The working principle of electromagnetic flowmeter

Electromagnetic flowmeter is a new type of flow measuring instrument developed rapidly with the development of electronic technology in the 1950s and 1960s. The electromagnetic flowmeter is a meter that is based on Faraday's law of electromagnetic induction and is used to measure the volumetric 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 acidic, alkali, salt and other corrosive media; electromagnetic flowmeters of various slurry flow measurement, formation A unique application area.
Structurally, the electromagnetic flowmeter consists of an electromagnetic flow sensor and a converter. The sensor is mounted on an industrial process pipe. Its function is to linearly convert the liquid volume flow value flowing into the pipe into an induced potential signal and send this signal to the converter through the transmission line. The converter is mounted not far from the sensor, it amplifies the flow signal sent by the sensor and converts it into a standard electrical signal output proportional to the flow signal for display, accumulation and regulation control.
 
The basic principle of electromagnetic flowmeter
(1) Measurement principle
According to Faraday's law of electromagnetic induction, when a conductor moves in a magnetic field to cut a magnetic line of force, an induced potential e is generated at both ends of the conductor, the direction of which is determined by the right-hand rule, the magnitude and magnetic induction of the magnetic field B, and the conductor is within the magnetic field. The length L is proportional to the speed u of the conductor. If B, L, and u are perpendicular to each other, then
e=Blu (3-35)
Similar to this. In a uniform magnetic field with magnetic induction B, a non-magnetic tube with an inner diameter D is placed perpendicular to the direction of the magnetic field. When the conductive liquid flows at the flow velocity u in the tube, the conductive fluid cuts the magnetic field lines. If a pair of electrodes are installed on the pipe section perpendicular to the diameter of the magnetic field (Fig. 3-17), it can be proved that as long as the flow velocity distribution in the pipe is axisymmetric, an induced electromotive force is also generated between the two electrodes:
e=BD (3-36)
Where is the average flow rate on the section of the pipe. The volumetric flow rate of the pipe thus obtained is:
Qv=πDUˉ= (3-37)
It can be seen from the above formula that the volume flow rate qv is linear with the induced electromotive force e and the inner diameter D of the measuring tube, and inversely proportional to the magnetic induction intensity B of the magnetic field, independent of other physical parameters. This is the measurement principle of electromagnetic flowmeter.
It should be noted that in order for the formula (3-37) to be strictly established, the measurement conditions must satisfy the following assumptions:
1 The magnetic field is a uniformly distributed constant magnetic field;
2 the flow velocity of the measured fluid is axially symmetrically distributed;
3 The liquid to be tested is non-magnetic;
4 The conductivity of the liquid to be tested is uniform and isotropic.
Figure 3-17 Schematic diagram of electromagnetic flowmeter

1-pole; 2-electrode; 3-pipe
(2) Excitation mode The excitation mode is the way of generating magnetic field. It can be seen from the foregoing that in order to make the formula (3-37) strictly established, the first condition that must be satisfied 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 ways of excitation, namely DC excitation, AC excitation and low frequency square wave excitation. Now introduce them separately.
1. DC Excitation DC excitation uses DC to generate a magnetic field or an eternal magnet that produces a constant uniform magnetic field. The great advantage of this kind of DC excitation transmitter is that it is less affected by AC electromagnetic field interference, so the influence of self-inductance in liquid can be neglected. However, the use of a DC magnetic field tends to cause the electrolyte liquid passing through the measuring tube to be polarized, that is, the electrolyte is electrolyzed in the electric field to produce positive and negative ions. Under the action of the electric field force, the negative ions run to the positive pole and the 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 opposite polarity, which seriously affects the normal operation of the electromagnetic flowmeter. Therefore, DC excitation is generally only used to measure non-electrolyte liquids, such as liquid metal.
Figure 3-18 DC excitation mode

2. AC excitation At present, most of the electromagnetic flowmeters used in industry use AC frequency excitation of power frequency (50Hz) power supply, that is, its magnetic field is generated by sinusoidal alternating current, so the generated magnetic field is also an alternating magnetic field. The main advantage of the alternating magnetic field transmitter is that it eliminates the polarization of 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 the low-level AC signal than the DC signal.
If the magnetic induction of the alternating magnetic field is
B=Bm sint (3-38)
Then the induced electromotive force generated on the electrode is
e=Bm Dsint (3-39)
The measured volume flow rate is
Qv= D (3-40)
Bm in the formula - the large value of the magnetic field magnetic induction intensity;
- the angular frequency of the excitation current, = 2f;
T-time;
f - power frequency.
It can be known from equation (3-40) 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 outputted on the two electrodes is proportional to the flow rate qv. This is the basic working principle of the AC magnetic field electromagnetic flow transmitter.
It is worth noting that the use of 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 the flow signal and the interference signal, and how to effectively suppress and eliminate various interference signals, has become an important topic in the development of AC excitation electromagnetic flowmeter.
3. Low-frequency square wave excitation DC excitation mode and AC excitation mode 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

It can be seen from Fig. 3-19 that in a half cycle, the magnetic field is a constant DC magnetic field, which has the characteristics of DC excitation and is little affected by electromagnetic interference. From the whole time process, the square wave signal is an alternating signal, so it can overcome the polarization phenomenon caused by DC excitation. Therefore, low frequency square wave excitation is a better excitation method, and it has been widely used in electromagnetic flowmeters. To sum up, electromagnetic flowmeters have the following advantages:
1 electromagnetic flowmeter can avoid the orthogonal electromagnetic interference of the alternating magnetic field;
2 electromagnetic flowmeter eliminates power frequency interference caused by distributed capacitance;
3 electromagnetic flowmeter suppresses the eddy current caused by the alternating magnetic field in the pipe wall and the inside of the fluid;
4 electromagnetic flowmeter to eliminate the polarization phenomenon of DC excitation.

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