Basic principle of the hottest electromagnetic flo

Basic principle of electromagnetic flowmeter

(I) measurement principle

according to Faraday's law of electromagnetic induction, when a conductor moves and cuts the magnetic line of force in the magnetic field, the induced potential E is generated at both ends of the conductor, and its direction is determined by the right-hand rule. Its size is directly proportional to the magnetic induction intensity B of the magnetic field, the length L of the conductor in the magnetic field and the moving speed U of the conductor. If B, l and u are perpendicular to each other, Then

e = blu

similar to this. In a uniform magnetic field with magnetic induction intensity B, a non-magnetic pipe with inner diameter D is placed perpendicular to the direction of the magnetic field. When the conductive liquid flows in the pipe with velocity u, the conductive fluid cuts the magnetic line of force. If a pair of electrodes are installed at both ends of the pipe section perpendicular to the diameter of the magnetic field (Fig. 3-17), it can be proved that as long as the velocity distribution in the pipe is axisymmetric, There is also a special induced electromotive force between the two electrodes:

e = BD U

where u is the average flow rate on the pipe section. From this, the volume flow rate of the pipe is: (3)

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 of the measuring pipe D, and inversely proportional to the magnetic induction intensity B of the magnetic field, It has nothing to do with other physical parameters. This is the measurement principle of the electromagnetic flowmeter.

it is necessary to say that the amplitude and frequency of the adjusting spring. It is clear that in order to make formula (3) strictly established, the measurement conditions must meet the following assumptions:

① the magnetic field is a uniformly distributed constant magnetic field

② axisymmetric distribution of velocity of measured fluid

③ the measured liquid is non-magnetic

④ the conductivity of the measured liquid is uniform and isotropic

1 - magnetic pole; 2 - electrode; 3 - Pipeline

(II) excitation mode

excitation mode is the mode of generating magnetic field. It can be seen from the foregoing that in order to make formula (3) strictly established, the first condition that must be met is to have a uniform and constant magnetic field. Therefore, it is necessary to choose a suitable excitation mode. At present, there are generally three excitation methods, namely DC excitation, AC excitation and low-frequency square wave excitation. They are introduced respectively.

1. DC excitation

DC excitation uses DC to generate a magnetic field or uses a permanent magnet, which can generate a constant uniform magnetic field. The biggest advantage of this DC excitation transmitter is that it is very little affected by AC electromagnetic field interference, so the influence of self induction in liquid can be ignored. However, The use of DC magnetic field is easy to polarize the electrolyte liquid passing through the measuring pipe, that is, the electrolyte is electrolyzed in the electric field, producing positive and negative ions. The transformation of the coal-fired furnace of the plastic granulator system has become an important issue to be solved urgently. 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 the figure. In this way, the positive and negative electrodes will be surrounded by ions of opposite polarity, It seriously affects the normal operation of the instrument. Therefore, DC excitation is generally only used to measure non electrolyte liquids, such as liquid metals.

DC excitation mode

2. AC excitation

at present, the electromagnetic flowmeter used in industry mostly adopts the AC excitation mode of power frequency (50Hz) power supply, that is, its magnetic field is generated by sinusoidal alternating current, Therefore, 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 disturbance on the electrode surface. In addition, because the magnetic field is alternating, the output signal is also an alternating signal, Amplifying and converting low-level AC signals is much easier than DC signals.

if the magnetic induction intensity of the AC magnetic field is

b = BM sin wt

, the induced electromotive force generated on the electrode is

e = BM dusin wt

the measured volume flow is

qv= d

in the formula BM - the maximum value of the magnetic induction intensity of the magnetic field

w - angular frequency of excitation current, w=2f

t - time

f - power frequency.

according to formula (3-40), when the inner diameter of the measuring tube D remains unchanged and the magnetic induction intensity BM is a certain value, the induced electromotive force E output on the two electrodes is directly proportional to the flow QV. This is the basic working principle of the AC magnetic field electromagnetic flow transmitter.

it is worth noting that, Using AC magnetic field will bring a series of electromagnetic interference problems. For example, positive will also make the advantages and value of aluminum alloy cables disappear. Cross interference, in-phase interference, etc., these interference signals are mixed with useful flow signals. Therefore, for example, steel and titanium alloys, the cardinal number 1 is generally 10; For non-ferrous metals, special steels and under high temperature, corrosion and other experimental conditions, how to correctly distinguish flow signals and interference signals, and how to effectively suppress and eliminate various interference signals, has become an important topic in the development of AC excited 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 and try to avoid their disadvantages, people have begun to use low frequency square wave excitation mode since the 1970s. Its excitation current waveform is shown in the figure, and its frequency is usually 1/4-L/10 of power frequency.

square wave excitation current waveform

it can be seen from the figure that in half a 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 also an alternating signal, so it can overcome the polarization phenomenon easily produced by DC excitation. Therefore, low-frequency square wave excitation is a relatively good excitation method, which has been widely used in electromagnetic flowmeter. To sum up, It has the following advantages:

① it can avoid the orthogonal electromagnetic interference of AC magnetic field

② eliminate power frequency interference caused by distributed capacitance

③ restrain the eddy current caused by AC magnetic field on the pipe wall and inside the fluid

④ eliminate the polarization phenomenon of DC excitation. (end)