequency and at variable frequency; each fan was run for 2 hours, and the electricity meter showed that the variable frequency operation resulted in an additional 1 kWh of energy consumption compared to operation at the mains frequency. This is considered normal, as the inverter itself also consumes power. Strictly speaking, there is still some error in the measurement; it’s a bit less than 1 degree. Such measurements also have problems, as the output current of the inverter is not pure mains frequency; it contains higher harmonics, and ordinary instruments cannot measure it accurately. The output current of the inverter is more reliable when based on the current reading displayed on the inverter panel. Therefore, it is untrustworthy to claim that using an inverter can save energy, regardless of the load characteristics or application scenario. Moreover, using measurement data from ordinary instruments for energy-saving comparisons makes the promotional claims unreliable as well. In some cases, variable frequency drives are used first and foremost because the process requires motor drive and speed control, with energy savings being merely a side benefit. Furthermore, the return on investment cannot be ignored; the smaller the motor, the more pronounced this issue becomes. Can an inverter be used as a variable-frequency power supply? An inverter cannot replace a variable-frequency power supply! This is because the structure, principles, and applications of the two are different. An inverter is composed of circuits such as AC-DC-AC (modulated wave), and its output voltage has a pulse square wave shape with many harmonic components. Both the voltage and frequency change proportionally to each other, and they cannot be adjusted separately, which does not meet the requirements of an AC power supply. In principle, it cannot be used as a power supply; inverters are used to control the speed of three-phase asynchronous motors. A variable-frequency power supply is composed of circuits such as AC-to-DC conversion, DC-to-AC conversion, and filtering. The voltage and current waveforms it outputs are sine waves, which are very similar to those of an AC power supply; moreover, it can output any voltage and frequency as required by the user. It is itself used as a power supply.