Source: Thermal control circle
1. What is the eddy current effect? When the magnetic line of force generated by
inductor coil passes through the metal conductor, the metal conductor will generate induction current , and it is a closed loop, similar to the shape of a water eddy current, so it is called eddy current , which is also called eddy current effect, which is actually an extension of the principle of electromagnetic induction .
Note: Eddy current sensor requires that the body to be tested must be a conductor. There is a small coil in the sensor probe of
, which is controlled by the controller to generate an oscillating electromagnetic field. When approaching the body to be measured, an induced current will be generated on the surface of the body to be measured, and a reverse electromagnetic field will be generated. At this time, the eddy current sensor judges the distance between the object to be measured based on the strength of the reverse electromagnetic field.
2, working principle and structure of eddy current sensor
Eddy current sensor is mainly composed of a flat circular coil placed on the frame. This coil can be pasted on the frame, or a groove is made on the frame and the wire is wound inside the groove. The figure below shows the structural principle of the eddy current sensor. It adopts the structural method of winding the wires in a narrow groove of the polytetrafluoroethylene frame to form a coil.
1—Electric Eddy Current Coil; 2—Probe Housing; 3—Position Adjustment Thread on the Housing; 4—Printed Circuit Board; 5—Clamp Nut 6— Power Indicator ; 7—Threshold Indicator; 8—Output Shield Cable; 9—Cable Plug
According to the penetration of eddy current in the conductor, the sensor can be divided into two categories: high-frequency reflective and low-frequency transmissive , but it is still similar in terms of basic working principle. The common one in use is the high-frequency reflective tactile, and the focus is on this basis.
sensor coil is excited by a high-frequency signal, causing it to generate a high-frequency alternating magnetic field φi. When the measured conductor is close to the coil, an eddy current e intersecting the magnetic field at the surface of the conductor in the range of the magnetic field is generated, and this eddy current will generate an alternating magnetic field φe to hinder the change of the external magnetic field. From an energy perspective, there is an eddy current loss in the conductor under test (the magnetic loss is ignored when the frequency is high). The energy loss reduces the Q value of the sensor and the equivalent impedance Z. Therefore, when the distance d between the measured object and the sensor changes, the Q value of the sensor and the equivalent impedance Z and the inductance L change, so the displacement amount is converted into electricity. This is the basic principle of eddy current sensors.
3, Application of eddy current sensors in thermal control equipment
3.1, axial displacement measurement
For many rotating machinery, including steam turbine , fuel steam turbine , water turbine , centrifugal and axial flow compressor , centrifugal pump , etc., axial displacement is a very important signal. Excessive axial displacement will cause excessive mechanism damage. measurement of axial displacement can indicate the axial gap between the rotating member and the fixed member or relative instantaneous displacement changes to prevent damage to the machine.
axial displacement refers to the gap between the rotor inside the machine along the axis direction, relative to the thrust bearing . Some mechanical failures can also be determined by detecting axial displacement: 1. Wear and failure of the thrust bearing; 2. Wear and failure of the balance piston; 3. Loosening of the thrust flange; 4. Locking of the coupling, etc.
axial displacement (axial clearance) measurement is often mixed with axial vibration. Axial vibration refers to the rapid change in the distance between the surface of the sensor probe and the body to be measured in the axial direction. This is a kind of axis vibration, expressed by peak-to-peak value. It has nothing to do with the average gap. Some faults can cause axial vibration. For example, the compressor is just the kick and mismatch.
Turbine axial displacement installation example
3.2, vibration measurement
measure radial vibration, you can see the working state of bearing , and you can also see the rotor imbalance, which is not in medium mechanical failure. It can provide information required for mechanical condition monitoring of the following key or basic machinery: 1. Industrial turbine , steam/fuel steam; 2. Compressor, air/special purpose gas, radial/axial direction; 3. expander ; 4. Power generator turbine, steam/fuel steam/water conservancy; 5. electric motor , generator; 6. exciter ; 7. Gear box; 8. Pump; 9. Fan, fan; 10. Reciprocating machinery.
Vibration measurement can also be used for continuous monitoring of general small machinery. It can provide important information for the early identification of various mechanical failures:
1, synchronous vibration of the shaft, instability of the oil film;
2, rotor friction, loose components;
3, bearing sleeve loosens, compressor kicks vibrating;
4. Rolling parts bearings fail, radial preload, internal/external inclusion misalignment;
5. Bearing Babbitt alloy wears, bearing clearance is too large, radial/axial;
6. Equilibrium (gas barrier) piston wear/failure/equilibrium (gas barrier) piston wear/failure , coupling "locked";
7, shaft bent, shaft crack;
8, electric motor air gap uneven, gear bite problem;
9, turbine blade channel resonance, impeller passes phenomenon.
3.3. Eccentricity measurement
Eccentricity is a measurement of the degree of shaft bending at low speeds. This bending can be caused by the following situations:
1. Original mechanical bending · Bending caused by temporary temperature rise · Under a static state, there must be some downward bending, sometimes called gravity bending, bending caused by external forces.
2. Measurement of eccentricity is very important for evaluating the comprehensive mechanical state of rotating machinery. Especially for steam turbines equipped with a TSI system (TSI), eccentric measurement has become an indispensable measurement item during startup or shutdown. It allows you to see the amplitude of the axis bending due to heat or gravity. The eccentric position of the rotor, also known as the radial position of the shaft, is often used to indicate the wear of the bearing and the magnitude of the load. If the situation is caused by misalignment, it is also used to determine the azimuth angle of the shaft, which can indicate whether the rotor is stable.
Turbine eccentricity and key phase installation example
3.4, expansion difference measurement
For a steam turbine generator set, when it starts and stops, due to different metal materials, different thermal expansion coefficients, and different heat dissipation, the thermal expansion of the shaft may exceed the expansion of the shell; it may cause the rotational components and stationary components of the turbine to contact each other, resulting in the damage of the machine. Therefore, the measurement of expansion difference is very important.
Turbine high pressure expansion difference installation example
3.5. Speed measurement
For all rotating machinery, the speed of the rotating machinery shaft needs to be monitored. The speed is an important indicator to measure the normal operation of the machine. The superiority of the eddy current sensor to measure the speed is incomparable to any other sensor. It can respond to zero speed and high speed, and has very strong anti-interference performance.
Application of eddy current sensor to measure gear speed
Schematic diagram of measuring speed
Analysis:
Measurement of rotation speed is actually a measurement of the frequency of the periodic signal caused by rotor rotation.There are many ways to measure speed. We use the counting method to measure speed, that is, to calculate the speed according to the number of cycles of the signal being measured within a certain time interval. In this system, there are i=6 protruding teeth on the speed measuring disc. Every time the rotor rotates, the eddy current sensor will output 6 cycle signals. Assume that the unit is s, the number of gears is N, f is the frequency, the rotor speed n is r/min,
can be found from the following formula:
n=(f/N)*60
turbine speed and zero speed installation example
4, precautions when using eddy current sensor
4.1. Requirements for the body to be measured
In order to prevent the magnetic field generated by electric eddy current from affecting the normal output of the instrument, a certain range of non-conductive dielectric space must be left around the sensor head. If more than two sensors are to be installed at a certain position at the same time, it must be considered whether cross-interference will occur. The specified distance must be maintained between the two probes. The surface area of the body to be measured should be more than 3 times the diameter of the probe. When the requirement of 3 times cannot be met, it can be appropriately reduced, but this is at the expense of sensitivity. Generally, when the diameter of the probe is equal to the surface area of the body to be measured, the sensitivity will be reduced to 70%. Therefore, when the sensitivity requirement is not high, the measured surface area can be appropriately reduced.
4.2. Requirements for working temperature
Generally, the maximum temperature of imported eddy current sensors is not greater than 180℃, while domestic ones can only reach 120℃. And these data come from manufacturers, which are very unreliable. According to various relevant data analysis, in fact, when the working temperature exceeds 70℃, the sensitivity of the eddy current sensor will be significantly reduced, and may even cause damage to the sensor.
4.3. Requirements for initial gap
All models of eddy current sensors have a good reading of linearity , so the appropriate initial gap must be adjusted when installing the sensor.
