An integrated three-phase filter can alleviate the problem, although space limitations may still force designers to fabricate a custom filter to achieve the required size, power rating, and frequency response. Combining an integrated three-phase design with space-saving nanocrystalline core technology in the inductive components offers additional freedom in achieving a standardized solution.
EMI Sources and Regulations
Electrical and electronic equipment is increasingly permeating every aspect of our personal and professional lives, enabling people to work, provide and consume services, manage essential infrastructure, monitor and protect the environment, safeguard our security, and make our social lives possible. As the world becomes increasingly populated by these devices, and given the critical nature of many of their functions, ensuring coexistence is essential. Each device can be a source of EMI, which, if left unaddressed, can cause other nearby equipment to malfunction. Minimizing emitted EMI and ensuring immunity to external EMI are essential aspects of engineering these devices for electromagnetic compatibility (EMC).
Regulations aim to ensure that all products marketed locally are capable of coexisting with others. The European Union's EMC Directive (2014/30/EU) refers to the specifications and testing standards established by various technical bodies, such as the International Electrotechnical Commission (IEC), the CISPR (International Special Committee on Radio Interference), and the International Organization for Standardization (ISO).
Any electrical equipment can create various types of EMI. For example, high-frequency switching emissions from a power converter or signal energy from one piece of equipment coupling to another where it is unwanted, are perceived as EMI.
EMI can be conducted along transmission lines or radiated through the air between subsystems or different pieces of equipment. When electronic devices are connected via an unconnected AC power supply, the EMI generated by each unit can reach other line-fed equipment through the power line.
Conducted emissions are divided into two categories, depending on the path they propagate. Normal-mode (differential) noise refers to asynchronous emissions that occur between the power and signal lines. In this case, the direction of the emitted current is opposite on each line. On the other hand, common-mode noise occurs between the power or signal lines and ground. In this case, called synchronous emissions, the direction of the current is the same for each line.
EMI-RFI Filters:
Filters are applied to attenuate these unwanted differential and common-mode emissions. Normal noise (differential mode) is attenuated by a combination of in-line inductors and X-capacitors connected across the power lines, as shown in Figure 1a. Figure 1b shows how Y-capacitors are needed between the power lines and ground to treat common-mode emissions.
Figures 1a and 1b. Normal-mode and common-mode EMI filtering.
Figure 2 shows how both combine to create a complete EMI-RFI filter. Any mains-powered equipment containing switching devices, such as AC/DC converters, must contain this type of filter to meet minimum emission, immunity, and susceptibility targets, such as the European EMC Directive (2014/30/EU).
Figure 2. EMI-RFI filter composed of differential-mode and common-mode filters.
In the case of an appliance such as a refrigerator designed to operate on a single-phase AC power supply, the filter is inserted between the power outlet and the load, as shown in Figure 3, since many modern appliances use switching devices. This contrasts with the past, when there was only a compressor and a light source.
Figure 3. EMI-RFI filter in a single-phase application.
Three-phase – Triples the Challenge.
High-power industrial equipment that demands more energy than a single-phase AC source can supply is typically powered by a three-phase source. Examples include large industrial robots, motor drives, and large medical devices such as MRI scanners. Three-phase equipment naturally requires three channels of EMI-RFI filtering: one on each line. Filter suppliers commonly offer units that integrate three individual line filters into a single enclosure as a three-phase filter, providing a space-saving solution.
Filter Selection Criteria.
However, achieving the desired combination of frequency response and power rating within a form factor suitable for prevailing size constraints is not always easy and may push the project toward a custom filter. This can present several problems. Often, product teams find that their chosen supplier cannot assist within a reasonable timeframe. If the anticipated production volume is low, some suppliers may be unwilling to discuss a custom filter project.
Assuming a willing supplier can be found, developing the custom filter inevitably adds time and cost to the project. Engineers must first wait for samples to be produced. Subsequently, if testing reveals that the filter design needs to be changed, further delays are likely before production examples can be delivered. In addition, applications must comply with different EMC standards depending on the installation location. In purely industrial applications, the requirements are the least stringent, but in residential areas, the requirements are more stringent. In medical environments such as hospitals, ground current is not permitted or must be limited, and capacitor-free Y-filters are the only solution.
Space Savings:
Nanocrystalline materials allow the inductive components, or chokes, of EMI-RFI filters to be much smaller than is possible with conventional ferrite cores. Taking advantage of this space saving, a filter with a suitable frequency response can be obtained while meeting applicable space constraints. Furthermore, the nanocrystalline core material allows for attenuation across a higher frequency range due to the material's capacitance, as fewer turns of copper winding are needed to achieve the same inductance. Windings introduce parasitic capacitance, which reduces attenuation at high frequencies. Specifically, with modern broadband technology operating at higher switching speeds, these higher frequency ranges require more attenuation than in the past.
KEMET has used its nanocrystalline technology to create a family of 24 compact, ready-to-use three-phase filters. These filters can be specified with any of six capacitance values Y, from 0 pF to 470,000 pF, providing a selection of common-mode attenuation characteristics suitable for various equipment topologies. Figure 4 shows the effect on common-mode attenuation of each of the six different capacitance values.
Figure4. Frequency response of three-phase filters with different values of capacitance Y.
Furthermore, each configuration is available with one of four current ratings, from 30 A to 60 A. The range of current ratings and attenuation characteristics helps ensure that engineers can find an available model to meet their project requirements, avoiding the overhead associated with a custom project. In addition to benefiting from faster access to samples and easier production-quantity ordering, only minimal technical engagement with the supplier is required, and engineers can quickly and easily evaluate one or more alternatives within the family.
Compared to a similar three-phase EMI-RFI filter, the GTX series occupies less than 30% of the volume and achieves a similar reduction in overall filter mass (Figure 5).
Figure 5. Nanocrystalline core technology reduces the size and weight of the three-phase filter to less than 30%.
Conclusion:
Electrical equipment powered from the AC line must have a line filter for each power phase to comply with mandatory EMC regulations. If space is limited, engineers may need to seek a specialist to design a custom filter to provide the desired attenuation in a suitably sized filter. Nanocrystalline inductor cores enable high common-mode attenuation over a wide frequency range in a small, lightweight unit, allowing commercially available filters to meet the demanding needs of industrial automation, renewable energy, and medical scanner applications that require high attenuation with high current capacity.
By Patrik Kalbermatten, Senior Manager - Distribution Promotion, Product Management MSABG - Magnetics, Sensors and Actuators at KEMET, part of the YAGEO Group
