Electromagnetic flowmeter (EMF) is a new type of flow measuring instrument developed rapidly with the development of electronic technology in the 1950s and 1960s. Electromagnetic flowmeter is made according to Faraday's electromagnetic induction law. Electromagnetic flowmeter is used to measure the volume flow of conductive liquid. Due to its unique advantages, electromagnetic flowmeters have been widely used in the flow measurement of various conductive liquids in industrial processes, such as various corrosive media such as acids, alkalis, and salts; various slurry flow measurements of electromagnetic flowmeters have been formed. A unique application area.
In structure, electromagnetic flowmeter consists of electromagnetic flow sensor and converter. The sensor is installed in the industrial process pipeline. Its role is to linearly convert the volume flow rate of liquid flowing into the pipeline into an induced potential signal and send this signal to the converter through the transmission line. The converter is installed not too far from the sensor. It amplifies the flow signal from the sensor and converts it into a standard electrical signal output proportional to the flow signal for display, accumulation, and adjustment control.
The basic principle of electromagnetic flowmeter
(I) Measurement principle According to Faraday’s law of electromagnetic induction, when a conductor moves magnetic field lines in a magnetic field, an induced electric potential e is generated at both ends of the conductor. Its direction is determined by the right-hand rule, and its size and magnetic field strength B The length L of the conductor in the magnetic field is proportional to the moving speed u of the conductor. If B, L, and u are perpendicular to each other, then e=Blu(3-35)
This is similar to this. In a uniform magnetic field with a magnetic induction strength of B, a non-magnetic conductive tube with an inner diameter of D is placed perpendicular to the direction of the magnetic field. When the conductive liquid flows in the tube at a flow rate u, the conductive fluid cuts the magnetic flux. If a pair of electrodes are installed perpendicular to the diameter of the magnetic field on the cross-section of the pipe (Fig. 3-17), it can be proved that as long as the flow velocity distribution in the pipe is axisymmetric, there is a special induced electromotive force between the two electrodes:
e=BD(3-36)
In the formula, the average flow velocity on the cross section of the pipe. This results in the volume flow of the pipe being:
Qv=πDUˉ=(3-37)
It can be seen from the above equation that the volume flow qv is linearly related to the induced electromotive force e and the inner diameter D of the measuring tube, and is inversely proportional to the magnetic induction strength B of the magnetic field, and has nothing to do with other physical parameters. This is the measurement principle of electromagnetic flowmeter.
It should be noted that to make equation (3-37) strictly true, the measurement conditions must satisfy the following assumptions:
1 magnetic field is a uniform distribution of a constant magnetic field;
2 The flow velocity of the measured fluid is axisymmetric;
3 The measured liquid is non-magnetic;
4 The conductivity of the liquid to be measured is uniform and isotropic.
Figure 3-17 principle of electromagnetic flowmeter 1 - magnetic pole; 2 - electrode; 3 - pipe (2) Excitation mode Excitation mode is the way to generate magnetic field. From the foregoing, we can see that in order to make equation (3-37) strictly established, the first condition that must be met is to have a uniform and constant magnetic field. To do this, you need to choose a suitable excitation method. At present, there are generally three methods of excitation, namely, DC excitation, AC excitation and low frequency square wave excitation. Are introduced separately.
1. DC excitation DC excitation uses direct current to generate a magnetic field or permanent magnets, which produces a constant uniform magnetic field. The biggest advantage of this kind of DC excitation transmitter is that it is affected by the AC electromagnetic field disturbance very little, so it can ignore the influence of the self-induction phenomenon in the liquid. However, the use of DC magnetic field is easy to make the electrolyte liquid through the measurement pipeline is polarized, that is, the electrolyte is electrolyzed in the electric field to produce positive and negative ions. Under the action of the electric force, negative ions run to the positive pole and positive ions run to the negative pole. As shown in Figure 3-18. In this way, the positive and negative electrodes will be surrounded by ions of the opposite polarity, which will seriously affect the normal operation of the electromagnetic flowmeter. Therefore, DC excitation is generally only used to measure non-electrolyte liquids, such as liquid metals.
Figure 3-18 DC excitation method Excitation At present, most of the electromagnetic flowmeters used in industry use the power frequency (50Hz) power supply AC excitation method, that is, its magnetic field is generated by the sinusoidal alternating current, so the generated magnetic field is also an alternating magnetic field. The main advantage of alternating magnetic field transmitters is the elimination of polarization disturbances at the electrode surface. In addition, since the magnetic field is alternating, the output signal is also an alternating signal, and it is much easier to amplify and convert a low-level AC signal than a DC signal.
If the magnetic field strength of AC magnetic field is B=Bm sint(3-38)
The induced electromotive force generated on the electrode is e=Bm Dsint(3-39)
The measured volume flow is qv=D(3-40)
Where Bm - the maximum value of magnetic field magnetic induction;
——Angle frequency of excitation current, =2f;
t - time;
f - power frequency.
From equation (3-40), it can be seen that when the inner diameter D of the measuring tube is constant and the magnetic induction intensity Bm is a certain value, the induced electromotive force e output on the two electrodes is proportional to the flow qv. This is the basic working principle of AC magnetic field electromagnetic flow transmitter.
It is worth noting that using AC magnetic fields will bring a series of electromagnetic interference problems. For example, orthogonal interference. In-phase interference, etc. These interfering signals are mixed with useful traffic signals. Therefore, how to correctly distinguish between the flow signal and the interference signal, and how to effectively suppress and eliminate various interference signals, has become an important issue in the development of AC.
3. The low-frequency square-wave excitation DC excitation method and the AC excitation excitation method have their own advantages and disadvantages. In order to give full play to their advantages, try to avoid their shortcomings. Since the 1970s, people began to use low-frequency square-wave excitation. Its excitation current waveform is shown in Figure 3-19, and its frequency is usually 1/4-l/10 of the power frequency.
Figure 3-19 Square-wave excitation current waveform From Figure 3-19, it can be seen that in a half-cycle, the magnetic field is a constant-steady DC magnetic field, which has the characteristics of DC excitation, and is very little affected by electromagnetic interference. From the whole time course, the square wave signal is an alternating signal, so it can overcome the polarization phenomenon of DC excitation. Therefore, low-frequency square-wave excitation is a relatively good excitation method, which is currently widely used. To sum up, electromagnetic flowmeters have the following advantages:
1 electromagnetic flowmeter can avoid the orthogonal magnetic interference of AC magnetic field;
2 electromagnetic flowmeter to eliminate the power frequency interference caused by the distributed capacitance;
3 electromagnetic flowmeter to suppress the alternating magnetic field in the pipe wall and the fluid caused by the eddy current;
4 electromagnetic flowmeter to eliminate the polarization of DC excitation.
In structure, electromagnetic flowmeter consists of electromagnetic flow sensor and converter. The sensor is installed in the industrial process pipeline. Its role is to linearly convert the volume flow rate of liquid flowing into the pipeline into an induced potential signal and send this signal to the converter through the transmission line. The converter is installed not too far from the sensor. It amplifies the flow signal from the sensor and converts it into a standard electrical signal output proportional to the flow signal for display, accumulation, and adjustment control.
The basic principle of electromagnetic flowmeter
(I) Measurement principle According to Faraday’s law of electromagnetic induction, when a conductor moves magnetic field lines in a magnetic field, an induced electric potential e is generated at both ends of the conductor. Its direction is determined by the right-hand rule, and its size and magnetic field strength B The length L of the conductor in the magnetic field is proportional to the moving speed u of the conductor. If B, L, and u are perpendicular to each other, then e=Blu(3-35)
This is similar to this. In a uniform magnetic field with a magnetic induction strength of B, a non-magnetic conductive tube with an inner diameter of D is placed perpendicular to the direction of the magnetic field. When the conductive liquid flows in the tube at a flow rate u, the conductive fluid cuts the magnetic flux. If a pair of electrodes are installed perpendicular to the diameter of the magnetic field on the cross-section of the pipe (Fig. 3-17), it can be proved that as long as the flow velocity distribution in the pipe is axisymmetric, there is a special induced electromotive force between the two electrodes:
e=BD(3-36)
In the formula, the average flow velocity on the cross section of the pipe. This results in the volume flow of the pipe being:
Qv=πDUˉ=(3-37)
It can be seen from the above equation that the volume flow qv is linearly related to the induced electromotive force e and the inner diameter D of the measuring tube, and is inversely proportional to the magnetic induction strength B of the magnetic field, and has nothing to do with other physical parameters. This is the measurement principle of electromagnetic flowmeter.
It should be noted that to make equation (3-37) strictly true, the measurement conditions must satisfy the following assumptions:
1 magnetic field is a uniform distribution of a constant magnetic field;
2 The flow velocity of the measured fluid is axisymmetric;
3 The measured liquid is non-magnetic;
4 The conductivity of the liquid to be measured is uniform and isotropic.
Figure 3-17 principle of electromagnetic flowmeter 1 - magnetic pole; 2 - electrode; 3 - pipe (2) Excitation mode Excitation mode is the way to generate magnetic field. From the foregoing, we can see that in order to make equation (3-37) strictly established, the first condition that must be met is to have a uniform and constant magnetic field. To do this, you need to choose a suitable excitation method. At present, there are generally three methods of excitation, namely, DC excitation, AC excitation and low frequency square wave excitation. Are introduced separately.
1. DC excitation DC excitation uses direct current to generate a magnetic field or permanent magnets, which produces a constant uniform magnetic field. The biggest advantage of this kind of DC excitation transmitter is that it is affected by the AC electromagnetic field disturbance very little, so it can ignore the influence of the self-induction phenomenon in the liquid. However, the use of DC magnetic field is easy to make the electrolyte liquid through the measurement pipeline is polarized, that is, the electrolyte is electrolyzed in the electric field to produce positive and negative ions. Under the action of the electric force, negative ions run to the positive pole and positive ions run to the negative pole. As shown in Figure 3-18. In this way, the positive and negative electrodes will be surrounded by ions of the opposite polarity, which will seriously affect the normal operation of the electromagnetic flowmeter. Therefore, DC excitation is generally only used to measure non-electrolyte liquids, such as liquid metals.
Figure 3-18 DC excitation method Excitation At present, most of the electromagnetic flowmeters used in industry use the power frequency (50Hz) power supply AC excitation method, that is, its magnetic field is generated by the sinusoidal alternating current, so the generated magnetic field is also an alternating magnetic field. The main advantage of alternating magnetic field transmitters is the elimination of polarization disturbances at the electrode surface. In addition, since the magnetic field is alternating, the output signal is also an alternating signal, and it is much easier to amplify and convert a low-level AC signal than a DC signal.
If the magnetic field strength of AC magnetic field is B=Bm sint(3-38)
The induced electromotive force generated on the electrode is e=Bm Dsint(3-39)
The measured volume flow is qv=D(3-40)
Where Bm - the maximum value of magnetic field magnetic induction;
——Angle frequency of excitation current, =2f;
t - time;
f - power frequency.
From equation (3-40), it can be seen that when the inner diameter D of the measuring tube is constant and the magnetic induction intensity Bm is a certain value, the induced electromotive force e output on the two electrodes is proportional to the flow qv. This is the basic working principle of AC magnetic field electromagnetic flow transmitter.
It is worth noting that using AC magnetic fields will bring a series of electromagnetic interference problems. For example, orthogonal interference. In-phase interference, etc. These interfering signals are mixed with useful traffic signals. Therefore, how to correctly distinguish between the flow signal and the interference signal, and how to effectively suppress and eliminate various interference signals, has become an important issue in the development of AC.
3. The low-frequency square-wave excitation DC excitation method and the AC excitation excitation method have their own advantages and disadvantages. In order to give full play to their advantages, try to avoid their shortcomings. Since the 1970s, people began to use low-frequency square-wave excitation. Its excitation current waveform is shown in Figure 3-19, and its frequency is usually 1/4-l/10 of the power frequency.
Figure 3-19 Square-wave excitation current waveform From Figure 3-19, it can be seen that in a half-cycle, the magnetic field is a constant-steady DC magnetic field, which has the characteristics of DC excitation, and is very little affected by electromagnetic interference. From the whole time course, the square wave signal is an alternating signal, so it can overcome the polarization phenomenon of DC excitation. Therefore, low-frequency square-wave excitation is a relatively good excitation method, which is currently widely used. To sum up, electromagnetic flowmeters have the following advantages:
1 electromagnetic flowmeter can avoid the orthogonal magnetic interference of AC magnetic field;
2 electromagnetic flowmeter to eliminate the power frequency interference caused by the distributed capacitance;
3 electromagnetic flowmeter to suppress the alternating magnetic field in the pipe wall and the fluid caused by the eddy current;
4 electromagnetic flowmeter to eliminate the polarization of DC excitation.
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