EMC Filters Guide REO UK LTD

Transcription

1 EMC Filters Guide EO INDUCTIVE COMPONENTS AG Bruehler Strasse 100, D Solingen, Germany Tel: 0049-(0) Fax: 0049-(0) EO USA 8432 East 33rd Street, Indianapolis IN , USA Tel: Fax: EO (UK) TD Units 8-10 ong ane Industrial Estate, Craven Arms, Shropshire SY7 8DU UK Tel: Fax: sales@reo.co.uk Website: EO UK TD

2 Electromagnetic field theory 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 EO (UK) TD Units 8-10 ong ane Industrial Estate, Craven Arms, Shropshire SY7 8DU UK Tel: Fax: sales@reo.co.uk Website: 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 atin!) without any explanation. + N S -

3 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. Finally, a Scottish mathematician, James Clerk Maxwell, picked up on Faraday s ideas and was successful in developing a theory of electromagnetism which enabled testable predictions 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 Electric Field Magnetic Field Direction of Wave Quite interestingly Maxwell predicted that electromagnetism would be propagated through space at a finite rate and was struck by the similarity between the predicted speed of electromagnetism and the speed of light. From this connection sprang the idea that light was an electric phenomenon and the subsequent discovery of radio waves. Michael Faraday

4 What makes up the electromagnetic spectrum? 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 oscillation 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 corresponding variety of wavelengths. If the original charged particle vibrates rapidly, the frequency of the wave is high. Because there are many oscillations per second, the corresponding wavelength is short. Conversely, if the original charged particle vibrates slowly, the frequency of the wave is low and the corresponding wavelength is long. The whole range of frequencies and wavelengths is called the electromagnetic spectrum and different parts of the spectrum are given different names. These parts of the spectrum have different properties and, consequently, 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 susceptible to malfunctions caused by external interference. This is particularly relevant in the case of electronic equipment that is required to continue running for economic or safety reasons. Banking systems and aircraft computers are two notable examples. Frequency Hz long-wave radio short-wave radio microwaves millimeter waves infrared light VISIBE IGHT ultraviolet light x rays Wavelength M red orange yellow green blue indigo violet gamma rays The Electromagnetic Spectrum

5 Electromagnetic Compatibility (EMC) and the law There is now the European Directive 89/336/EEC, which specifically deals with EMC. ike 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: The electromagnetic disturbance it generates does not exceed a level allowing radio and telecommunications equipment and other apparatus to operate as intended; The apparatus has an adequate level of intrinsic immunity to electromagnetic 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: ADeclaration of Conformity must be kept and made available to the enforcement authority. ACE mark must be affixed to the apparatus, or its packaging, instructions or guarantee certificate. 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 environment. 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. ATechnical Construction File must be produced and be ready for inspection, if required. 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. EN 081 Emissions EN 082 Immunity Domestic Commercial ight Industrial 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 Part 1 Part 2 Part 3 Industrial Special antenna, such as cables, pcb tracks, internal wiring and mechanical structures. These elements can unintentionally transfer energy via electric, magnetic or electromagnetic fields, which couple with other circuits.

6 EMC Filters Instead of designing a filter stage for every new piece of electronic equipment 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 circuit should take into account the impedances of the source and load. The main components inside the filter are chokes, capacitors and resistors. The capacitors oppose the AC flow of electrons more at lower frequencies and less at higher frequencies. Inductors on the other hand react against the rate of change of current; they are more effective at opposing AC flow of electrons at higher frequencies. Therefore, a combination of series connected inductors and shunt connected capacitors is chosen to provide suppression over a wide frequency spectrum. The resistors serve to discharge the capacitors when the supply is disconnected and for damping resonances. The enclosure is normally produced from metal to provide good earth bonding. The filter circuit is designed to contend with two types of noise. There is COMMON MODE NOISE, which manifests itself as a current in phase in the live and neutral conductors and returns via the safety earth. This produces a noise voltage between live/neutral and earth. It is often caused by capacitive coupling to the case earth. The other is DIFFEENTIA MODE NOISE produced by current flowing along either the live or neutral conductor and returning by the other. This produces a noise voltage between the live and neutral conductors. The chokes fall into two groups; current compensated or common mode and series or differential mode types. The current compensated choke has two or three windings on a toroidal core. The direction of each winding is chosen to give an opposing current flow, hence balancing the flux. The result is that a much smaller choke can be used. Furthermore, the common mode currents, which are in phase in the two or three conductors, have an additive effect, thus presenting higher impedance against the common mode noise. Source Symmetrical Interference current Common Mode ( asymmetrical interference ) Differential Mode ( Symmetrical Interference ) Earth Cx 1 Cx 2 Asymmetrical Interference current oad Cp = parasitic capacitance Cy Cy The differential mode chokes are larger due to the higher current handling requirement. Using a core made from a highly permeable material will reduce its size. A typical EO filter built into metal enclosure (lid removed). The main components; capacitors and inductors can be clearly seen.

7 Testing Capacitors also fall into two groups; X Class and Y Class. The X Class capacitors are connected between live and neutral, or between phases, to reduce differential noise. They are tested to withstand mains voltage. Y Class capacitors on the other hand are more critical because they are connected between live/neutral and earth to reduce common mode noise. Because of this they have to be tested to ensure that they cannot fail to short circuit. Needless to say they are more expensive. For higher levels of attenuation, several stages of chokes and capacitors can be added and this is known as a multistage filter. Another important factor is the earth leakage current. The larger the Y Class capacitor the more the Hz current that will leak to earth, raising the potential of the filter enclosure. The maximum permissible leakage current depends on the application but to give an indication the maximum earth leakage for double insulated equipment, such as hedge trimmers and drills, is 0.25 ma. Equipment that is permanently connected to the mains supply may have a leakage current of up to 5mA. Industrial equipment normally has higher leakage current limits but in each case particular care must be taken to ensure that earthing regulations are observed. The frequency range of interest for emissions from most products is 1kHz to 30MHz. The emissions are measured using a spectrum analyser and then they are compared with the Class B limits for domestic or light industrial applications or Class Alimits for industrial applications (Standard EN ). The spectrum analyser will give a sweep of the entire spectrum range and indicate the emission levels, usually as a print-out. The limits are also indicated and frequencies with unacceptable emission levels are highlighted. The picture shows a screen capture from a spectrum analyzer. Note that the sweep range is 1 khz to 30 MHz and the red line (quasi-peak) and purple line (average) indicating the limits for light industrial and domestic use (Class B). The two frequency curves for the equipment under test are quasi peak in blue and average in green. The vertical scale is in dbv. In order to conform the curves should not exceed their corresponding limits.this is the format for the results as prescribed in EN for conducted emissions. The electromagnetic field strengths associated with the emissions measurements range from a few millionths of a volt per meter to a few thousandths of a volt per meter. It is difficult to represent this scale in a linear manner and so a logarithmic scale is adopted. The spectrum analyser plot has decibels as the unit of measurement along its vertical axis. Adecibel is a ratio of two power values, which in this case is the increase in power caused by the emission compared with the power of the reference voltage. For convenience, because power is proportional to the square voltage, the ratio of the voltages is squared instead of measuring power. Screen capture from a spectrum analyzer

8 Points to consider when selecting a filter The performance and hence quality of a filter may be assessed by looking at its insertion loss characteristics. This data is published by all reputable filter manufacturers and the following is an example. Damping [db] symmetrical asymmetrical Frequency [MHz] The curves can be used to assess whether a particular filter will provide sufficient suppression for a known interference pattern. However, precise performance can only be established by testing under real conditions. The higher the db value the greater the degree of suppression achieved. The current rating is also very important especially in instances where the load produces high peak currents and the MS is not a true indication of the real current. Higher than expected currents can cause core saturation, in the inductors, thus drastically reducing the filter s effectiveness. Most filter manufacturers will take this into account when designing a filter but this data is not normally published. The acceptable level of earth leakage current for the installation must also be known and the filter selected, accordingly. The components in a filter are classified for a range of climatic and mechanical requirements, including lower temperature limit, over temperature limit and relative humidity. Unless otherwise stated EO filters conform with Environment Category IEC ower temperature limit - 25C The current has to be derated for ambient temperatures of greater than C = (85-)/ The permissible current rating is subject to ambient temperature. Below C a filter can be used at its rated current. The current reduces for higher temperatures. At 85C the current is reduced to zero. (For 25/85/21 ) Standards Over temperature limit + 85C All EO filters are built to the following standards:- VDE 0565 Part 1-3 DIN EN 130 IEC /85/21 2 [ 2 ] C elative Humidity 95% 21 days/year Ambient 85C All materials are U listed and in many cases filters are U approved, or approval can be obtained in a short time.

9 Installation Standard Filters - Single Phase Afilter should be positioned as close as possible to the connection to the supply; ideally before the mains switch in the front panel and fuses. Otherwise the connecting cables could provide a coupling path via stray induction to the unfiltered cables. Wiring on each side of the filter should be well separated and extend straight out from the filter s terminals. If this is not practical the output cables should be run at 90 degrees to the input cables to reduce the likelihood of coupling. CNW 101 Single phase, 2 V, cost effective unit for general purpose. CNW 101/3 CNW 101/6 CNW 101/10 CNW 101/16 CNW 101/20 3 A 6 A 10 A 16 A 20 A CNW 102 Single phase, 2 V, high performance unit suitable for most applications. CNW 102/3 CNW 102/6 CNW 102/10 CNW 102/16 CNW 102/20 3 A 6 A 10 A 16 A 20 A The filter should be correctly rated for the local supply voltage. CNW 201 CNW 116 Agood ground connection is required between the filter casing and earth. The ground connections must have large contact surfaces and be made onto bare metal, not painted. 230V / 4V 2 line bookcase style filter. CNW 201/16 CNW 201/30 CNW 201/ CNW 201/63 16 A 30 A A 63 A Single phase, 2 V, optimised unit for use with motor drives. CNW 116/8 CNW 116/12 CNW 116/20 CNW 116/30 8 A 12 A 20 A 30 A Cx1 Cx2 C y ' N() C y N'(')

10 Standard Filters - Three Phase CNW 103 CNW 104 CNW 105 CNW phase, 3 x 4 V, 3 line mains filter with high attentuation. 3 phase, 3 x 4 V, 3 line mains filter with very high attentuation. 3 phase, 3 x 4 V, 4 line mains filter with high attentuation. 3 phase, 3 x 4 V bookcase style filters, with very high attentuation. CNW 103/3 CNW 103/6 CNW 103/10 CNW 103/16 CNW 103/25 CNW 103/36 CNW 103/ CNW 103/ CNW 103/120 CNW 103/1 3 x 3 A 3 x 6 A 3 x 10 A 3 x 16 A 3 x 25 A 3 x 36 A 3 x A 3 x A 3 x 120 A 3 x 1 A CNW 104/3 CNW 104/6 CNW 104/10 CNW 104/16 CNW 104/25 CNW 104/36 CNW 104/ CNW 104/ CNW 104/120 CNW 104/1 3 x 3 A 3 x 6 A 3 x 10 A 3 x 16 A 3 x 25 A 3 x 36 A 3 x A 3 x A 3 x 120 A 3 x 1 A CNW 105/3 CNW 105/6 CNW 105/10 CNW 105/16 CNW 105/25 CNW 105/36 CNW 105/ CNW 105/ CNW 105/120 CNW 105/1 4 x 3 A 4 x 6 A 4 x 10 A 4 x 16 A 4 x 25 A 4 x 36 A 4 x A 4 x A 4 x 120 A 4 x 1 A CNW 204/7 CNW 204/16 CNW 204/30 CNW 204/42 CNW 204/55 CNW 204/75 CNW 204/100 CNW 204/130 CNW 204/1 3 x 7 A 3 x 16 A 3 x 30 A 3 x 42 A 3 x 55 A 3 x 75 A 3 x 100 A 3 x 130 A 3 x 1 A INE 3 Cx1 Cx2 Cy2 Cy1 ' ' 3' ' OAD INE 3 Cx1 Cy1 Cx2 Cx3 Cy2 Cy3 ' ' 3' ' OAD INE 3 N Cx1 Cy Cx2 ' ' 3' N' ' OAD CNW 106 CNW 203 CNW 3 3 phase, 3 x 4 V, 4 line mains filter with very high attentuation. CNW 106/6 CNW 106/10 CNW 106/16 CNW 106/25 INE 3 N 4 x 6 A 4 x 10 A 4 x 16 A 4 x 25 A Cx1 Cx2 Cx Cy1 Cy2 ' ' 3' N' ' OAD 3 phase, 3 x 4 V bookcase style filters, with very high attenuation. CNW 203/16/SE 3 x 16 A CNW 203/25/SE 3 x 25 A CNW 203/36/SE 3 x 36 A CNW 203//SE 3 x A INE 3 Cx1 Cx2 Cy ' ' 3' OAD Over voltage protection device, 3 x 4V for easy installation into new or existing equipment. Efficient protection against high voltages, lightning strikes and transients. Attenuation of interference down to approximately db at 1MHz. Max. Surge ka CNW For details of our full range of EMC filters and custom design solutions please; Call sales@reo.co.uk Visit our website