Harmonic problems of heating noise in inverter application and countermeasures

by:V&T Technologies     2019-11-09
In modern industrial control, the electric drive system using frequency converter to control the motor has remarkable energy saving effect, convenient adjustment and control, simple maintenance, and can be networked and controlled remotely, it can form a control system with PLC and other advantages. These advantages of frequency converters make them increasingly widely used in the field of industrial automatic control. This paper analyzes the fault problem in inverter application and introduces the treatment method. 1 Some problems in Inverter Application 1. 1 harmonic problem, the device that plays a switching role in the main circuit of the inverter must generate harmonics during the process of turning on and off the circuit. Lower harmonics usually have a great impact on the motor load, causing torque pulsation; The higher harmonics increase the leakage current of the inverter output cable, which makes the motor output insufficient. Harmonic interference can also lead to misoperation of relay protection devices, resulting in inaccurate measurement of electrical instruments and even failure to work properly. 1. 2 Noise and vibration problems, using inverter speed regulation, will generate noise and vibration, because the inverter output waveform contains higher harmonic components. With the change of operating frequency, the fundamental wave component and the higher harmonic component all change in a large range, which is likely to resonate with the inherent mechanical vibration frequency of the motor, this kind of resonance is the source of noise and vibration. 1. 3 heating problem, the inverter generates heat due to internal loss during operation. This kind of heat main circuit accounts for 98% and the control circuit accounts for about 2%. At the same time, the ambient temperature is too high in summer, so that the inverter temperature can rise up to 80 ~ 90 ° C, because the inverter is an electronic device, including electronic devices and electrolytic capacitors, the temperature is too high to cause component failure, so that the LCD screen data can not be displayed, the phenomenon of inverter protection action often occurs. Therefore, the harmonics output by the inverter must be suppressed within the allowable range, and noise and vibration must be eliminated or weakened to dissipate heat to extend the service life of the inverter. 2 Analysis and Processing of some problems in Inverter Application 2. 1 The treatment of harmonic problems, the treatment of harmonic problems is to cut off the propagation path of interference and suppress higher harmonics on the interference source. The ways to cut off interference are: 1)Cut off the path of propagation interference from the common grounding wire. The grounding of the power line and the grounding of the control line should be separated, that is, the grounding terminal of the power device is connected to the ground line, and the grounding terminal of the control device is connected to the metal casing of the device disk. 2)The signal line is far away from the interference source current wire. Wiring separation is effective in eliminating this kind of interference, that is, separating high-voltage cables, power cables, control cables from instrument cables and computer cables. The ways to suppress higher harmonics on interference sources are: 1)Increase the internal impedance of the inverter power supply. Generally, the internal impedance of the power supply equipment can play a role in buffering the reactive power of the DC filter capacitor of the inverter. The larger the internal impedance, the smaller the harmonic content. This internal impedance is the short-circuit impedance of the transformer. Therefore, when selecting the inverter power supply, it is best to select a transformer with large short-circuit impedance. 2)Install filter. LC type passive filter is installed in front of the inverter to filter out higher harmonics, usually 5 and 7 harmonics. 3)Install reactor. The line reactor is installed on the front side of the inverter to suppress the overvoltage on the power supply side. 4)Set the active filter. Active filtering automatically generates a current with the same amplitude and the opposite phase as the harmonic current, which can effectively absorb the harmonic current. 2. 2 Handling of noise and vibration problems 1) When the low harmonic component in the inverter output resonates with the inherent mechanical frequency of the rotor, the noise increases; When the higher harmonic components in the inverter output resonate with the iron core, casing, bearing frame, etc. near their respective inherent frequencies, the noise increases. The noise generated by the inverter drive motor, especially the harsh noise, is related to the switching frequency controlled by PWM, especially in the low frequency zone. To solve this problem, the AC reactor is generally connected to the output side of the inverter. If there is a margin for electromagnetic torque, the u/f can be set to be smaller to stabilize and reduce noise. 2) When the frequency converter works, the magnetic field caused by the higher harmonics in the output waveform generates electromagnetic excitation force on many mechanical components, and resonance will occur when the frequency of the excitation force is close to or recombine with the inherent frequency of these mechanical components. The main influence on vibration is the lower harmonic component, which has a great influence on PAM mode and square wave PWM mode. However, when WM is adopted, the low harmonic component is small and the influence is small. The method of reducing or eliminating vibration is to connect a human ac reactor on the output side of the inverter to absorb the higher harmonic current components in the output current of the inverter. When using the PAM mode or the square wave PWM mode inverter, the WM mode inverter can be used to reduce the pulsating torque, which can weaken or eliminate the vibration and prevent the mechanical part from being damaged due to vibration. 2. 3 for the treatment of heating problems, the operating environment temperature of the general inverter is generally required to be-L0 ℃ ~ 50 ℃. In order to ensure the reliable operation of the inverter and prolong the service life of the inverter, the inverter must be cooled. In winter, the internal fan of the inverter can be used to take away the heat inside the inverter box; In summer, the temperature itself is 40 ℃, and the internal heat taken away by the internal fan of the frequency converter can only raise the temperature of the indoor and frequency converter boxes, at this time, the best way is to make a few Φ 500 holes evenly and properly on the upper and lower sides of the wall next to the inverter box in the power distribution room of the machine, and at the same time ensure that there is a certain space around the inverter in the control cabinet, maintain good natural ventilation. If this is not enough, you can turn on the fan, or install an exhaust fan and a wind tunnel in the hole to force the heat generated by the inverter out of the room. Finally, air conditioning can be considered to force the cooling of the space environment where the inverter is installed.
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