First, some problems in the application of frequency converter
1.1
Harmonic problems
The devices that play a switching role in the main circuit of the inverter must generate harmonics in the process of turning on and off the circuit. The lower harmonics usually have a greater impact on the motor load, causing torque ripple; The higher harmonics increase the leakage current of the output cable of the inverter, so that the motor output is insufficient. Harmonic interference can also lead to the malfunction of the relay protection device, so that the electrical meter measurement is inaccurate or even unable to work normally.
1.2
Noise and vibration issues
The use of frequency converter speed regulation will produce noise and vibration, because the output waveform of the frequency converter contains higher harmonic components. With the change of operating frequency, the fundamental component and the higher harmonic component change in a wide range, and it is likely to resonate with the natural mechanical vibration frequency of the motor, which is the source of noise and vibration.
1.3
Fever problems
The inverter generates heat due to internal loss during operation, and this heat accounts for 98% of the main circuit and about 2% for the control circuit. At the same time, the ambient temperature is too high in summer, so that the temperature of the inverter rises, the temperature can be as high as 80-90 °C, because the inverter is an electronic device, containing electronic devices and electrolytic capacitors, etc., the temperature is too high to cause the failure of components, so that the LCD screen data can not be displayed, and the phenomenon of inverter protection action often occurs.
Therefore, the harmonics output by the inverter must be suppressed within the allowable range, and the noise and vibration must be eliminated or weakened at the same time, and the heat dissipation of the inverter must be carried out to prolong the service life of the inverter.
2. Analysis and treatment of some problems in the application of frequency converter
2.1
Dealing with harmonic problems
The problem of harmonics is to cut off the propagation path of interference and suppress the higher harmonics on the source of interference.
Interrupting the propagation of interference can be by:
1) cut off the way of the common grounding wire propagation interference, the grounding of the power line and the grounding of the control line should be separated, that is, the grounding terminal of the power unit is connected to the grounding wire, and the grounding terminal of the control device is connected to the metal shell of the device disk.
2) The separation of the conductor wiring of the signal line away from the interference source current is effective in eliminating this interference, that is, the high-voltage cable, power cable, control cable and instrument cable and computer cable are separated from the wiring.
The ways to suppress higher harmonics on the interferer source are:
1) Increase the internal impedance of the power supply of the inverterUsually the internal impedance of the power supply equipment can play the role of 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 choosing the power supply of the inverter, it is best to choose a transformer with a large short-circuit impedance.
2) Install the filter before the inverter to install the LC type passive filter to filter out the higher harmonics, usually the 5th and 7th harmonics.
3) Install the reactor and install the line reactor on the front side of the inverter, which can suppress the overvoltage on the power supply side.
4) Set 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
Dealing with noise and vibration problems
1) When the lower harmonic component in the output of the inverter resonates with the natural mechanical frequency of the rotor, the noise increases; When the higher harmonic components in the output of the inverter resonate with the core, casing, bearing frame, etc., near their respective natural frequencies, the noise increases.
The noise generated by the inverter-driven motor, especially the harsh noise, is related to the switching frequency controlled by the PWM, especially in the low-frequency region. To solve this problem, an 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 smaller to smooth out and reduce noise.
2) When the inverter is working, the magnetic field caused by the higher harmonics in the output waveform produces electromagnetic driving force for many mechanical parts, and the frequency of the driving force is close to or coincides with the natural frequency of these mechanical parts. The lower harmonic components have a greater impact on vibration, and have a greater impact on PAM and square wave PWM. However, when the SPWM method is used, the harmonic components of the lower order are small, and the influence is also smaller.
The way to reduce or eliminate vibration is to connect a human AC reactor on the output side of the inverter to absorb the higher harmonic current components of the inverter's output current. When using a PAM or square wave PWM inverter, the SPWM inverter can be used to reduce the ripple torque, which can reduce or eliminate the vibration and prevent the mechanical part from being damaged due to vibration.
2.3
Treatment of fever problems
The operating environment temperature of the general-purpose inverter is generally required to be -l0°C - 50°C. In order to ensure that the inverter works reliably and prolongs the service life of the inverter, the inverter must be dissipated. In winter, the built-in fan of the inverter can be used to take away the heat inside the inverter box; The temperature itself is 40 °C in summer, and the internal heat taken away by the built-in fan of the inverter can only increase the temperature of the room and the inverter box, and the best way at this time is to use the window or the wall of the inverter box in the distribution room close to the inverter box to evenly and appropriately make a few holes of φ500mm, and at the same time ensure that there is a certain space around the inverter in the control cabinet to maintain good natural ventilation. If this is not possible, 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 out of the room. Finally, air conditioning can be considered to force cooling the space environment where the inverter is installed.
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