Electromagnetic FlowmeterClassified by excitation method:
To generate a uniform and constant magnetic field, it is necessary to choose a suitable excitation method. According to the excitation current method, there are DC excitation, AC (power frequency or other frequencies) excitation, low-frequency rectangular wave excitation, and dual frequency rectangular wave excitation.
1. DC excitation
The direct current excitation method uses direct current or magnets to generate a constant and uniform magnetic field. This type of DC excitation transmitter is minimally affected by AC electromagnetic field interference, thus the effect of self inductance in the liquid can be ignored. However, using a direct current magnetic field can easily polarize the electrolyte liquid passing through the measurement pipeline, that is, the electrolyte is electrolyzed in an electric field, producing positive and negative ions. Under the action of the electric field force, negative ions run towards the positive electrode and positive ions run towards the negative electrode, which will cause the positive and negative electrodes to be surrounded by ions of opposite polarity, seriously affecting the normal operation of the instrument. Therefore, DC excitation is generally only used to measure non electrolyte liquids, such as liquid metal flow rates (at room temperature and at high temperatures for liquid steel, lithium, potassium), etc.
2. Communication excitation
In industry, most of them use power frequency (50Hz) AC excitation to generate alternating magnetic fields, avoiding polarization interference on the surface of DC excitation electrodes. However, using AC excitation can bring a series of electromagnetic interference problems (such as orthogonal interference, in-phase interference, zero drift, etc.). Nowadays, AC excitation is being replaced by low-frequency square wave excitation.
3. Low frequency square wave excitation
There are two types of low-frequency square wave excitation waveforms: binary (positive negative) and ternary (positive zero negative zero), with frequencies typically ranging from 1/2 to 1/32 of the power frequency. Low frequency square wave excitation can avoid orthogonal electromagnetic interference caused by AC magnetic fields, eliminate power frequency interference caused by distributed capacitance, suppress eddy currents caused by AC magnetic fields on pipe walls and inside fluids, and eliminate polarization phenomena caused by DC excitation.

8、 Troubleshooting:
During operation, various faults may cause inaccurate measurements. Generally, faults that occur during operation can be divided into two categories. One type is caused by the malfunction of the flowmeter itself or damage to its components; One type of fault is caused by changes in external conditions, such as unstable output, countless flows, excessive errors, etc. Here are several simple troubleshooting methods:
a、 Unstable output: 1. Unstable flow field; 2. The liquid detected by the sensor contains gas and large solid blocks; 3. Electrical connection virtual connection; 4. Poor grounding; 5. Solution to electrode leakage: 1. Modify the pipeline or install fake sensors; 2. Normal phenomena; 3. Check the wiring and connect the wires properly; 4. Connect the ground wire properly; 5. Repair the sensor.
b、 Liquid flow without output: 1. Reverse the two core wires of the signal transmission cable between the converter and the liquid flow; 2. The power supply is not connected or has poor contact; 3. There is leakage in the sensor instrument pipeline, housing, and end face. Solution: 1. Reverse the wire head; 2. Connect the power supply and maintain good contact; 3. Repair the sensor.
c、 Liquid does not flow but has output: 1. There is a broken circuit in the signal transmission cable connection between the converter and the liquid; 2. The signal cable is disconnected from the electrode connection; 3. Surface contamination or deposition of insulation layer on the electrode; 4. Poor grounding or open circuit. Solution: 1. Connect the cable properly; 2. Open the sensor and reconnect it; 3. Clean the surface of the electrode; 4. Connect the ground wire properly.
d、 Excessive error: 1. Zero point too high; 2. Not filled with liquid; 3. Excessive distortion of the power supply; 4. Poor grounding. Solution: 1. Adjust the zero point again; 2. Improve pipeline conditions, with sensors always filled with liquid; 3. Improve the power supply conditions to meet normal working conditions; 4. Connect the ground wire properly.