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EMC Filters Guide REO (UK) LTD Units 8 -10 Long Lane Industrial Estate, Craven

EMC Filters Guide REO (UK) LTD Units 8 -10 Long Lane Industrial Estate, Craven Arms, Shropshire SY7 8DU UK Tel: 01588 673411 Fax: 01588 672718 Email: sales@reo.co.uk Website: www.reo.co.uk REO UK LTD REO INDUCTIVE COMPONENTS AG Bruehler Strasse 100, D-42657 Solingen, Germany Tel: 0049-(0) 2 12-88 04-0 Fax: 0049-(0) 2 12-88 04-188 REO USA 8432 East 33rd Street, Indianapolis IN46226-6550, USA Tel: 001 317 8991395 Fax: 001 317 8991396 Electromagnetic field theory What makes up the electromagnetic spectrum? Electromagnetic compatibility (EMC) and the law Where does electromagnetic interference come from? EMC filters Testing Points to consider when selecting a filter Standards Installation Standard filters Hans Christian Oersted was a professor of science at Copenhagen University. In 1820 he arranged a science demonstration to friends and students in his home. He planned to demonstrate the heating of a wire by an electric current, and also to carry out demonstrations of magnetism, for which he provided a compass needle mounted on a wooden stand. While performing his electric demonstration, Oersted noted to his surprise that every time the electric current was switched on, the compass needle moved. He kept quiet and finished the demonstrations, but in the months that followed worked hard trying to make sense out of the new phenomenon. But he couldn’t! The needle was neither attracted to the wire nor repelled from it. Instead, it tended to stand at right angles (see drawing below). In the end he published his findings (in Latin!) without any explanation. N S + - Electromagnetic field theory REO (UK) LTD Units 8-10 Long Lane Industrial Estate, Craven Arms, Shropshire SY7 8DU UK Tel: 01588 673411 Fax: 01588 672718 Email: sales@reo.co.uk Website: www.reo.co.uk 2 5 7 8 9 12 13 14 15 16 Finally, a Scottish mathematician, James Clerk Maxwell, picked up on Faraday’s ideas and was successful in developing a theory of electromag- netism which enabled testable predic- tions to be made. Inspired by Faraday’s lines of force, he developed a model that unified magnetic and electrical forces. Maxwell unified the study of electricity and magnetism in four tidy equations. In essence he discovered that electric and magnetic fields were intrinsically related to one another, with or without the presence of a conductive path for electrons to flow. Stated simply Maxwell’s discovery was this:- A changing electric field produces a perpendicular magnetic field and A changing magnetic field produces a perpendicular electric field Michael Faraday Electric Field Magnetic Field Direction of Wave Quite interestingly Maxwell predicted that electromagnetism would be propa- gated through space at a finite rate and was struck by the similarity between the predicted speed of electromagnet- ism and the speed of light. From this connection sprang the idea that light was an electric phenomenon and the subsequent discovery of radio waves. It was the scientist Michael Faraday who first studied in detail the phenomena involving the interaction between electricity and magnetism. Amongst his many achievements, he is credited with the construction of the first electric motor and the discovery of both the principle and the method whereby a rotating magnet can be used to create an electric current in a coil of wire (still the basis of modern electricity generating plants). He also observed the way in which iron filings arrange around a magnet. They appear to follow lines of magnetism leading out from one magnetic pole and back to the other pole. He suggested that the effect of a magnet on a wire carrying a current is the result of lines of force . Faraday spent the latter part of his life working to devise an experiment that would confirm his theory of electromagnetic fields but without success. The electromagnetic spectrum is a family of waves that travel through space by way of the production of electric and magnetic fields. Changing electric fields are set up by the oscilla- tion of charged particles and these changing electric fields induce changing magnetic fields in the surrounding space. Changing magnetic fields then set up more changing electric fields and so on. The net result is that the wave energy travels across space. All electromagnetic waves travel at the same speed through the same medium or substance but they have a variety of frequencies which provide a correspon- ding variety of wavelengths. If the original charged particle vibrates rapidly, the frequency of the wave is high. Because there are many oscilla- tions per second, the corresponding wavelength is short. Conversely, if the original charged particle vibrates slowly, the frequency of the wave is low and the correspon- ding wavelength is long. What makes up the electromagnetic spectrum? The whole range of frequencies and wavelengths is called the electromag- netic spectrum and different parts of the spectrum are given different names. These parts of the spectrum have different properties and, conse- quently, they have different uses. Therefore, it can be seen that there is the need for the coexistence of all kinds of radio services, which use the electromagnetic spectrum to convey information, with technical processes and products emitting electromagnetic energy as an undesirable by-product. Furthermore, the problems of EMC are not limited to interference with radio services because electronic equipment of all kinds is becoming more suscep- tible to malfunctions caused by external interference. This is particularly relevant in the case of electronic equip- ment that is required to continue running for economic or safety reasons. Banking systems and aircraft computers are two notable examples. 10 3 10 0 10 -3 10 -9 10 -12 10 6 10 9 10 12 10 15 10 18 10 21 Wavelength M Frequency Hz long-wave radio short-wave radio microwaves millimeter waves infrared light ultraviolet light x rays gamma rays VISIBLE LIGHT red orange yellow green blue indigo violet The Electromagnetic Spectrum There is now the European Directive 89/336/EEC, which specifically deals with EMC. Like a number of other documents produced by the European Commission this is a new approach directive, which sets out the essential requirements that must be satisfied before products can be marketed anywhere within the EC. It also says how evidence of conformity will be provided. In the case of EMC the essential requirements are that electrical and electronic equipment shall be constructed so that: Electromagnetic Compatibility (EMC) and the law Standards The generic standards relating to EMC are divided into two sections, one for immunity and one for emissions, each of which has separate parts for different environment classes. The electromagnetic disturbance it generates does not exceed a level allowing radio and telecommunica- tions equipment and other apparatus to operate as intended; The apparatus has an adequate level of intrinsic immunity to electro- magnetic disturbance to enable it to operate as intended. The manufacturers or their authorised representative are required to attest that the requirements of the Directive have been met. This requires two things: Compliance In essence, for goods to comply with the EMC Directive they should be tested but in reality this is difficult because laboratory conditions are not the same as the real working environ- ment. Also it is quite possible that numerous types of other equipment could be connected to the goods and it is impossible to cover all possible configurations. However, where possible the goods should be tested either in house or by a competent body against the relevant standards. A Technical Construction File must be produced and be ready for inspection, if required. A Declaration of Conformity must be kept and made available to the enforcement authority. A CE mark must be affixed to the apparatus, or its packaging, instruc- tions or guarantee certificate. Where does Electromagnetic Interference come from? Electromagnetic interference (EMI) can manifest in a variety of ways and the emission source is usually frequency dependent. The interference can be conducted, through mains cables and earthing connections, or radiated. Most electronic hardware contains elements which act in a similar manner to an antenna, such as cables, pcb tracks, internal wiring and mechanical struc- tures. These elements can unintention- ally transfer energy via electric, magnetic or electromagnetic fields, which couple with other circuits. Domestic Commercial Light Industrial Industrial Special EN 50 081 Emissions EN 50 082 Immunity Part 1 Part 2 Part 3 Instead of designing a filter stage for every new piece of electronic equip- ment that is manufactured, there is a convenient solution available in the form of a ready made module that can be connected between the mains supply and the electronic unit. The components are selected by the filter manufacturer to give the best reduction of mains conducted interference for most situations, across the frequency spectrum, using the optimum selection of components. More importantly filters comply with the numerous safety rules and the approval costs have been spread over a large number of units because the filters can be universally applied. Filters work by providing an impedance mismatch between the power line and the equipment, which reflects the generated noise back to its source. In order to maximise the impedance mismatch the choice of filter uploads/Ingenierie_Lourd/ emc-filters-guide.pdf