1 Way of interference generation
Interference comes from sources of interference and may be present both inside and outside the meter. Outside the instrument, some high-powered electrical equipment and power equipment may become sources of interference, and power transformers, relays, switches, and power lines inside the instrument 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 signal under test, which is mainly generated in the following manner.
111 electromagnetic induction
Electromagnetic induction, that is, magnetic coupling. There are strong alternating magnetic fields 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 creates an interference in the circuit, and the closed circuit of the secondary meter will generate an induced potential in the varying magnetic field, and the induced potential can be expressed by the equation. This induced potential is connected in series with the useful signal. When the signal source is far away from the secondary meter, the interference is more prominent. In order to reduce the induced potential, items such as B, A or cos must be minimized, so it is necessary to keep the wires away from these strong electrical equipment and power grids, to adjust the direction of the wiring and to reduce the loop area of ​​the conductor. Only by twisting the two signal lines at a short pitch, the magnetic induction potential can be reduced to the original 110.
112 electrostatic induction
Electrostatic induction is the coupling of electricity. In the opposite two 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 source of interference is the formation of interference in the loop through capacitive coupling, which is the result of the interaction of the two electric fields.
In the middle, the potential of the wire 1 induces a voltage to the ground on the wire 2. When the two signal wires 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. A potential difference will be generated on the two signal lines, sometimes up to tens of millivolts or more. When the signal line is twisted, the potential difference generated by the electric field on the two signal lines is greatly reduced. After using electrostatic shielding, the induced potential can be reduced to 11.
113 additional thermoelectric potential and chemical potential
The chemical potentials generated by different metal contacts, the thermoelectric potential generated by friction, and the corrosion of metals may also become interferences in the electrical circuit. Such interferences mostly occur in the form of direct current. The thermoelectric potential is easily generated at the terminal block or the reed relay.
114 vibration
When the wire moves in a magnetic field, an induced electromotive force is generated. Therefore, it is necessary to fix the signal wires in a vibrating environment.
1.2 Common mode interference E
cEc is the interference superimposed between any input of the secondary meter and the ground, and is mainly generated in the following manner.
121 ground potential is different
In the earth, there is often a potential difference between different points, especially in the vicinity of high-power electrical equipment. When the insulation performance of these equipments is poor, this potential difference is greater. In the use of the instrument, the input loop often has multiple grounding points intentionally or unintentionally, so that the potential difference of different grounding points is introduced into the meter, and the ground potential difference can sometimes reach 110V or more, and simultaneously appears in 2 The root signal line is as shown.
2 The common mode interference between the signal source and the secondary instrument can induce the common voltage Ec to the ground at the two input terminals by 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 secondary instrument when the signal source 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 wires have a common voltage Ec to the ground, when the secondary instrument input terminal has leakage impedance Z3 and Z4 to the ground. Ec generates leakage currents Ic1 and Ic2 through the leakage path to the ground, as shown in 3b).
Since the common mode interference does not overlap with the signal, it does not directly affect the meter. But it measures the leakage current generated by the system to the ground. This leakage current can directly act on the meter through the coupling of the resistor, causing interference. Thus an interference voltage will be generated at both inputs.
After understanding the different sources of interference, you can take appropriate measures to eliminate or avoid them for different situations. Since all sources of interference affect the meter through a certain coupling channel, interference can be suppressed by cutting off the coupled coupling channel.
1 Way of interference generation
Interference comes from sources of interference and may be present both inside and outside the meter. Outside the instrument, some high-powered electrical equipment and power equipment may become sources of interference, and power transformers, relays, switches, and power lines inside the instrument 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 signal under test, which is mainly generated in the following manner.
111 electromagnetic induction
Electromagnetic induction, that is, magnetic coupling. There are strong alternating magnetic fields 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 creates an interference in the circuit, and the closed loop of the secondary meter will generate an induced potential in the varying magnetic field, and the induced potential can be expressed by the equation. This induced potential is connected in series with the useful signal. When the signal source is far away from the secondary meter, the interference is more prominent. In order to reduce the induced potential, items such as B, A or cos must be minimized, so it is necessary to keep the wires away from these strong electrical equipment and power grids, to adjust the direction of the wiring and to reduce the loop area of ​​the conductor. Only by twisting the two signal lines at a short pitch, the magnetic induction potential can be reduced to the original 110.
112 electrostatic induction
Electrostatic induction is the coupling of electricity. In the opposite two 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 source of interference is the formation of interference in the loop through capacitive coupling, which is the result of the interaction of the two electric fields.
In the middle, the potential of the wire 1 induces a voltage to the ground on the wire 2. When the two signal wires 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. A potential difference will be generated on the two signal lines, sometimes up to tens of millivolts or more. When the signal line is twisted, the potential difference generated by the electric field on the two signal lines is greatly reduced. After using electrostatic shielding, the induced potential can be reduced to 11.
113 additional thermoelectric potential and chemical potential
The chemical potentials generated by different metal contacts, the thermoelectric potential generated by friction, and the corrosion of metals may also become interferences in the electrical circuit. Such interferences mostly occur in the form of direct current. The thermoelectric potential is easily generated at the terminal block or the reed relay.
114 vibration
When the wire moves in a magnetic field, an induced electromotive force is generated. Therefore, it is necessary to fix the signal wires in a vibrating environment.
1.2 Common mode interference E
cEc is the interference superimposed between any input of the secondary meter and the ground, and is mainly generated in the following manner.
121 ground potential is different
In the earth, there is often a potential difference between different points, especially in the vicinity of high-power electrical equipment. When the insulation performance of these equipments is poor, this potential difference is greater. In the use of the instrument, the input loop often has multiple grounding points intentionally or unintentionally, so that the potential difference of different grounding points is introduced into the meter, and the ground potential difference can sometimes reach 110V or more, and simultaneously appears in 2 The root signal line is as shown.
2 The common mode interference between the signal source and the secondary instrument can induce the common voltage Ec to the ground at the two input terminals by 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 secondary instrument when the signal source 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 wires have a common voltage Ec to the ground, when the secondary instrument input terminal has leakage impedance Z3 and Z4 to the ground. Ec generates leakage currents Ic1 and Ic2 through the leakage path to the ground, as shown in 3b).
Since the common mode interference does not overlap with the signal, it does not directly affect the meter. But it measures the leakage current generated by the system to the ground. This leakage current can directly act on the meter through the coupling of the resistor, causing interference. Thus an interference voltage will be generated at both inputs.
After understanding the different sources of interference, you can take appropriate measures to eliminate or avoid them for different situations. Since all sources of interference affect the meter through a certain coupling channel, interference can be suppressed by cutting off the coupling channel of the interference.
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