Common problems and necessary countermeasures for inverter application

In modern industrial control, the electric drive system that uses the inverter to control the motor has significant energy-saving effect, convenient adjustment and control, simple maintenance, centralized remote control by network, and control system with PLC. These advantages of the frequency converter make it increasingly widely used in the field of industrial automation. This paper analyzes the faults in the inverter application and introduces the processing methods.

1, the problem in the application of the inverter

1.1 Harmonic problems

In the main circuit of the inverter, the device functioning as a switch must generate harmonics during the process of switching the circuit. Lower harmonics usually have a greater impact on the motor load, causing torque ripple; while higher harmonics increase the leakage current of the inverter output cable, resulting in insufficient motor output. Harmonic interference can also cause malfunction of the relay protection device, making the electrical meter measurement inaccurate or even unable to work properly.

1.2 Noise and vibration problems

Using frequency converter speed regulation will generate noise and vibration, because the output waveform of the inverter contains higher harmonic components. As the operating frequency changes, the fundamental component and the higher harmonic component vary over 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 fever problem

The inverter generates heat due to internal losses during operation. This heat main circuit accounts for 98%, and the control circuit accounts for about 2%. At the same time, the ambient temperature in the summer is too high, so that the temperature of the inverter rises, and the temperature can be as high as 80~90 °C. Since the inverter is an electronic device, it contains electronic components and electrolytic capacitors. The temperature is too high, which may cause component failure, so that the LCD screen The data cannot be displayed, and the inverter protection action often occurs.

Therefore, the harmonics of the inverter output must be suppressed within the allowable range, while eliminating or attenuating noise and vibration, and dissipating heat from the inverter to extend the life of the inverter.

2. Analysis and treatment of some problems in the application of frequency converter

2.1 Treatment of harmonic problems

The treatment of harmonic problems is to cut off the propagation path of the interference and suppress the higher harmonics on the interference source.

The ways to cut off interference are:

1) The way to cut off the common grounding wire to propagate the 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 ground line, and the grounding terminal of the control device is connected to the metal casing of the device disk.

2) Separation of the wiring of the signal line away from the source of the interference source is effective in eliminating such interference, that is, separating the high-voltage cable, the power cable, the control cable, the instrument cable, and the computer cable separately.

The ways to suppress higher harmonics on the interferer are:

1) Increase the internal impedance of the power supply of the inverter. Usually, the internal impedance of the power supply can buffer 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 of the transformer. impedance. Therefore, when selecting the inverter power supply, it is best to choose a transformer with a short circuit impedance.

2) Install the filter Install an LC-type passive filter in front of the inverter to filter out the higher harmonics, usually filtering out the 5th and 7th harmonics.

3) Install the reactor Install the line reactor on the front side of the inverter to suppress the overvoltage on the power supply side.

4) Setting the active filter Active filtering is to automatically generate a current with the same amplitude and opposite phase as the harmonic current, so that the harmonic current can be effectively absorbed.

2.2 Treatment of noise and vibration problems

1) When the low-order harmonic component of the inverter output resonates with the inherent mechanical frequency of the rotor, the noise increases; when the higher harmonic component of the inverter output is connected with the iron core, the casing, the bearing frame, etc. When resonance occurs near the respective natural frequencies, the noise increases.

The noise generated by the drive motor of the inverter, especially the harsh noise, is related to the switching frequency of the PWM control, especially in the low frequency region. To solve this problem, an AC reactor is usually connected to the output side of the inverter. If the electromagnetic torque has a margin, the u/f setting can be set smaller to smooth and reduce the noise.

2) When the inverter is working, the magnetic field caused by the higher harmonics in the output waveform generates electromagnetic force for many mechanical components. When the frequency of the power is close to or coincides with the natural frequency of these mechanical components, resonance will occur. The main influence on the vibration is the lower harmonic component, which has 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 small.

The method of mitigating or eliminating vibration is to connect an AC reactor on the output side of the inverter to absorb the higher harmonic current component in the inverter output current. When the inverter is used in PAM mode or square wave PWM mode, the SPWM mode inverter can be used to reduce the pulsating torque, so that the vibration can be weakened or eliminated, and the mechanical part can be prevented from being damaged by vibration.

2.3 Treatment of fever problems

The operating environment temperature of general-purpose inverters is generally required to be -10 °C ~ +50 °C. In order to ensure reliable operation of the inverter and extend the service life of the inverter, the inverter must be dissipated. In the winter, the internal fan of the inverter can be used to take away the heat inside the inverter box; the summer temperature itself has 40 °C, and the internal heat taken by the built-in fan of the inverter can only increase the temperature of the indoor and inverter boxes. At this time, the best way is to use the window or the machine power distribution room to properly and properly play a few φ500mm holes in the upper and lower walls of the inverter box, and ensure that there is a certain space around the inverter inside the control cabinet. Good natural ventilation. If this is not possible, you can turn on the fan, or add an exhaust fan and air duct to 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 in which the inverter is installed.

3, the conclusion

It is necessary to strengthen the research on the fault problem in the application of the frequency converter. This is of great significance for the normal use of the frequency converter, potential gain and efficiency.

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