Eddy current sensors can be divided into two types: high-frequency reflection type and low-frequency transmission type. Eddy current sensors, the working principle of which will be explained below:

The eddy current sensor works based on the principle of electromagnetic induction, but it is completely different from electromagnetic induction, and the interference of electromagnetic induction should be avoided in actual measurement. The formation of eddy current: It is now assumed that the core in a coil is made of a whole piece of ferromagnetic material. This core can be regarded as composed of many closed filaments perpendicular to the magnetic flux, thus forming many closed loops . When alternating current is applied to the coil, since the magnetic flux passing through the core changes periodically with the current, there must be an induced electromotive force in these closed loops. Under the effect of this electromotive force, many vortex-shaped currents are formed, which are called eddy currents. The working principle of the eddy current sensor is shown below:

When a high-frequency current i passes through the coil, a high-frequency magnetic field is generated around the coil, which acts on the metal body, but due to the skin effect, it cannot penetrate the metal body with a certain thickness, but only acts on the thin layer on the metal surface . Under the action of the alternating magnetic field, an induced current Ie is generated on the metal surface, which is an eddy current. The induced current also generates an alternating magnetic field and reacts on the coil in the opposite direction to the original magnetic field of the coil. The two magnetic fields are superimposed on each other, which changes the impedance Z of the original coil. The change of Z is only related to the resistivity ρ, magnetic permeability u, excitation electromagnetic intensity i, frequency f, coil geometry r, and the coil and metal conductor The distance between them. The impedance of the coil can be expressed by the following functional formula: Z = F (ρ, u, i, f, d). When the material of the object to be measured is constant, ρ and u are constants, and i, f and d in the meter are also fixed values, so Z becomes a single-valued function of the distance d.

3. Practical application

The eddy current sensor is widely used because of its large linear range of measurement, high sensitivity, simple structure, strong anti-interference ability, and is not affected by oil pollution and other media, especially non-contact measurement. It is mainly used in the following monitoring projects in thermal power plants:

1. Rotor speed: During the operation of the unit, the rotor speed is continuously monitored, and an alarm signal or a shutdown signal is issued when the speed is higher than the given value. Its working principle: According to the working principle of the eddy current sensor, a cyclically changing pulse amount will be generated between the approaching eddy current probe and the running rotor gear. Measuring this cyclically changing pulse amount can achieve Rotor speed monitoring.

2. Zero speed of the rotor: Zero speed is an indication that the unit is running below a minimum rotation speed. This is to prevent the gravity bending of the rotating shaft of the unit during stop. The working principle is the same as that of the rotor speed.

3. Eccentricity: The eccentricity of the rotor is an indicator of its bending under thermal stress. It is observed when the gear mechanism is turned, and it provides reliable and accurate monitoring data for rotor misalignment. The eddy current probe can continuously monitor the peak-to-peak value of eccentricity, and this value is synchronized with the key phase pulse. Its working principle: The eccentric probe is installed at the journal of the front bearing box of the steam turbine, and its core part is an inductance coil. When the large shaft rotates, if there is an eccentricity, the distance between the shaft and the inductance coil will change periodically, so that the inductance of the inductance coil will change periodically. By measuring this inductance change value, the shaft can be measured. Eccentricity.

4. Key phase: The key phase is a physical quantity describing the position of the rotor at a certain moment. The key phase probe and the eccentric probe together monitor the eccentricity of the large shaft, which can accurately reflect the specific phase angle of the eccentricity of the large shaft. Its working principle: The key phase measurement is to set a groove or convex groove on the measured shaft, which is called the key phase mark. When this groove or convex groove is turned to the position of the probe, it is equivalent to the distance between the probe and the measured surface is changed. The sensor will generate a pulse signal, and the pulse signal will be generated for each rotation of the shaft. The position of the axis in each revolution. Therefore, by comparing the pulse signal with the vibration signal of the shaft, the phase angle of the vibration can be determined.

5. Vibration: The eddy current probe mainly monitors the relative vibration of the main shaft relative to the bearing seat. Its working principle: the change of the distance between the coil of the eddy current probe and the measured metal body can be transformed into the change of the three electrical parameters of the coil's equivalent inductance, equivalent impedance and quality factor, and then matched with the corresponding preamplifier , These three electrical parameters can be further transformed into voltage signals, which can realize the measurement of vibration. There are several reasons for vibration:

(1) Vibration caused by the center of the unit is not correct. If the vacuum drops during the operation of the unit, the temperature of the exhaust steam will rise and the rear bearings will rise, thus destroying the vibration caused by the center of the unit.

(2) Vibration due to unbalanced rotor mass.

(3) Vibration due to elastic bending of the rotor.

(4) Vibration due to the unstable oil film of the bearing.

(5) Vibration due to friction inside the steam turbine.

(6) Vibration due to water impact.

(7) The steam turbine vibrates when it reaches the critical speed.

6. Axial displacement: The axial displacement refers to the internal rotor of the unit along the axis direction, relative to the gap between the thrust bearings. By measuring the axial displacement, the axial gap or relatively instantaneous axial change between the rotating part and the fixed part can be indicated. Its working principle is the same as that of vibration measurement, but it should be noted that the measurement of axial displacement is often confused with axial vibration. Axial vibration refers to the rapid change of the distance between the surface of the sensor probe and the measured object in the axial direction, which is expressed by the peak-to-peak value, and it has nothing to do with the average gap.

7. Expansion difference: When the unit is running, the rotor will expand when heated, because the rotor is limited by the thrust bearing, so it can only be extended to the low pressure side in the axial direction. Due to the small size of the rotor and the direct impact of steam, the temperature rise and thermal expansion are relatively fast, while the volume of the cylinder is large, and the temperature rise and thermal expansion are relatively slow. Before the thermal expansion of the rotor and the cylinder has not stabilized, the thermal expansion value existing between them is simply referred to as the expansion difference. Regulations on the direction of differential expansion: when the unit is started or when the load is increased, it is a heating process of steam to metal. The rotor heats up faster than the cylinder. The expansion value greater than the cylinder is called the positive expansion difference. During shutdown or load reduction, it is a cooling process. The rotor cools faster than the cylinder, so the rotor shrinks faster, that is, the axial expansion of the rotor is less than the expansion of the cylinder, which is called the negative expansion difference.

4. Installation of eddy current sensor

1. Precautions for installation

(1) The installation gap of the probe.

(2) The safe distance between the probe head and the mounting surface.

(3) Sealing and insulation of cable adapters.

(4) The probe is corrosion resistant.

(5) The minimum distance between each probe.

(6) The firmness of the probe mounting bracket.

(7) Installation of cables and extension cables in the probe.

(8) High temperature and high pressure environment of the probe.

2. Factors affecting sensor characteristics:

(1) The influence of the surface flatness of the measured object on the sensor

The irregular surface of the measured object will bring additional errors to the actual measurement. Therefore, the surface of the measured object should be flat and smooth, and there should be no defects such as protrusions, holes, scores, and grooves.

(2) The influence of the magnetic effect on the sensor surface

The eddy current effect is mainly concentrated on the surface of the measured object. If the residual magnetic effect is formed during processing, as well as uneven quenching, uneven hardness, and uneven crystal structure, etc., the sensor characteristics will be affected.

(3) The effect of the surface coating of the measured object on the sensor

The effect of the coating on the surface of the test object on the sensor is equivalent to changing the material of the test object. Depending on the material and thickness of the coating, the sensitivity of the sensor will change slightly.

(4) The effect of the surface size of the measured object on the sensor

Because the magnetic field range generated by the probe coil is fixed, the eddy current formed on the surface of the measured body is also fixed. In this way, there are certain requirements on the surface size of the measured body. Generally, when the surface of the measured object is a flat surface, with the point facing the center line of the probe as the center, the diameter of the measured surface should be greater than 1.5 times the diameter of the probe head. When the measured object is a circular axis and the center line of the probe is perpendicular to the axis, the diameter of the measured axis is generally more than 3 times the diameter of the probe head, otherwise the sensitivity of the sensor will decrease, the smaller the surface of the measured object, the sensitivity The more drops. In experimental tests, when the surface size of the measured object is the same as the diameter of the probe head, its sensitivity will drop to about 72%. The thickness of the measured body will also affect the measurement result. The depth of the eddy current field in the measured body is determined by the frequency, material conductivity, and magnetic permeability. Therefore, if the measured object is too thin, it will cause insufficient eddy current effect and reduce the sensitivity of the sensor.

3. Sensor installation requirements

(1) Requirements for working temperature

The maximum allowable temperature of the general eddy current sensor is ≤180? C. In fact, if the operating temperature is too high, not only the sensitivity of the sensor will be significantly reduced, but also the sensor will be damaged. Therefore, the sensor must be installed when measuring the high, medium and low shaft vibration In the bearing shell, only the special high-temperature eddy current sensor is allowed to be installed near the steam seal.

(2) Requirements for the measured body

In order to prevent the magnetic field generated by the eddy current from affecting the normal output of the instrument, a certain range of non-conductive medium space must be left around the sensor head during installation. If two or more sensors need to be installed in a certain part during the test, in order to avoid cross interference, a certain distance should be maintained between the two sensors. In addition, the surface area of ​​the measured body should be more than 3 times the diameter of the probe, the surface should not have scars, small holes and gaps, and the surface plating is not allowed. The material of the measured body should be consistent with the material calibrated by the probe and the front device.

(3) Requirements for probe bracket

The probe is fixed on the bearing seat by a bracket. The bracket should have sufficient rigidity to increase its natural frequency to avoid or reduce the excited natural vibration of the bracket when the measured body vibrates.

(4) Requirements for initial clearance

The eddy current sensor should be at a certain gap voltage (the gap between the top of the sensor and the measured object, the indication on the meter is generally voltage), the reading has a good linearity, so it must be adjusted properly when installing the sensor Initial gap.

After the rotor rotates and the unit is loaded, the rotor will move relative to the sensor. If the sensor is installed on the top of the bearing, the clearance will be reduced; if it is installed in the horizontal direction of the bearing, the clearance depends on the direction of rotation of the rotor; when the steering is fixed, the clearance depends on whether it is installed on the right or left. In order to obtain a suitable working clearance value, the displacement value and displacement direction of the journal of the rotor from the static state to the rotating state of the unit under load should be estimated during installation, so as to be considered when adjusting the initial clearance. According to field experience, the rotor rises from static to working speed, and the journal lift is about 1/2 of the bearing bush clearance; the horizontal displacement is related to the bearing bush form, the clearance on both sides of the bearing bush, and the working state of the unit's sliding pin system. The general displacement value is 0.05- 0.20mm.

When adjusting the initial gap of the sensor, in addition to the above factors, the maximum vibration value and the original rotor sway value must also be considered. The initial gap of the sensor should be greater than 1/2 of the maximum amplitude of the rotating shaft and the original sway value of the rotating shaft.

3. Installation steps

(1) Before inserting the probe into the installation hole, make sure that there is no debris in the hole, and the probe can rotate freely without being entangled with the wire.

(2) In order to avoid scratching the end of the probe or the monitoring surface, a non-metal gap gauge can be used to measure the gap of the probe.

(3) The probe gap can also be adjusted by the electric steam method connecting the probe wire to the extension cable and the pre-probe.

When the probe gap is adjusted properly, tighten the locknut. At this time, it should be noted that excessive tightening will damage the thread. After the probe is fixed, the probe's wire should also be firm. The length of the extension cable should be the same as the length required by the front end. Any lengthening or shortening will cause measurement error.

The front device should be placed in a cast aluminum box to avoid mechanical damage and pollution. Do not allow extra cables to be attached to the box. Without changing the length of the cable from the probe to the front end, multiple front ends are allowed to be installed in the same box. Cable routing. Use proper isolation and shielding grounding to minimize the interference to the signal.

4. Installation of extension cable

The extension cable is an important part of the eddy current sensor as the middle part connecting the probe and the pre-probe. Therefore, the installation of the extension cable should ensure that it is not easily damaged during use, and the high temperature environment of the extension cable should be avoided. The connection between the probe and the extension cable should be locked, and the connector should be wrapped with a heat-shrinkable tube to avoid grounding and prevent the connector from loosening. When laying the extension cable, the cable radius should not be too small to damage the cable. It is generally required that the diameter of the extension cable reel should not be less than 55mm.

5. Installation of front-end device

The preamplifier is the signal processing part of the entire sensor system. It is required to install it in a place away from the high temperature environment. The surrounding environment should be free of obvious steam and water droplets, non-corrosive vapor, dryness, low vibration, preamplifier A place where the ambient temperature does not differ much from room temperature. When installing, do not contact the metal part of the front housing with the cabinet or the ground. During installation, it is necessary to avoid other interference signals from affecting the measurement circuit.

6. Locking of speed, zero speed, eccentricity, key phase sensor installation gap

These four sensors can be installed using feeler gauge to measure the installation gap. Insert a feeler gauge that sets the thickness of the installation gap between the end face of the probe and the measured surface. The installation gap of these four sensors is about 1.3mm. When the feeler gauge is pressed against the end face of the probe and the measured surface, the probe can be tightened.

7. Locking the installation gap of shaft vibration sensor

Connect the probe, extension cable, and preamplifier, and connect the power supply to the sensor system. Use a high-precision multimeter to monitor the output voltage of the preamplifier. At the same time, adjust the gap between the probe and the measured surface. The output voltage of the preamplifier When it is between 10-11vDC, tighten the two fastening nuts of the probe to fix the probe.

8. Zero lock of shaft displacement

(1) The measurement principle of the shaft displacement monitoring system:

The shaft displacement monitoring system uses the relationship between the output voltage of the eddy current sensor and the vertical distance of the metal surface to be measured within a certain range to convert the displacement signal into a voltage signal and send it to the monitor to achieve the purpose of monitoring and protection.

(2) Zero position lock of shaft displacement sensor:

Factors that must be referenced for the zero position lock of the shaft displacement sensor:

a) Clearance â–³ of thrust pad of large shaft

b) The position of the large shaft (that is, the large shaft thrust disk has been leaned against the working surface or non-working surface of the thrust pad)

c) Calibration data of displacement monitors and sensors

Known: △ = 0.36mm, the range of the shaft displacement monitor is ± 1.25mm, and the large shaft thrust plate leans against the working surface.

The shaft displacement is locked with the calculated value of the zero voltage of the sensor, which is more accurate and reliable. Taking the 11mm sensor as an example, it is known that: △ = 0.36mm, the large-axis thrust disk leans on the working surface, the range of the shaft displacement monitor is ± 1.25mm, the sensor sensitivity F = 4.0V / mm, and the zero installation voltage Vo = 10.0V . Then the calculation of zero voltage X:

X = Vo-F × 1/2 × △ = 9.28V

After the final zero lock, the monitor should show -0.18mm.

Note: If the large-axis thrust disk rests on the non-working surface of the thrust pad, X should be calculated as follows:

X = Vo + F × 1/2 × △ = 10.72V

Finally, install the lock sensor according to the calculated X value, the monitor display should be 0.18mm

(3) The problems that should be paid attention to when the sensor zero lock is installed on site:

a) Without considering the thrust pad clearance, the meter will produce a measurement error of 1/2 × △ mm.

b) Reverse the push shaft gap of 1/2 × △ mm, the meter will produce a measurement error of △ mm.

5. Common faults and treatment methods of eddy current sensors

1. Common faults:

(1) The eddy current probe is damaged.

(2) The connection between the probe wire and the extension cable is loose.

(3) The connection between the extension cable and the front device is loose.

(4) The front-end device and extension cable are faulty.

(5) The extension cable is grounded.

(6) The connection between the probe lead and the extension cable is not well insulated and grounded.

2. Processing method:

(1) Replace the eddy current probe.

(2) Tighten the connector of the probe lead and extension cable.

(3) Tighten the connecting screw between the extension cable and the front device.

(4) Replace the front-end device and extension cable.

(5) Replace the extension cable or wrap its damaged grounding part with insulation tape.

(6) Wrap the connector of the probe lead and the extension cable with a heat shrinkable tube.

3. Several suggestions for common troubleshooting methods:

(1) When replacing the eddy current probe, care should be taken to avoid scratching the probe, and the connecting wire must not be wound multiple times.

(2) The connection between the probe lead and the extension cable is a lock with a locking function. When tightening, avoid excessive force to avoid damage to the lock.

(3) Do not use excessive force to tighten the connection screw between the extension cable and the front device, so as not to cause screw slippage.

(4) The replaced preamplifier should be consistent with the probe and extension cable model. The preamplifier should be placed in a cast aluminum box to avoid other interference signals affecting the measurement accuracy.

(5) When replacing the extension cable, it should be noted that the diameter of the coil of the cable should not be too small to avoid damage to the cable. Generally, the coil diameter should not be less than 55mm.

(6) When processing the connection between the probe lead and the extension cable, use a heat-shrinkable tube to wrap it. Do not use electrical tape, as the oil mist will dissolve the contaminants on the tape and contaminate the connector. When you need to open the connection between the probe lead and the extension cable, use a blade to cut a small hole in the metal joint. In this process, be careful of scratching the cable.

4. Dangerous points that may occur when dealing with common faults:

(1) Workers should supervise each other by accidentally touching other operating equipment.

(2) When replacing the vibration probe during operation, high temperature burns should be worn, and protective gloves should be worn.

(3) When replacing the extension cable, the diameter of the coil is too small to damage the cable, and the diameter of the coil should not be less than 55mm.

(4) When the probe is damaged when the probe is replaced, the staff should take corresponding protective measures.

(5) Due to the damage to the probe caused by excessive temperature, the working temperature of the probe should generally be less than 180C. Only the special high-temperature eddy current sensor is allowed to be installed near the steam seal.

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