Classification of electromagnetic flowmeters

Electromagnetic flowmeters classified by excitation method

To produce a uniform and constant magnetic field, you need to choose a suitable excitation method. Divided by the excitation current mode, there are DC excitation, AC (power frequency or other frequency) excitation, low frequency rectangular wave excitation and dual frequency square wave excitation.

1. DC excitation

DC excitation uses direct current or permanent magnets to generate a constant uniform magnetic field. The greatest advantage of this kind of DC excitation transmitter is that it is affected by the AC electromagnetic field disturbance very little, so the influence of the self-induction phenomenon in the liquid can be ignored. However, the use of a DC magnetic field makes it easy to polarize the electrolyte liquid passing through the measuring tube, ie, the electrolyte is electrolyzed in the electric field to generate positive and negative ions. Under the action of the electric field force, negative ions run toward the positive electrode, and positive ions run toward the negative electrode, which will result in positive The negative electrode is surrounded by oppositely polarized ions, which seriously affects the normal operation of the instrument. Therefore, DC excitation is generally only used to measure non-electrolyte liquids, such as liquid metal flow (mercury at room temperature and liquid steel, lithium, and potassium at high temperatures).

2. AC excitation

Most of the electromagnetic flowmeters used in industry use alternating current excitation (50Hz) power to generate alternating magnetic field, which avoids polarization interference on the surface of DC excitation electrodes. However, using AC excitation can bring about a series of electromagnetic interference problems (such as orthogonal interference, in-phase interference, zero-point drift, etc.). AC excitation is now being replaced by low frequency square wave excitation.

3. Low frequency square wave excitation

The low-frequency square-wave excitation waveform has two values ​​(positive-negative) and three values ​​(positive-zero-negative-zero), and its frequency is usually 1/2 to 1/32 of the power frequency. The low frequency square wave excitation can avoid the orthogonal electromagnetic interference of the AC magnetic field, eliminate the power frequency interference caused by the distributed capacitance, suppress the eddy current caused by the AC magnetic field inside the pipe wall and the fluid, and eliminate the polarization phenomenon of the DC excitation.