With the continuous development of power electronics technology, variable frequency speed control has become the mainstream method for AC motor speed control. E-Neng frequency converters, with their excellent speed control performance and significant energy-saving effect, have been widely used in industrial and civil control fields.
As is well known, the thyristors inside frequency converters of various brands are in a high-speed switching state. Continuous switching will generate continuous interference current, accompanied by a large amount of electrical pollution such as radiation and harmonics, which will interfere with engineering projects, cause the measurement and control system to malfunction, seriously damage the stability of the system, and even the frequency converter itself will be affected by interference, causing "bootstrapping" speed regulation failure.
Therefore, we will analyze the sources, propagation methods, and practical solutions for inverter interference, which we have frequently encountered in recent years, based on our experience in handling these issues, for your reference.
Interference source analysis and suppression
1. Input terminals (R, S, T)
The frequency converter rectifies AC power into DC power through a three-phase full-bridge diode circuit. Its input current is a large pulse, not a sine wave, and contains high-order harmonics. These high-order harmonics are generally within the 20th harmonic and may affect other equipment on this power supply line, which is considered conducted interference.
Common countermeasures: Add an input reactor (for low harmonics), add an input filter or magnetic ring (for high harmonics), or change the wiring points of other electrical equipment to be away from the RST terminal.
2. Output terminals (U, V, W)
At the PWM output of the frequency converter, the voltage is a modulated square wave, and the current is a sinusoidal dogtooth wave containing abundant high-order harmonics. These harmonics are proportional to the carrier frequency in various ways, and when the carrier frequency is high, they can also become radio frequency interference. This is a combination of conducted and inductive interference.
Common countermeasures include: adding shielding sleeves to the inverter output lines and grounding at one end on the inverter side; adding an output reactor, output filter, or magnetic ring; changing the wiring method to keep it away from other circuit lines; and reducing the carrier frequency.
3. Radio frequency interference
It mainly occurs in three places:
A. Internal switching power supply of the frequency converter (40-50KHz)
B. Surge electromagnetic process during IGBT commutation in the main circuit (related to switching speed, leakage inductance of the main circuit, and carrier frequency)
C. PWM output line (as described above).
General countermeasures: Options A and B are generally related to the design; if they pass EMC safety testing, there is no problem. Special handling by external users typically involves adding a metal shielding layer and changing the inverter's installation orientation.
4. Grounding
Problems with poor grounding:
A. If the connection is not good or the grounding resistance is too high (the grounding resistance should be less than 10Ω), the inverter will generate induced voltage during operation.
B. Mixing three-phase and single-phase electricity, or sharing or mixing the neutral and ground wires of single-phase electricity, creates a potential difference and generates interference voltage.
C. Multiple grounding, such as the inverter and motor being grounded in different places, or the control cabinet and inverter being grounded in different places, will generate step voltage or voltage abnormality and cause interference.
General countermeasures include: ensuring proper single-point grounding; separating the circuit current from the grounding current; separating the main circuit power supply from the control circuit power supply; and applying salt water to the buried ground wire to enhance conductivity.
5. Wiring
Notes and inspection items:
A. The RST and UVW lines should not be bundled together. They should be kept at a distance of 20-60cm (depending on the magnitude of the high current) to prevent output harmonic interference from feeding back to the input and affecting other equipment.
B. Main circuit traces and control signal traces must not be in the same slot or run parallel to each other. If they must be routed together, they must be routed perpendicularly. At the same time, pay attention to using shielded or twisted-pair cables to ensure that the area enclosed by the return conductors is minimized.
C. Note that high-frequency lines and low-frequency lines should be arranged separately.
D. The grounding wire should not be too thin or too long.
E. Other equipment control circuits should avoid drawing power from the RST terminal as much as possible, and instead draw power from a remote power source.
6. Isolation
Ensure proper isolation of input and output signal lines, paying particular attention to the isolation between expansion cards and external circuits. Consider the potential levels of different systems, such as whether there is isolation between high-voltage and low-voltage power, and between power and control signals. Are they grounded? Are the wiring of the customer's power supply and signals consistent with the inverter's signal logic? Are they matched? If necessary, take isolation measures, such as using optocouplers, isolation transformers, etc., or modify the interface circuit design.
7. Shielding
A. If the signal line is long, passes near a high-voltage circuit, has a high frequency, is a weak voltage signal, or is near contact switching equipment, shielding measures should be taken, such as using shielded wires, to prevent interference.
B. Power lines, especially UVW lines, should be placed in metal conduits whenever possible and necessary to prevent interference with other equipment.
C. For high-frequency operating components such as switching power supplies, a metal shielding layer (mesh) can be added for shielding if necessary.
8. Power Grid
A. When the power grid has high-order harmonics or waveform distortion, or instantaneous drops, it is best to add input reactors, input filters, or magnetic rings.
B. Pay attention to the balance of the three-phase load of the power grid, as well as the mixed connection of three-phase three-wire system, three-phase four-wire system, and three-phase five-wire system, so as to avoid the formation of abnormal ground current and voltage.
C. When the power supply transformer at the upper end of the frequency converter is more than 10 times the capacity of the frequency converter and the distance is relatively short, an input reactor should be added to improve the input current waveform and reduce harmonic interference.
Inverter interference problems can be roughly divided into three categories.
A. Inverter's own interference
B. Interference from external equipment on the frequency converter
C. Interference from the frequency converter to external equipment
Interference issues with frequency converters mainly manifest in the operation of the motor. For example, the motor may suddenly stop during operation, run at varying speeds, have unstable speeds, be unable to stop, or be uncontrollable. These are all examples of interference affecting the frequency converter.
Case Analysis and Handling
1. Inverter self-interference
An EN600 frequency converter was being used on-site. While the machine was running, pressing the stop button had no effect, and the converter could not be stopped. Inspection revealed that the converter's ground wire was only connected to the neutral wire of the transformer in the distribution cabinet, while the transformer's neutral wire was not connected to the earth. After communicating with an electrician to ground the transformer's neutral wire, the frequency converter returned to normal operation.
The above incident illustrates a lack of attention to grounding connections. According to national electrical engineering regulations, equipment must be manufactured with a strictly separate grounding and neutral wire. The neutral wire in the distribution cabinet should have a dedicated terminal, and the grounding wire should also have a dedicated grounding screw. Because this user only connected the transformer's neutral wire to the "N" terminal, and the grounding wire was not connected, even though the control wire was shielded and the shielding layer was connected to the grounding screw, it was not connected to the earth and therefore failed to provide shielding. This caused the frequency converter to malfunction due to interference, resulting in the motor failing to stop. Connecting the neutral and grounding wires in the distribution cabinet restored normal operation. Many users connect the grounding and neutral wires, but this method has drawbacks. If the neutral wire is disconnected, starting the equipment may electrify the machine tool, posing a threat to personal safety. It is better to directly connect the grounding wire in the distribution cabinet to the earth.
This type of interference is a type of interference inherent to the frequency converter itself.
2. Interference from external equipment to the frequency converter
When a shutdown command is given to the frequency converter at a certain site, the motor sometimes fails to stop. The control line shielding is found to be properly grounded, reducing the frequency converter carrier frequency has no effect, and adding magnetic ring filters to the input and output terminals of the frequency converter also has little effect.
Finally, an inspection revealed that the distribution cabinet where the frequency converter was installed was too close to the distribution room. When the power distribution equipment in the distribution room was working, a large current flowed through it, which generated a strong magnetic field around the current, thus interfering with the normal operation of the frequency converter. After moving the equipment away from the distribution room, normal operation was restored.
This type of interference is caused by external equipment affecting the frequency converter.
3. Interference from frequency converters to external equipment
At a certain site, after giving the inverter a start command, the motor did not run. The inverter's frequency source was checked and found to be an external 4-20mA input. After the 4-20mA signal was input to the inverter, the monitoring panel displayed a frequency of 0.00. A multimeter was used to measure the inverter's output terminals, and no output was found. Connecting a capacitor in parallel to the inverter's output terminals and restarting the device restored normal operation. This indicates that the signal source was interfered with.
This type of interference is caused by the frequency converter to external devices.
Nowadays, frequency converters are widely used, and various interference problems, such as the interference events in the above case, are frequently encountered. Analyzing and researching methods to suppress interference is a very important topic. Regardless of the type of interference, higher harmonics are the most common cause of interference from frequency converters. Therefore, when installing and using frequency converters, anti-interference measures must be taken for the control circuit. These measures must be implemented correctly to ensure the safe and reliable operation of the engineering system.
Shenzhen E-Energy Electric Technology Co., Ltd. is a national high-tech enterprise with independent intellectual property rights. It focuses on the research, development, production and sales of products in the field of industrial automation. Its main products include frequency converters, servo controllers, PLCs and new energy systems.
Established in 2004, Yineng Electric has passed ISO9001:2015 quality management system certification and EU CE certification. It has won honors such as the National Innovation Fund, Shenzhen Strategic Emerging Industries Fund, Product Innovation Award, and Most Investable Award, and has been repeatedly named one of the "Top Ten Domestic Brands of Low-Voltage Frequency Converters".
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