Outside the meter, some high-power electrical equipment and power equipment may become sources of interference, and power transformers, relays, switches, and power cords inside the meter may also become sources of interference. The introduction of interference is mainly as follows. 1.1 Series mode interference E n It is the interference superimposed on the measured signal, mainly caused by the following methods. 111 electromagnetic induction Electromagnetic induction, that is, magnetic coupling. There is a strong alternating magnetic field in the surrounding space of high-power transformers, AC motors, high-voltage power grids, etc. used in the project. The connecting wires between the signal source and the secondary instrument and the wiring inside the secondary instrument pass the alternating magnetic field The magnetic coupling in the circuit forms interference in the circuit. The closed loop of the secondary instrument will generate an induced potential in this changing magnetic field. The induced potential can be expressed by the formula. This induced potential is connected in series with the useful signal. When the signal source is far away from the secondary instrument, this interference situation is more prominent. In order to reduce the induced potential, items such as B, A, or cos must be reduced as much as possible, so it is necessary to keep the wires away from these heavy-duty electrical equipment and power grids, adjust the routing direction, and reduce the area of ​​the wire loop. Only by twisting the two signal wires at a short pitch, the magnetic induction potential can be reduced to the original 110. 112 static induction Static induction is the coupling of electricity. In the two opposing objects, if the potential of one of them changes, the potential of the other object also changes due to the capacitance between the objects. The interference source forms interference in the loop through capacitive coupling, which is the result of the interaction of the two electric fields. In this case, the potential of the wire 1 will induce a voltage to the ground on the wire 2 E. When two signal lines are laid in parallel with the power line, the distributed capacitance is not equal because the distance between the power line and the two signal lines is not equal. There will be a potential difference between the two signal lines, sometimes reaching tens of millivolts or even greater. When the signal line is twisted, the potential difference generated by the electric field on the two signal lines can be greatly reduced. After using electrostatic shielding, the induced potential can be reduced to 11. 113 additional thermoelectric and chemical potentials The chemical potential generated by different metal contacts, the thermoelectric potential generated by friction, and the corrosion of the metal can also become interference when in the electrical circuit. Most of this interference occurs in the form of direct current. It is easy to generate thermoelectric potential in the terminal board or reed relay. 114 vibration When a wire moves in a magnetic field, an induced electromotive force is generated. Therefore, it is necessary to fix the signal wire in a vibrating environment. 1.2 Common mode interference E cEc is the interference superimposed between any input terminal of the secondary instrument and the ground, mainly caused by the following methods. 121 Different ground potential In the ground, there is often a potential difference between different points, especially near high-power electrical equipment. When the insulation performance of these equipment is poor, this potential difference is greater. In the use of the instrument, there are often multiple ground points intentionally or unintentionally, so that the potential difference between different ground points is introduced into the instrument. This ground potential difference can sometimes reach more than 110V, and it appears at the same time in 2 The root signal line, as shown. 2 The common mode interference between the signal source and the secondary instrument can induce a common voltage Ec to ground at the two input terminals through electrostatic coupling, which appears in the form of common mode interference. 122 signal source is unbalanced bridge 3a) is a schematic diagram of the connection between the signal source and the secondary instrument when it is an unbalanced bridge. When the bridge power supply is grounded, except for the unbalanced voltage signal of the diagonal of the bridge, that is, the signal source voltage Ea, the two signal conductors have a common voltage Ec to ground, when the secondary instrument input terminal has leakage impedances Z3 and Z4 to ground , Ec generates leakage currents Ic1 and Ic2 through the leakage path to ground, as shown in 3b). Since the common mode interference does not overlap with the signal, it does not directly affect the instrument. However, it forms a leakage current to the ground through the measurement system. This leakage current can directly act on the instrument through the coupling of the resistance, causing interference. Therefore, an interference voltage will be generated at the two input terminals. After understanding the different sources of interference, you can take appropriate measures to eliminate or avoid different situations. Because all interference sources affect the instrument through a certain coupling channel, the interference can be suppressed by cutting off the coupling channel of the interference. 2 Suppression of interference There are many common anti-interference measures. If you want to suppress the interference, you must make a comprehensive analysis of the interference. You must take measures in three aspects: eliminating or suppressing the interference source, destroying the interference path, and weakening the sensitivity of the receiving circuit to noise interference. Solving the poor contact of connectors, virtual welding, etc. is a proactive measure to eliminate interference sources; in addition, for DC signals, a filter circuit can be added to the input end of the instrument to minimize the interference of the signal mixed in the actual process; in the actual process In addition, the isolation method should also be used to avoid the formation of interference fields as much as possible. Pay attention to keep the signal wires away from the power lines, and signal lines with different signal amplitudes should not be worn in the same conduit. Reasonable wiring should reduce the generation of stray magnetic fields Electrical components such as transformers are magnetically shielded. But in fact, many interference sources are difficult or impossible to eliminate. In this case, it is necessary to take protective measures according to the type of interference in the instrument application to suppress the interference. 2.1 Suppression of series mode interference The superposition of series mode interference and signal is difficult to eliminate once it is generated, and it should be prevented. The measures are generally as follows. Twisting of 211 signal wire Twisting the signal wires together can greatly reduce the area surrounded by the signal loop, and the induction potential En is greatly reduced by the formula. In addition, the twisting of the signal wires makes the distance between the two signal wires and the interference source approximately equal, and the distributed capacitance It can also be approximately equal, that is, C120. From the equation, the induced potential Ec is greatly reduced. Therefore, the twisting of the signal wire can greatly reduce the interference of series mode caused by the magnetic field and electric field into the loop through inductive coupling. 212 shield In order to prevent the interference of the electric field, the signal wire can be wrapped with a layer of metal mesh as a shielding layer, and then an outer layer of insulating layer can be wrapped on it to select the metal shielding wire as the signal transmission wire. The purpose of shielding is to isolate the coupling of the field and suppress the interference of various fields. However, after the shielding is adopted, the shielding layer must be properly grounded to reduce the distributed capacitance between the interference source and the signal wire, and to minimize the interference. If the shielding layer is a non-ferromagnetic material, there is no shielding effect on the magnetic field at a power frequency of 50 Hz. The signal wire can be inserted into the iron tube to make the wire magnetically shielded. 2.2 Common mode interference E C suppression Ec is the interference superposed between any input terminal of the secondary instrument and the ground, which is mainly caused by the difference in ground potential. To prevent common mode interference, a combination of shielding and grounding is usually used to suppress the interference. For safety reasons, the secondary instrument and the signal source housing are usually connected to the ground to maintain zero potential. The signal source circuit and the instrument system also need to be stably grounded. As shown, the two points are grounded. Due to the difference in ground potential, common mode interference occurs. Therefore, system grounding usually adopts single-point grounding on the signal source side or secondary instrument circuit, as shown. In order to improve the anti-interference ability of the instrument, the instrument manufacturer generally floats the amplifier to cut off the leakage path of the common mode interference, so that the interference cannot enter. In addition, in fact, the signal source side is also impossible to be insulated from the ground, using 4a) grounding It is impossible to completely eliminate the interference introduced by the ground potential difference, so in order to improve the anti-interference ability of the secondary instrument, the grounding method shown in 4b) is often used. In practical applications, shielding and grounding are usually combined to solve most interference problems. If the shield layer is grounded on both the signal side and the instrument side, the ground potential difference will form a loop through the shield layer. Since the ground resistance is usually much smaller than the resistance of the shield layer, a potential gradient will form on the shield layer and pass The distributed capacitance between the shielding layer and the signal wire is coupled into the signal circuit, so the shielding layer must also be grounded. In addition, the grounding of the signal conductor shield should be on the same side as the system ground, as shown in 4. That is, when the ungrounded signal source is connected to the grounded secondary instrumentation amplifier, the shielding layer should be connected to the common end of the amplifier as shown in 4a), and when the signal source is grounded and the amplifier is floating, the shielding layer should be as shown in 4b). It is connected to the common end of the signal source. In fact, the housing of the secondary instrument needs to be grounded for safety. And there must be distributed capacitance and leakage resistance between the input end of the instrument and the shell. It is impossible for the floating ground to completely cut off the leakage path. Therefore, when necessary, usually double-layer shielded floating ground protection is used. That is, an inner shield layer is set in the outer shell of the secondary instrument. The inner shield layer is not electrically connected to the signal input terminal and the outer shell. The inner shield layer leads a wire to connect with the shield layer of the signal wire. It is grounded at one point, so that the input protection shield and signal shield of the secondary instrument are stable to the signal source and are in an equipotential state, which can greatly improve the anti-interference ability of the secondary instrument. In the above, the anti-interference measures that are often used in actual engineering are introduced for the ways in which interference is generated in instrument applications. In actual use, the interference situation at the industrial production site is complex, and it is often difficult to solve the problem with an anti-interference method. Various methods such as twisting, shielding, grounding, filtering, and isolation of signal lines should be used in combination for different situations In order to obtain satisfactory results. (Organized by China Educational Equipment Purchasing Network) Ningbo XISXI E-commerce Co., Ltd , https://www.petspetscleaning.com