In modern industrial control, the use of frequency converter to control the electric drive system of the electric motor has the advantages of significant energy saving effect, convenient adjustment and control, simple maintenance, network centralized remote control, and PLC control system. These advantages of the inverter make it increasingly widely used in the field of industrial automation. This article analyzes the problems in the application of frequency converters and introduces the processing methods.
1 Some Problems in Frequency Converter Application
1.1 Harmonic issues
In the main circuit of the frequency converter, the device that acts as a switch must generate harmonics in the process of turning the circuit off and on. The lower harmonics usually have a greater influence on the motor load and cause torque ripple. Higher harmonics increase the leakage current of the output cable of the inverter, which results in insufficient motor output. Harmonic interference can also lead to malfunctions of relay protection devices, making the metering of electrical instruments inaccurate or even impossible.
1.2 Noise and Vibration Issues
Using the inverter to regulate the speed will produce noise and vibration, because the inverter output waveform contains higher harmonic components. With the change of operating frequency, the fundamental wave components and higher harmonic components all change in a wide range, and it is likely to resonate with the inherent mechanical vibration frequency of the motor, which is the source of noise and vibration.
1.3 Heating problems
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 temperature of the inverter rises and the temperature can reach as high as 80~90°C. Since the inverter is an electronic device containing electronic devices and electrolytic capacitors, the temperature is too high and the components can fail, causing the LCD screen Data cannot be displayed, and inverter protection actions often occur.
Therefore, the harmonics of the inverter output must be suppressed within the permissible range, and at the same time, the noise and vibration must be eliminated or reduced to dissipate the heat of the inverter to prolong the service life of the inverter.
2 Analysis and Handling of Some Problems in Frequency Converter Application
2.1 Handling of Harmonic Problems
The treatment of harmonic problems is to cut off the transmission path of interference and suppress the higher harmonics on the interference source.
The ways to cut off the interference are:
1) Ways to cut off the common ground wire to spread interference The grounding of the power line and the grounding of the control line should be separated, that is, the ground terminal of the power device is connected to the ground wire, and the ground terminal of the control device is connected to the metal shell of the device disk.
2) Separation of the signal line from the interference source current is effective for eliminating such interference, that is, the high voltage cable, the power cable, the control cable, the instrument cable, and the computer cable are separately routed.
The ways to suppress the higher harmonics on the interference source are:
1) Increase the internal impedance of the power supply of the inverter Usually the internal impedance of the power supply can act to buffer the reactive power of the DC filter capacitor of the inverter. The greater the internal impedance, the smaller the harmonic content. This internal impedance is the short circuit of the transformer. impedance. Therefore, when choosing the inverter power supply, it is better to choose a transformer with a large short circuit impedance.
2) Install the filter In front of the inverter, install an LC type passive filter to filter out higher harmonics, usually filtering out the 5th and 7th harmonics.
3) Install reactors Install line reactors on the front side of the inverter to suppress overvoltage on the power supply side.
4) Setting up the active filter The active filter automatically generates a current with the same amplitude and opposite phase as the harmonic current so that the harmonic current can be effectively absorbed.
2.2 Handling of Noise and Vibration Problems
1) When the harmonic components of the frequency converter output resonate with the inherent mechanical frequency of the rotor, the noise increases; when the harmonic components of the inverter output are in the core, the casing, the bearing frame, etc. When resonance occurs near the respective natural frequency, the noise increases.
The noise generated by the drive motor of the inverter, especially harsh noise, is related to the switching frequency of the PWM control, especially in the low frequency region. To solve this problem, connect the AC reactor to the output side of the inverter. If the electromagnetic torque has a margin, u/f can be set smaller to suppress and reduce noise.
2) When the inverter is operating, the magnetic field caused by the higher harmonics in the output waveform generates electromagnetic force for many mechanical components. The resonance occurs when the frequency of the pilot power is close to or coincides with the natural frequency of these mechanical components. The major influence on the vibration is the lower harmonic components, which have a greater influence on the PAM mode and the square-wave PWM mode. However, when the SPWM method is used, the low-order harmonic components are small and the influence is also reduced.
The method of reducing or eliminating the vibration is to connect the AC reactor at the output side of the inverter to absorb the higher harmonic current components in the output current of the inverter. When using the PAM or square-wave PWM inverter, the SPWM inverter can be used to reduce the pulsating torque, which can reduce or eliminate the vibration and prevent the mechanical part from being damaged due to vibration.
2.3 Treatment of heat problems
The general operating environment temperature of the inverter is generally required to be between -10°C and +50°C. In order to ensure the reliable operation of the frequency converter and extend the service life of the frequency converter, the inverter must be cooled. In winter, the built-in fan of the inverter can be used to take away the heat inside the inverter cabinet; the summer temperature itself has 40°C, and the internal heat taken away by the built-in fan of the inverter can only increase the temperature of the indoor and inverter cabinets. At this time, the best way is to use windows or in the machine power distribution room immediately below the wall of the inverter box and play a few φ500mm evenly and properly, while ensuring that there is a certain space around the inverter in the control cabinet. Good natural ventilation. If this is not enough, you can turn on the fan, or install an exhaust fan and air duct at the hole to force the heat generated by the inverter to be forced out of the room. Finally, air conditioning may be used to force cooling of the space environment where the inverter is installed.
3 Conclusion
It is necessary to strengthen the research on faults in the application of frequency converters. This is of great significance to the normal use of frequency converters and tapping potential and increasing efficiency.
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