(าป)Measuring principle
According to Faraday's law of electromagnetic induction, when a conductor in cutting movement in the magnetic field lines, the conductor at both ends that have induced the potential e, the direction set by the Right-hand Rule, the size of the magnetic field strength of the magnetic induction B, in the magnetic field within the conductor L and the length of the conductor is directly proportional to velocity u, B, L, u three perpendicular to each other,
eBlu (335)
And this is similar. Magnetic induction intensity in the uniform magnetic field B, perpendicular to the direction of the magnetic field by putting a diameter of not permeability to the D pipeline, when the conductive liquid to flow in pipelines in the u flow, conductive fluid on the cutting magnetic line. If the pipeline On the vertical section of the magnetic field in diameter installed at both ends of a pair of electrodes (3-17) can be proved that as long as the velocity distribution channels within the symmetrical distribution of Weizhou, the two have special electrodes between induced EMF:
eBD  (336)
In For the pipeline section on the average velocity. This could be in the pipeline for the volume of traffic
qv    337
By the on-visible, with the volume of traffic qv EMF e sensors measure the diameter D and a linear relationship with the magnetic field strength of the magnetic induction is inversely proportional to B, has nothing to do with other physical parameters. This is the electromagnetic flowmeter measurement principle.
It is worth noting that, if-(3-37) the establishment of strict, we must meet the following conditions to measure the assumption that
Uniform distribution of the magnetic field is a constant magnetic field
Tested the fluid flow rate of axial symmetry
Measured non-magnetic properties of liquid
Measured the electrical conductivity of liquid evenly and isotropic
Figure 3-17 electromagnetic flowmeter principle sketch
1 - Pole; 2 - electrode; 3 - Pipeline
()Exciting way
Exciting way have a magnetic field that is the way by the above we can see that for the type (3-37) the establishment of strict, the first must meet the conditions is to have a uniform constant magnetic field. For this reason, it is necessary to select a suitable way excitation . At present, there are three general Lai touch, that is, DC excitation, AC excited and low-frequency square-wave excitation. Respectively are to be introduced.
1DC excitation
DC exciting way to generate the magnetic field or a DC permanent magnet, it can produce a constant uniform magnetic field. This DC excitation of the biggest advantages of the transmitter is affected by the exchange of electromagnetic interference is very small, so the liquid can ignore the phenomenon of self-inductance Impact, but easy to use DC magnetic field by measuring the electrolyte liquid pipeline was polarization, that is, electrolytes in electric field was electrolysis, negative ions are generated. In the role of the electric field, the negative ions toward the cathode, the positive ions toward the anode. If As shown in Figure 3-18. This will lead to positive and negative electrodes were of the opposite polarity surrounded by plasma, which has seriously affected the normal work of instrumentation, so DC excitation is generally used to measure non-electrolyte fluids, such as liquid metal, and so on.
Figure 3-18 DC exciting way
2AC excited
At present, the industrial use of electromagnetic flowmeter, the most frequently used (50 Hz) power AC excited, that is, its magnetic field by the sine of the alternating current, so the magnetic field is also an alternating magnetic field. Alternating magnetic field transmission The main advantage is to remove the electrode surface of the polarization in the disturbance. Addition, since the alternating magnetic field is, therefore, the output signal is alternating signal amplification and conversion of low-level exchange of signals than the DC signal much easier.
If the exchange magnetic field strength of the magnetic induction to
BBm sin  t 338
The electrodes were induced to EMF
eBm D sin t 339
As measured volume flow
qv= D  340
Type in Bm - magnetic field strength of the magnetic induction Max
DDExciting Current angular frequency 2  f
tDDTime
fDDPower frequency
By-(3-40) tells us that when the measurement of the same diameter D, magnetic induction intensity Bm for a certain value, the output of the two electrode-induced emf e qv with the flow in direct proportion. This is the exchange of the magnetic field of electromagnetic flow transmitter Basic working principle.
It is worth noting that AC magnetic field will bring a series of electromagnetic interference. Such as orthogonal interference. Phase interference, and so on, these signal interference with the flow of useful signals mixed together. Therefore, how to correctly distinguish between traffic signals and signal interference, And how to effectively curb various kinds of interference and rule out the signal, on the exchange of excitation electromagnetic flowmeter developed important subject.
3Low-frequency square-wave excitation
DC exciting way and the exchange of Li Huizi advantages and disadvantages of each method, in order to give full play to their strengths, try to avoid their shortcomings, since the 1970s, people began to use low-frequency square-wave excitation way. It's exciting current waveform in Figure 3-19 by the Show, their frequency is usually the frequency of 1/4- l/10.
Figure 3-19 square wave excitation current waveform
Figure 3-19 from that, in half a cycle, the magnetic field is of Steady DC magnetic field, it has the characteristics of DC excitation, the electromagnetic interference little effect. From the time the whole process, square-wave signal is a reference to the change Signal, it can overcome DC Li Zi Yi of the polarization. Therefore, the low-frequency square-wave excitation is a relatively good excitation, is now in the electromagnetic flowmeter on a wide range of applications. Summarize, it has the following advantages
To avoid the exchange of orthogonal magnetic field of electromagnetic interference
Capacitance caused by the elimination of the frequency interference
Inhibit the exchange magnetic field in the internal wall and fluid caused by eddy
Ruled out the DC excitation polarization |
