Use Shielded Cables and Proper Grounding
The shielding and grounding of cables are designed based on the basis of electromagnetic interference suppression. In turn, the shielding layer is applied mainly to absorb and reflect the electromagnetic field inside the cable to decrease its external radiation without compromising the fact that there should not be any allowance for the external electromagnetic waves into its internal conduction path. The best possible shielding effect requires strict control in these aspects:
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Shielding Material Selection: It normally employs copper braided mesh, aluminum foil, and synthetic composite materials. Copper braided mesh yields a better low-frequency interference shielding effect, and the shielding by using aluminum foil is suitable for high-frequency interference. It is suggested that double-shielded cables be used for the cables connecting the inverter to the motor, and the use of braided mesh and foil shielding should cope with a wide range of interference sources.
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Shielding Layer Grounding Method: The grounding methods of shielding layers include single-ended grounding and double-ended grounding. Single-ended grounding can avoid the propagation of low-frequency interference through the ground loop, while double-ended grounding has a better effect of shielding in high-frequency. Generally speaking, if the length of a cable is short (like less than 5 meters), single-ended grounding can be used. However, for wiring over longer distances-say, more than 10 meters-double-ended earthing is recommended: low impedance grounding coupling to both ends will prevent ground loop effects.
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Grounding Impedance Control: The grounding resistance should be as small as possible to avoid high-frequency interference reflecting from the ground back into the cable. The grounding resistance for shielded cables should be less than 1 ohm for those that connect three-phase motors and inverters. The section of the shielding layer exposed to open air should be as short as possible (<2cm) to avoid forming a radiation point.
Choose the Right Filter
With the right selection and configuration, filters are highly effective in suppressing electromagnetic interference in various frequency bands in three-phase motor systems. The design requirements of the filtering schemes in the power line, signal line, and control line are different, but with the right knowledge and tools, you can confidently manage these issues:
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Power Line Filter: The filter can be installed at the power input to prevent power noise from entering internal equipment and suppress noise generated by the motor system in propagating backward to the power grid. Most of the three-phase filters make use of LCL or LC-T structures that are capable of offering excellent attenuation in the range of 150kHz to 30MHz. For common-mode interference, the filter should possess a CMRR of up to 60 dB.
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Output Filter: The coupling between the inverter and the motor may create high-frequency components. Employment of the sine wave filter or du/dt filter will be able to restrain high-frequency harmonic components within a certain limit and hence reduce stress on motor winding insulation as well as the energy radiated outside.
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Signal Line Filter: The majority of filtering in both digital and analog signal lines uses the RLC filter structure. It’s crucial to design accurately based on different signal frequencies. For instance, on communication lines such as RS485/RS232, the preference for low capacitance and high impedance EMI filters is a testament to the precision required in your work to reduce signal attenuation while ensuring the integrity of the signal.
Optimization of the filter selection must also take into account the real power, frequency response characteristics, and actual environmental noise conditions. As much as possible, install the filter near the interference source, such as inverter input or output. Ensure that the grounding resistance of the filter is less than 0.1 ohms to guarantee effective filtering performance.
Reasonable Wiring and Layout
Electromagnetic interference mainly depends on one of the most basic factors: the rationality of cable wiring. Wiring should consider physical distances between cables, the method of coupling, and the ordering method. The following are some key suggestions for optimizing wiring:
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Separation of Strong and Weak Currents: It’s crucial to keep the strong current (power lines, motor lines) adequately apart from the weak current (signal lines, control lines). This means that during the actual installation, a distance of at least 30 centimeters should be maintained, and putting strong and weak current cables together in one trunking is not advisable.
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Shielding Isolation Measures: When strong and weak currents share the same trunking, it’s essential to implement shielding isolation with the help of metal partitions. These metal partitions should be well-grounded to ensure their effectiveness. Wiring using conduits or metal trunking is a good method to reduce radiation interference from outside to signal lines.
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Cable Crossing Angle: Where the crossing of cables cannot be avoided, it is recommended that the intersection angle is close to 90 degrees to minimize the coupling area between lines and, hence, diminish the induced current generation.
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Cable Length Optimization: Cable length is one of the most important factors affecting the amplitude of interference. If the cable is too long, it will raise parasitic capacitance and inductance, thus creating resonance phenomena. It is recommended to arrange cables along the shortest path and install grounding clamps at intervals in order to guide high-frequency interference to the ground.
Use EMC-Compatible Inverters and Motors
The selection of inverters and motors directly determines the level of EMC. At an early stage in a system, the selection of equipment with good EMC design can reduce some electromagnetic interference sources. The following are points to be considered when selecting equipment:
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Choose EMC-Compliant Inverters: Inverters shall be provided with integrated EMI filtering and shall conform to the standard IEC 61800-3 or higher to ensure that the level of electromagnetic radiation and conducted interference in the industrial environment is within limits.
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Reasonable Carrier Frequency Setting: As the carrier frequency of the inverter increases, its output voltage waveform approaches a sine wave. However, this also leads to an increase in EMI intensity. Therefore, it’s crucial to strike a balance between EMI and motor stability. The recommended carrier frequency range is between 4kHz-8kHz.
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Select Motors with Good Shielding: Your choice of motors can significantly impact the system’s anti-interference capability. Motors with built-in grounding structures and metal enclosures are effective in suppressing high-frequency electromagnetic waves, preventing leakage. By optimizing the terminal structure, you can further reduce grounding resistance during connection, enhancing the system’s anti-interference capability.
Ensure a Good Grounding System
The grounding system forms the ground from where all EMI suppression measures apply. In fact, the quality of a system’s electromagnetic compatibility depends directly on the quality of the grounding system. Some design considerations for a grounding system are given below:
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Grounding Method Selection: As professionals involved in the design and maintenance of motor systems, you have the power to select the most suitable grounding method. For a small motor system, a single-point grounding method can be adopted to avoid interference due to ground-loop currents. For a large complex system, your choice of multi-point grounding can prevent common-mode interference owing to the difference in ground potentials among different points of grounding.
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Grounding Impedance Control: Your decisions regarding grounding resistance and impedance control carry significant weight. In a strict sense, the grounding resistance should be controlled within the designed range. In high-frequency systems, quite a few times, more important than the grounding resistance is the grounding impedance. To effectively reduce the grounding impedance, the wires used for grounding can be shortened, the grounding area can be increased, and multi-strand braided wires can be used for grounding.
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Equipotential Grounding Network: Employing an equipotential grounding network mesh grounding or floor grounding for large-scale industrial systems, the potential difference in all grounding points, thereby preventing any interference current which may be generated due to a potential difference.
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Grounding Material Selection: Use materials with low resistivity and high anticorrosion performance, such as copper wires. Avoid the use of aluminum material in order not to cause electrochemical corrosion caused by the potential difference in contact.
