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The working principle of electromagnetic flowmeter |
| Electromagnetic flowmeterㄗEletromagnetic FlowmetersㄛShortEMFㄘThe 20th century, 50 to 60 years with the rapid development of electronic technology and developed a new type of flow measurement instruments﹝Electromagnetic flowmeterAccording to Faraday's law of electromagnetic induction is madeㄛElectromagnetic flowmeterUsed to measure the volume of conductive liquid flow meter. Because of its unique advantagesㄛElectromagnetic flowmeterHas been widely used in various industrial processes, the flow measurement of conductive liquid, such as various acids, alkalis, salt and other corrosive media˙Electromagnetic flowmeterVarious slurry flow measurement, has developed a unique application areas﹝ In the structureㄛElectromagnetic flowmeterBy the electromagnetic flow sensor and converter of two parts. Sensors installed in the industrial process piping, it is the role of the liquid flowing into the pipeline to transform the linear volume flow into the induced electric potential signal, and this signal through a transmission line to the converter. Converters installed in a place not too far away from the sensor, it sent the traffic sensor signal amplification and converted into a standard electrical signal proportional to the flow signal output to the display, accumulation and regulation and control﹝ Electromagnetic flowmeterThe basic principle (A) the measurement principle According to Faraday's law of electromagnetic induction, when a conductor in a magnetic field sports cutting magnetic line, the conductor ends that generate induced electric potential e, the direction by the right-hand rule to determine its size and the magnetic flux density B, the conductor in a magnetic field The length L and the conductor is proportional to the velocity u, and if B, L, u three mutually perpendicular, then eˊBlu (3ㄜ35) And this is similar. Magnetic induction B in a uniform magnetic field, perpendicular to the magnetic field put a non-magnetic diameter D pipeline, when the conductive liquid in the pipeline to flow velocity u, conductive fluid to cut magnetic lines. If the cross section perpendicular to the field pipe diameter installed at both ends of a pair of electrodes (Figure 3-17) can be proved that as long as the distribution of axisymmetric pipe flow velocity distribution, especially between the two electrodes generating induced electromotive forceㄩ eˊBD (3ㄜ36) Where the average cross section for the pipe flow. Thus the volume of available pipe flowㄩ qvˊ羽DU‘ˊ ㄗ3ㄜ37ㄘ Can be seen from the above equation, the volume flow rate qv and the measurement of electromotive force e, and a linear relationship between tube diameter D, and the magnetic field is inversely proportional to the magnetic flux density B, and other physical parameters. That isElectromagnetic flowmeterThe measurement principleㄝ Should be noted that, to make the equation (3-37) strictly, the measurement conditions must satisfy the following assumptionsㄩ Ｋ a constant magnetic field is uniform magnetic field˙ Ｌ the measured velocity of the fluid distribution of axial symmetry˙ Ｍ non-magnetic fluid to be measured˙ Ｎ measured the electrical conductivity of the liquid homogeneous and isotropic﹝ Figure 3-17 Schematic of electromagnetic flowmeter 1 - pole; 2 - electrodes; 3 - Pipeline (B) excitation Excitation produces a magnetic field that way. We can see from the above, for the type (3-37) strictly true, the first condition that must be met is to have a uniform constant magnetic field. To do this, you need to select an appropriate excitation. At the moment, there are three ways encourage touch, that DC excitation, excitation and low frequency square wave AC excitation. Now be introduced, respectivelyㄝ 1. DC excitation DC excitation mode with the DC magnetic field or by a permanent magnet, it produces a constant uniform magnetic field. The greatest advantage of DC excitation of the transmitter is affected by the exchange of electromagnetic interference is very small, so the liquid can be ignored in the self-inductance phenomenon. However, using DC magnetic field by measuring the pipe easy to make the electrolyte liquid is polarized, that is, by electrolysis in the electrolyte in the electric field, resulting in positive and negative ions. Under the action of forces in the field, ran to the positive ions, positive ions ran negative. As shown in Figure 3-18. This will lead to positive and negative electrodes were surrounded by ions of opposite polarity, seriously affecting the electromagnetic flowmeter to work properly. Therefore, the DC excitation is generally used for measuring non-electrolyte liquids such as liquid metal, etcㄝ Figure 3-18 DC excitation mode 2. AC excitation At present, the industrial useElectromagnetic flowmeterㄛMost used frequency (50Hz) Power Supply AC excitation, that is, its magnetic field is generated by a sinusoidal alternating current, so the magnetic field is generated by an alternating magnetic field. Alternating magnetic field of the transmitter main advantage is the elimination of electrode polarization in the interference. In addition, because the magnetic field is alternating, so the output signal is an alternating signal, amplify and convert low level AC signal is much easier than the DC signalㄝ If the exchange of the magnetic induction intensity BˊBm sint ㄗ3ㄜ38ㄘ The electrode induced electromotive force is generated eˊBm Dsint ㄗ3ㄜ39ㄘ The measured volume flow qv= D ㄗ3ㄜ40ㄘ Where Bm - maximum value of magnetic flux density˙ 求求Excitation current of angular frequencyㄛˊ2f˙ t求求Time˙ f求求Power frequencyㄝ From (3-40) shows that, when measuring the same tube diameter D, magnetic induction Bm is a constant, the output of the two electrodes and flow qv induced emf is proportional to e. This is the AC magnetic field of electromagnetic flow transmitter's basic working principleㄝ It is noteworthy that, with the alternating magnetic field will bring a series of electromagnetic interference. Such as orthogonal interference. Phase interference, the interference signal and the useful traffic signals mixed together. Therefore, how to correctly distinguish traffic signals and interference signals, and how to effectively suppress and eliminate all kinds of interference signals, to become the AC excitationElectromagnetic flowmeterDeveloped an important issue﹝ 3ㄝLow-frequency square-wave excitation DC and AC excitation methods have advantages and disadvantages of AIDS Action, in order to give full play to their strengths and avoid their weaknesses, since the 70s, people began to use low-frequency square wave excitation. Excitation current waveform is shown in Figure 3-19, the frequency is usually frequency of 1/4-l/10ㄝ Figure 3-19 square wave excitation current waveform Can be seen from Figure 3-19, the half cycle, the DC magnetic field is constant steady magnetic field, it has the characteristics of DC excitation by electromagnetic interference small. From the whole time looking, square-wave signal is an alternating signal, it can overcome the DC excitation polarization easy to produce AIDS. Therefore, the low-frequency square wave excitation excitation is a better way, is currently inElectromagnetic flowmeterOn a wide range of applications. RecapㄛElectromagnetic flowmeterHas the following advantagesㄩ ＫElectromagnetic flowmeterAC magnetic field orthogonal to avoid electromagnetic interference˙ ＬElectromagnetic flowmeterCaused by the distributed capacitance to eliminate frequency interference˙ ＭElectromagnetic flowmeterInhibition of AC magnetic field inside the wall and fluid due to eddy current˙ ＮElectromagnetic flowmeterDC excitation polarization excludedㄝ |