Reactors in the inverter system - Database & Sql Blog Articles

1 Introduction

The rapid development of power electronics technology has made AC frequency conversion speed regulation the mainstream of electric transmission, and its application range is increasingly wide. However, since the inverter is used in various electrical environments, the inverter does not take appropriate protective measures, which will affect the stability and reliability of the inverter operation. It turns out that proper selection of distribution transformers and inverters can effectively prevent the impact of overvoltage and surge current generated by operating AC inlet switches, and at the same time reduce harmonic interference from the power grid. It can reduce the pollution of the grid generated by the harmonics generated by the inverter and improve the power factor of the inverter. Therefore, it is necessary to discuss the functions of various types of reactors used in inverters and the selection of reactors.

2 There are three types of reactors for use with the inverter: 1) The input reactor ACL, also known as the input reactor, can limit the current surge caused by sudden changes in the grid voltage and operating overvoltage, suppress the mutual transmission of harmonics, and effectively protect Inverter and improve its power factor. The harmonic current input to the grid when the inverter is connected to and disconnected from the line reactor is shown in Figure 1. It can be seen from Fig. 1 that the harmonic wave can be effectively suppressed after the reactor is connected. 2) The output reactor OCL, also called the line reactor, is connected between the output end of the inverter and the load (motor), which can passivate the steepness of the inverter output voltage (switching frequency) and reduce the power in the inverter. The disturbance and impact of the component can also reduce the overvoltage of the motor winding caused by the cable capacitance, which can extend the distance between the inverter and the motor, improve the service life of the motor, and suppress the noise of the inverter. 3) The smoothing reactor DCL is also called DC reactor. It is connected between the DC rectification link and the inverter link of the inverter system. DCL can make the inverter link run more stably and improve the power factor of the inverter. The connection of the three reactors in the inverter is shown in Figure 2. 3 When the line reactor needs to be installed, the line reactor can prevent harmonic interference from the power grid and reduce the pollution of the grid caused by the harmonic current generated by the rectifier unit. Adding a line reactor can reduce the current rise rate, smooth the spikes contained in the power supply voltage, or the voltage sag caused by the smoothing of the bridge rectifier circuit, which will greatly reduce the fault of the inverter. Therefore, access to the line reactor is beneficial to improve the operation of the inverter. According to the operating experience, the inverter must be installed with a line reactor to ensure reliable operation of the inverter. Selection of 4 Line Reactors The selection of line reactors can be determined as expected from the voltage drop across the windings of each phase of the reactor. Generally, the voltage drop is selected to be 1.5% to 3% of the phase voltage of the grid side, and can also be selected according to the data specified by the inverter. The inductance L of the reactor is calculated by the voltage current method. 5 Output reactor selection The output reactor selection can be determined as expected on the voltage drop across each phase winding of the reactor. Generally, the voltage drop is selected to be 0.5% to 2% of the phase voltage of the grid side. It can also be selected according to the data specified by the inverter. At the same time, the distance between the inverter and the motor should be considered to select the capacitance of the cable. The selection of 6 smoothing reactors In the frequency conversion system with DC link, the DC reactor can be connected after the rectifier to effectively improve the power factor. When properly matched, the power factor can be increased to 0.95. In addition, the DC reactor can make the inverter The device is stable in operation and can limit the short-circuit current. Therefore, inverters of more than 55kW produced by many manufacturers are supplied with DC reactors. 7 Inductance calculation of three-phase AC line reactor When the rated voltage drop ΔUL of the reactor is selected and the rated operating current In is determined, the inductance L of the reactor can be calculated. Reactor inductance L = ΔUL / (2πfIn) where: ΔUL - unit V; f - power frequency (Hz); Note: the inductance of the AC reactor with iron core should not be measured by the bridge, because the measurement The power supply frequency of the inductor bridge is generally 1000Hz, so the inductor bridge can only be used to measure the air core reactor. For AC reactors made of silicon steel sheets, the inductance can be measured by the AC voltmeter of the power frequency source, the ammeter method. 8 Conclusion Three kinds of reactors have a great effect on the stable and reliable operation of the inverter system. Among them, the line reactor has the largest effect, and the line reactor is usually required. The domestic reactors are of good quality and low cost, which is worthy of promotion and application.