The frequency converter can generate harmonics with high power, which have strong interference to other equipment in the system. Its interference pathway is consistent with the general electromagnetic interference pathway, mainly divided into electromagnetic conduction, radiation, and inductive coupling. Specifically:
① Generate electromagnetic noise on directly driven motors, causing an increase in iron and copper losses, and conducting interference to the power supply, which is then transmitted to other equipment in the system through the distribution network;
② Generate electromagnetic radiation to surrounding electronic and electrical equipment;
③ The frequency converter induces coupling with adjacent lines, causing interference voltage or current, which affects the normal operation of electrical equipment.
Practice has shown that among electromagnetic conducted interference, electromagnetic radiation interference, and inductive coupling interference, the most significant interference to field instruments and control systems is line conducted noise, followed by electromagnetic radiation interference, and finally inductive coupling interference.
The frequency converter itself does not generate a lot of electromagnetic interference through radiation
1. The electrical equipment that is susceptible to electromagnetic interference mainly includes:
(1) Weak signal analog measurement circuits, such as thermocouples, thermistors, strain gauges, chemical pH measurement, etc.
(2) Although the measurement signal of the analog measurement circuit is greater than 1 V, when the lead is long or a resolution of 0.1% is required.
(3) Analog signal circuits with wide frequency bands, such as audio appliances.
(4) Video circuits, such as closed-circuit television, computer monitors, etc.
(5) When the data transmission line is not shielded or the wiring is not suitable (but generally RS232, RS485 communication, and Ethernet networks have good anti-interference properties).
(6) Proximity sensors using high-frequency oscillators, especially capacitive types.
(7) Wireless communication equipment.
2. Effective measures to suppress electromagnetic interference:
① The silt signal line cannot be parallel to unshielded motor cables or unfiltered power lines. There are generally the following rules: when the parallel length of the two is greater than 1 m, the distance between the signal line and the power line is less than 300 mm; when the parallel length is 1 Yao 10 m, the distance between the two is greater than 300 mm; if the parallel length is greater than 10 m, the distance between the two should increase proportionally, and if the parallel length is 40 m, the distance between the two should be 40/10 y 0.3 m to 1.2 m. The distance between the signal line and the frequency converter should be at least 300 mm. If the distance is too close, shielded signal lines should be used, but not including the signal lines connected to the frequency converter.
② The grounding frequency converter control cabinet should be equipped with a common grounding busbar. The ground of the power supply and other circuits should be connected to the common ground terminal. The ground terminal of the frequency converter itself cannot be used as the common ground terminal. The two ends of the ground wire in the motor cable (four core cable) should be connected to the frequency converter and motor ground terminals respectively. The control terminal of the frequency converter and other circuits should be grounded through the 0V terminal. The ground wire of the input terminal of the frequency converter should be as short as possible. If it is required to isolate the control circuit from ground, it can be grounded through a 100 μ F capacitor or by using a shielded wire with the shielding layer grounded.
③ In general, frequency converters do not need to be equipped with power filters. But when there are other electromagnetic interference prone devices or shielded motor cables with a length exceeding 40 meters or unshielded motor cables with a length exceeding 10 meters in the power supply circuit, causing an increase in electromagnetic wave interference voltage in the power line, a power filter should be installed.
④ The power side capacitor reduces the electromagnetic interference current flowing into the power line to 1/10 or less of its original value. Capacitors should be placed as close as possible to the frequency converter and the wiring should be short. The outgoing line of the motor passes through the ferrite ring, causing a local increase in impedance of the wire passing through, preventing electromagnetic interference power from passing through. If the wire is wound a few times around the loop, the total inductance and impedance values will increase with the square of the number of turns, and it is generally recommended to make 2 or 3 turns.
⑤ Note that the grounding wire must be outside the loop.
⑥ Although the carrier frequency is high and the interference is large, the current waveform is good and the noise is small, so it can be appropriately selected to achieve a balance between the two. When the motor power is high and the output line is long, in order to reduce the interference effect, the carrier frequency can be appropriately lowered, which is more advantageous for suppressing electromagnetic interference. The distance between the frequency converter and the motor and the selected carrier frequency.
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