Electromagnetic Interference Suppression Work Plan for Frequency Converters

    1. Specific situations requiring suppression of electromagnetic interference from frequency converters:

       The input and output of the frequency converter generate harmonics, especially with significant impact on output interference.On-load tap changer testeruses microcomputer technology, without disassembling transformers or lifting switches, to test the performance of on-load tap changers of various power transformers. The electromagnetic interference energy generated by the frequency converter is mainly propagated through motor cables, power lines, and grounding wires. When the capacity of the frequency converter is greater than or equal to 10% of the power transformer capacity, and the output line length exceeds 100 m, and there are highly sensitive electronic devices, instruments, etc. nearby, and the motor power ranges from tens to hundreds of kilowatts, the electromagnetic interference problem cannot be ignored. Effective suppression methods (Electromagnetic Compatibility, EMC) must be adopted to ensure that the frequency converter and other equipment do not affect each other.

       2. Electrical equipment susceptible to electromagnetic interference mainly includes:

     (1) Weak-signal analog measurement circuits, such as thermocouples, thermistors, strain gauges, chemical pH measurement, etc.

     (2) Analog measurement circuits where the measured signal is greater than 1 V, but when the leads are long or a resolution of 0.1% is required.

     (3) Analog signal circuits with wide bandwidth, such as audio equipment.

     (4) Video circuits, such as closed-circuit television, computer monitors, etc.

     (5) Data transmission lines without shielding or with improper routing (but generally RS232, RS485 communication and Ethernet networks have good anti-interference performance).

     (6) Proximity sensors using high-frequency oscillators, especially capacitive types.

     (7) Radio communication equipment.

       3. Effective measures to suppress electromagnetic interference:

      (1) General measures are as follows.

       Signal lines: Signal lines must not be parallel to unshielded motor cables or unfiltered power lines. Generally, the following rules apply:

        When the parallel length is greater than 1 m, the distance between the signal line and the power line should be less than 300 mm; when the parallel length is 1 to 10 m, the distance should be greater than 300 mm; if the parallel length exceeds 10 m, the distance should increase proportionally. For example, if the parallel length is 40 m, the distance should be 40/10 * 0.3 m = 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 this does not include signal lines connected to the frequency converter.

        Grounding: The frequency converter control cabinet should be equipped with a common grounding bus. The power ground and other circuit grounds should all be connected to the common grounding terminal. The grounding terminal of the frequency converter itself cannot be used as the common grounding terminal. The ground wire in the motor cable (four-core cable) should be connected to the grounding terminals of the frequency converter and the motor respectively. The control terminals of the frequency converter and other circuits should be grounded through the 0V terminal. The grounding wire at the input end of the frequency converter should be as short as possible. If the control circuit needs to be isolated from the ground, a 100 μF capacitor can be used for grounding, or shielded wires with the shield grounded can be used.

       Generally, the frequency converter does not need a power filter. However, when other electromagnetic interference-sensitive equipment is connected to the power supply circuit, or the shielded motor cable length exceeds 40 m, or the unshielded motor cable length exceeds 10 m, causing increased electromagnetic interference voltage in the power line, a power filter should be installed.

      The capacitor on the power supply side reduces the electromagnetic interference current flowing into the power line to 1/10 or less of the original. The capacitor should be placed as close to the frequency converter as possible, and the wiring should be short. The motor output wire passes through the ferrite ring, which locally increases the impedance of the wire passing through, preventing electromagnetic interference from passing through. If the wire is wound around the ring several times, the total inductance and impedance will increase with the square of the number of turns, generally 2 to 3 turns is appropriate.

      Note that the grounding wire must be outside the ring.

    (2) Enhanced measures are as follows.

      Lowering the carrier frequency: Although high carrier frequency causes more interference, it results in better current waveforms and lower noise, so it can be appropriately selected to balance both. When the motor power is large and the output line is long, to reduce interference impact, the carrier frequency can be appropriately lowered.