In modern industrial control, electric drive systems using frequency converters to control motors offer significant energy savings, convenient adjustment and control, simple maintenance, networked centralized and remote control capabilities, and the ability to form control systems with PLCs. These advantages have led to the increasingly widespread application of frequency converters in industrial automation control. This article analyzes common faults encountered in frequency converter applications and introduces solutions.
1. Some problems in the application of frequency converters
1.1 Harmonic Problems
The switching devices in the main circuit of a frequency converter generate harmonics during the switching process. Lower-order harmonics usually have a greater impact on the motor load, causing torque pulsation; while higher-order harmonics increase the leakage current of the frequency converter's output cable, resulting in insufficient motor output. Harmonic interference can also cause malfunctions of relay protection devices, inaccurate measurement of electrical instruments, or even failure to function properly.
1.2 Noise and Vibration Issues
Using a frequency converter for speed control will generate noise and vibration because the output waveform of the frequency converter contains high-order harmonic components. As the operating frequency changes, the fundamental component and high-order harmonic components vary over a wide range, which may resonate with the inherent mechanical vibration frequency of the motor, and this resonance is the source of noise and vibration.
1.3 Overheating Issues
Inverters generate heat during operation due to internal losses, with the main circuit accounting for 98% of this heat and the control circuit accounting for about 2%. Additionally, in summer, high ambient temperatures cause the inverter temperature to rise, reaching as high as 80-90℃. Since inverters are electronic devices containing electronic components and electrolytic capacitors, excessively high temperatures can easily cause component failure, making the LCD screen unable to display data and frequently triggering inverter protection trips.
Therefore, it is necessary to suppress the harmonics output by the frequency converter within the allowable range, while eliminating or reducing noise and vibration, and cooling the frequency converter to extend its service life.
Analysis and handling of some problems in the application of frequency converters
2.1 Handling Harmonic Problems
The solution to harmonic problems is to cut off the propagation path of interference and suppress higher harmonics at the source of interference.
The transmission pathways of interference can be cut off as follows:
1) Cut off the path of interference propagation through the shared 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 should be connected to the grounding wire, and the grounding terminal of the control device should be connected to the metal casing of the device panel.
2) Separating the signal lines from the current source of interference is an effective way to eliminate this interference. That is, separate the high-voltage cables, power cables, control cables from the instrument cables and computer cables.
Methods for suppressing higher harmonics on interference sources include:
1) Increase the internal impedance of the inverter power supply. The internal impedance of the power supply equipment typically buffers the reactive power of the inverter's DC filter capacitor. The higher the internal impedance, the lower the harmonic content. This internal impedance is the short-circuit impedance of the transformer. Therefore, when selecting a power supply for the inverter, it is best to choose a transformer with a high short-circuit impedance.
2) Install a filter: Add an LC type passive filter before the frequency converter to filter out high-order harmonics, usually the 5th and 7th harmonics.
3) Installing a line reactor: Installing a line reactor on the front side of the frequency converter can suppress overvoltage on the power supply side.
4) Setting an active filter: An active filter automatically generates a current with the same amplitude but opposite phase to the harmonic current, thereby effectively absorbing the harmonic current.
2.2 Handling of Noise and Vibration Problems
1) When the low-order harmonic components in the inverter output resonate with the rotor's natural mechanical frequency, the noise increases; when the high-order harmonic components in the inverter output resonate with the iron core, housing, bearing bracket, 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 of the PWM control, and is particularly noticeable in the low-frequency range. To solve this problem, an AC reactor is generally connected to the output side of the inverter. If there is sufficient electromagnetic torque margin, the u/f ratio can be set smaller to smooth out and reduce the noise.
2) When the frequency converter is working, the magnetic field caused by the higher harmonics in the output waveform generates electromagnetic forces on many mechanical components. When the frequency of the driving force is close to or coincides with the natural frequency of these mechanical components, resonance will occur. The lower harmonic components have a greater impact on vibration, especially in PAM and square wave PWM modes. However, when using SPWM mode, the lower harmonic components are smaller, and their impact is also smaller.
One way to reduce or eliminate vibration is to connect an AC reactor to the output side of the frequency converter to absorb the high-order harmonic current components in the output current. When using a PAM or square wave PWM frequency converter, an SPWM frequency converter can be used instead to reduce pulsating torque, thereby weakening or eliminating vibration and preventing mechanical parts from being damaged by vibration.
2.3 Handling of overheating issues
The operating temperature range for general-purpose frequency converters is typically -10℃ to +50℃. To ensure reliable operation and extend the lifespan of the frequency converter, heat dissipation is essential. In winter, the internal fan can be used to remove heat from the converter enclosure. However, in summer, with temperatures already reaching 40℃, using the internal fan will only raise the room temperature and the converter enclosure temperature. The best solution is to use windows or drill several φ500mm holes evenly and appropriately above and below the converter enclosure in the electrical control room, ensuring sufficient space around the converter for good natural ventilation. If this is insufficient, fans can be turned on, or exhaust fans and ducts can be installed at the openings to forcibly extract the heat generated by the frequency converter outdoors. Finally, air conditioning can be considered to forcibly cool the space where the frequency converter is installed.
3. Conclusion
Strengthening research on fault problems in frequency converter applications is essential, as it is of great significance for the normal use of frequency converters and for tapping potential and increasing efficiency.
