Detailed explanation of frequency converter interference problems and treatment methods
With the popularization of frequency converter applications, the interference problem caused by frequency converters has become more and more prominent. This article will detail the relevant knowledge of EMC for end users and solve the problem that some engineering equipment is normal when tested in the factory, but it is installed on site. The causes of problems related to interference will appear, and corresponding solutions will be given for these interference phenomena.
1. The concept of electromagnetic compatibility
Electromagnetic compatibility (EMC) refers to the ability of a device or system to operate as required in its electromagnetic environment without causing intolerable electromagnetic interference to any equipment in its environment.
Therefore, EMC includes two requirements. On the one hand, it means that the electromagnetic interference caused by the equipment to the environment during normal operation cannot exceed a certain limit; on the other hand, it means that the electromagnetic interference existing in the environment of the equipment has a certain limit. degree of immunity, i.e. electromagnetic susceptibility.
In the International Electrotechnical Commission standard IEC, electromagnetic compatibility (EMC) is defined as a system or device that can operate normally in the electromagnetic environment in which it is located without causing interference to other systems and devices.
EMC includes two parts: EMI (electromagnetic interference) and EMS (electromagnetic tolerance). The so-called EMI electromagnetic interference is the electromagnetic noise generated by the machine itself in the process of performing its due functions that is detrimental to other systems; and EMS refers to the machine. The ability to perform its intended function without being affected by the surrounding electromagnetic environment.
The normal working ability of various electrical or electronic equipment to meet the design requirements with a specified safety factor in a common space with a complex electromagnetic environment. Also called electromagnetic compatibility.
Its meaning includes:
Ø Mutual consideration between electronic systems or equipment in the electromagnetic environment.
Ø The electronic system or equipment can work normally according to the design requirements in the natural electromagnetic environment.
EMC is very common in our daily life, industrial equipment, etc. It can be said that it is everywhere. For example, when we are making a landline phone call and there is a mobile phone nearby when receiving text messages, the receiver will make a buzzing noise. This is common. EMC phenomenon. There are also daily microwave ovens that must use electromagnetic shielding on the front panel and shell. Lightning in nature, static electricity on human hands, etc. are all common EMC phenomena.
In addition, in industrial sites, equipment such as frequency converters, rectifiers, and welders interfere with PLCs, walkie-talkies interfere with other equipment, and so on.
1. Three elements of electromagnetic compatibility
For electromagnetic compatibility problems to occur in the system, three factors must exist, namely electromagnetic interference sources, coupling pathways, and sensitive equipment. Therefore, when encountering electromagnetic compatibility problems, we must start from these three factors to find the root cause of interference. In engineering practice, multiple measures are often taken to solve electromagnetic compatibility problems.
(1) Source of electromagnetic interference
Sources of electromagnetic interference are divided into two types: natural and man-made.
Natural interference sources mainly include various phenomena occurring in the atmosphere, such as noise generated by thunder and lightning. Natural interference sources also include cosmic noise from the sun and outer space, such as solar noise, interstellar noise, galactic noise, etc.
There are many sources of human interference, such as electric bells, gas rectifiers, mobile phones, electric heaters, walkie-talkies, soft starters, frequency converters, servos, rectifiers, contactors, intermediate relays, switches, fluorescent lamps, engine ignition systems, and arc welding. Machines, thyristors, inverters, corona discharge, various industrial, scientific and medical high-frequency equipment, noise caused by electric railways and nuclear electromagnetic pulses generated by nuclear explosions, etc.
(2) Coupling approach
That is, the path or medium through which electromagnetic interference is transmitted. Electromagnetic interference propagation pathways are generally divided into two types: conduction coupling and radiation coupling. The detailed division of coupling pathways is shown in Figure 1.
Figure 1 Division of coupling pathways of electromagnetic interference
1) Conductive coupling
Conducted coupling is one of the main coupling pathways between interference sources and sensitive equipment.
Conducted coupling must have a complete circuit connection between the interference source and the sensitive equipment. The electromagnetic interference is transmitted from the interference source to the sensitive equipment along this connection circuit, causing electromagnetic interference.
The connection circuit of conductive coupling includes interconnecting wires, power lines, signal lines, ground conductors, conductive components of the equipment, common impedance, circuit components, etc.
Conductive coupling can be divided into three basic methods according to its coupling mode:
Ø Capacitive coupling
Ø Inductive coupling
In actual engineering, these three coupling methods exist at the same time and are related to each other.
Among them: circuit coupling is the most common and simplest conductive coupling method. The simplest circuit conductive coupling model is shown in Figure 2.
Figure 2 General form of circuit conductive coupling
When circuit 1 is acted upon by voltage U1, the voltage is added to the common impedance Z12 via Z1.When circuit 2 is open, the voltage coupled from circuit 1 to circuit 2 is
When the currents of two circuits flow through a common impedance, the voltage formed by the current of one circuit on the common impedance will affect the other circuit. This is common impedance coupling. The coupling through the common ground impedance is shown in Figure 3. indicates that for this kind of coupling, the ground wire should be shortened and thickened as much as possible to reduce the impedance of the common ground wire.
Figure 3 Common impedance interference when two devices are connected to a common ground wire at the same time
Multiple grounding points will produce interference voltages due to different impedances to the ground. The way to solve this problem is to use one point of grounding.
The one-point grounding diagram of multiple frequency converters is shown in Figure 4.
Figure 4 One-point grounding diagram of multiple frequency converters
Solution to common impedance interference:
Make two current loops or systems independent of each other. The signals are independent of each other and circuit connections are avoided to avoid circuit coupling.
Limit the coupling impedance to make the coupling impedance as low as possible. When the coupling impedance approaches zero, it is called circuit decoupling. In order to make the coupling impedance small, the wire resistance and wire inductance must be made as small as possible.
Circuit decoupling: that is, the electrical connection between different current loops is only made at a single point. At this point, it is impossible for circuit interference current to flow, thus achieving the purpose of circuit decoupling between current loops.
Isolation: Isolation technology is often used in related systems with widely different levels (such as between signal transmission equipment and high-power electrical equipment)
2) Capacitive coupling
The Capacitive Coupling (The Capacitive Coupling) is also called electrical coupling, which is caused by the electric field interaction between two circuits. The capacitive coupling model and its equivalent circuit between two circuits composed of a pair of parallel wires are shown in Figure 5.
Figure 5 Coupling model and equivalent circuit
The equivalent formula for Un is as follows:
When if R is low impedance and satisfies
Then, the above formula can be simplified to UN≈jωC12RU1
The interference effect of capacitive coupling is equivalent to connecting a current source with an amplitude In=jωC12U1 between conductor 2 and ground.
This formula is the most important formula describing capacitive coupling between two conductors, and it clearly shows that the voltage picked up (coupled) depends on the relevant parameters.
It is assumed that the voltage U1 and operating frequency f of the interference source cannot be changed, leaving only two parameters C12 and R that reduce capacitive coupling.
Methods to reduce coupling capacitance:
The electrical parameters of the interference source system should make the voltage change amplitude and rate of change as small as possible;
The interfered system should be designed to have as low resistance as possible;
The coupling part of the two systems should be arranged so that the coupling capacitance is as small as possible. For example, in wire and cable systems, the spacing should be as large as possible, the conductors should be as short as possible, and parallel routing should be avoided;
The interference source and interference object can be electrically shielded. The purpose of shielding is to cut off the power line path between the conductor surface of the interference source and the conductor surface of the interference object to minimize the coupling capacitance.
3) Inductive coupling
Inductive coupling (Inductive Coupling) is also called magnetic coupling, which is caused by the interaction of magnetic fields between two circuits. When a current I flows in a closed circuit, the current produces a magnetic flux proportional to the current.
The value of the inductance depends on the geometry of the circuit and the loop area, direction and distance of the interference source and the sensitive circuit, as well as whether the interference source and the sensitive circuit are shielded. The formula for electromotive force is:
Methods to suppress inductive coupling:
The electrical parameters of the interference source system should make the amplitude and rate of current change as small as possible;
The victim system should have high impedance;
To reduce the mutual inductance between the two systems, make the wires as short as possible and as large as possible, avoid parallel wiring, and reduce the area enclosed by the current loop when using a two-wire structure;
Set up magnetic shielding for interference sources or interference objects to suppress interference magnetic fields.
Use balancing measures to make most of the interference magnetic fields and coupled interference signals cancel each other out.If the interfered wire loop is placed in the interference field in such a way that the cutting magnetic field lines are minimal and the two wires are perpendicular, the coupled interference signal will be minimal; in addition, if the interference source wires are balanced and twisted, the magnetic fields generated by the interference currents can be offset.
4) Radiation interference
When a sensitive device is relatively far away from the interference source, the interference propagates outward in the form of electromagnetic waves through the surrounding media, and the interference electromagnetic energy is emitted to the surrounding space according to the rules of the electromagnetic field.
The main ways of radiation coupling include the combination of transmitting pairs of antennas, cables, wires, and chassis.
Radiation coupling is usually divided into three types, namely antenna-to-antenna coupling, field coupling and cable coupling, and wire-to-wire coupling.
Among them, industrial sites mainly involve the coupling of fields and lines, which refers to the inductive coupling of the space electromagnetic field to the wires existing therein, thereby forming distributed electromagnetic interference sources on the wires;
In addition, the cables of the equipment are generally composed of the connection wires of the signal loop, the power supply wire of the power loop, and the ground wire. Each of these wires consists of input impedance, output impedance and return wire to form a loop. Therefore, equipment cables are the parts of the equipment's internal circuits that are exposed outside the chassis. They are most susceptible to coupling by the radiation field of the interference source and induce interference voltage or interference current, which enters the equipment along the wires to form radiated interference.
For situations where the wire is relatively short and the electromagnetic wave frequency is relatively low, readers can regard the loop formed by the wire and impedance as an ideal closed loop. Interference caused by electromagnetic fields through closed loops belongs to closed loop coupling.
For situations where the cable is relatively long and the electromagnetic wave frequency is relatively high, the induced voltage on the wire is uneven and consistent. It is necessary to equate the induced voltage into many distributed voltage sources and use transmission line theory to deal with it.
Measures to suppress radiation interference:
Radiation shielding: insert a metal shield between the interference source and the interference object to block the spread of interference.
Distance isolation: Increase the distance between the interference source and the interfered object. This is because in the near field area, the field intensity is proportional to the square or cube of the distance. When the distance increases, the field attenuates quickly.
Shielding methods are generally required for field radiation interference to cables. For equipment such as frequency converters, RFI filters are used to weaken conducted interference and at the same time weaken radiation interference. In addition, wiring must comply with the wiring rules of the frequency converter.
(3) Sensitive equipment
In actual engineering. The coupling pathways through which sensitive equipment is attacked by electromagnetic interference are conduction coupling, radiation coupling, inductive coupling and their combinations. Sensitive equipment (Victim) refers to devices, equipment, subsystems or systems that will be harmed and suffer electromagnetic hazards when exposed to electromagnetic energy emitted by electromagnetic interference sources, resulting in performance degradation or failure. Many devices and equipment are both sources of electromagnetic interference and sensitive equipment. Common sensitive equipment in industrial sites include PLCs, field instruments, etc.
2. Common types of interference at industrial sites
Common types of interference in industrial sites include surge, harmonic interference, fast pulse group interference, electrostatic interference and radiation interference, etc.
Surge is also called sudden wave. In layman's terms, it is an instant overvoltage that exceeds the normal operating voltage. Essentially, a surge is a violent pulse that occurs in just a few millionths of a second.
Possible causes of surges include heavy equipment, short circuits, power switching, or large engines. Current experiments have proven that products containing surge blocking devices can effectively absorb sudden huge energy to protect connected equipment from damage.
Among them, surges caused by lightning strikes are the most harmful. During lightning discharges, dangerous overvoltages may occur within a range of 1.5 to 2km centered on the lightning strike. The (external) surge caused by lightning strikes is characterized by a single-phase pulse type with huge energy. The voltage of an external surge can quickly rise from a few hundred volts to 20,000V in a few microseconds, and can be transmitted over a considerable distance.
According to ANSI/IEEE C62.41-1991, the instantaneous surge can reach 20,000V and the instantaneous current can reach 10,000A. According to statistics, surges outside the system mainly come from lightning and the impact of other systems, accounting for about 20%. The surge voltage of lightning is shown in Figure 6.
Figure 6 Waveform of surge voltage
The hazards caused by lightning strikes mainly include the following aspects:
Induced lightning surge overvoltage: The high-speed changing electromagnetic field generated by lightning strikes. The electric field radiated by lightning acts on the conductor, inducing a very high overvoltage. This type of overvoltage has a steep front and rapidly decays.
Direct lightning surge overvoltage: Direct lightning strikes on the power grid. Due to the huge instantaneous energy and extremely destructive power, there is no equipment that can protect direct lightning strikes.
Lightning strike conducted surge overvoltage: It is conducted from distant overhead lines. Since the equipment connected to the power grid has different suppression capabilities for overvoltage, the conducted overvoltage energy weakens with the extension of the line.
Oscillating surge overvoltage: The power line is equivalent to an inductor, and there is distributed capacitance between the earth and nearby metal objects, forming a parallel resonant circuit. In TT and TN power supply systems, when a single-phase ground fault occurs, due to high-frequency The components resonate and produce a very high overvoltage on the line, which mainly damages the secondary instruments.
For locations with thunderstorms lasting more than 15 days, a lightning protection device must be installed, and a varistor should be installed on the frequency converter, which can effectively prevent damage to equipment caused by surges and overvoltage of the power supply.
Due to the operation of circuit breakers, the input and removal of loads, or system faults, the system parameters change, resulting in the internal electromagnetic energy conversion or transmission transition process of the power, which will cause overvoltage within the system. The causes of internal overvoltage in power systems can be roughly divided into:
Ø Insertion and removal of large power loads;
Ø Induction and removal of inductive load;
Ø Insertion and removal of power factor compensation capacitor;
Ø Short circuit fault;
In addition, surges also have an impact on the system when the coils of the contactor and intermediate relay are closed. The surge of the contactor coil may reach thousands of volts. It is recommended to install surge suppression components on these devices, such as RC and bidirectional peak diodes. Wait, the surge when the coil is pulled in is shown in Figure 7.
Figure 7 Surge when the contactor coil is closed
2. Harmonic interference
The main circuit of the frequency converter generally consists of AC-DC-AC. The externally input 380 V/50 Hz industrial frequency power supply is rectified into a DC voltage signal by a three-phase bridge thyristor, and then filtered by a filter capacitor and inverted by a high-power transistor switching device. into an AC signal with variable frequency.
In the rectifier circuit, the waveform of the input current is an irregular rectangular wave. The waveform is decomposed into the fundamental wave and each harmonic according to the Fourier series. The higher harmonics among them will interfere with the input power supply system. In the inverter output circuit, the output current signal is a pulse waveform modulated by the PWM carrier signal. Currently, low-voltage inverters generally use IGBT high-power inverter devices, and the PWM carrier frequency is 2.5~20 kHz. Similarly, the output circuit current signal It can also be decomposed into the fundamental wave containing only sine waves and other harmonics, and the high-order harmonic current directly interferes with the load. In addition, high-order harmonic currents are also radiated into space through cables, interfering with adjacent electrical equipment.
The voltage and current waveforms input by the frequency converter are shown in Figure 8.
Figure 8 Input side waveform of the inverter
The harmonics generated by the frequency converter can be reduced by installing incoming line reactors, DC reactors, passive filters and other equipment of the frequency converter.
3. Fast burst interference
EFT is the abbreviation of Electrical Fast Transient Burst Immunity Test. When instantaneous disturbances caused by lightning, ground faults, power switch operations, or inductive load operations such as relays in the circuit interfere with the entire control loop, they will interfere with the control box (and PLC and other devices). Characteristics of this type of interference The pulses appear in groups, the repetition frequency of the pulses is high, the rise time of the pulse waveform is short, and the energy of a single pulse is low. Therefore, it is possible that the switching of inductive loads by mechanical switches in a certain circuit may cause interference to other electrical and electronic equipment in the same circuit. The schematic diagram of the generation of transient voltage and fast pulse group when the contacts are closed and opened is shown in Figure 9.
Figure 9 Generation of pulse group
The oscilloscope graph of the measured contactor contacts is shown in Figure 10.
Figure 10 Oscilloscope waveform of contactor opening
Since the leading edge tr of a single pulse waveform of the pulse group reaches 5ns and the pulse width reaches 50ns, the pulse group interference is destined to have extremely rich harmonic components.
The harmonic frequency with larger amplitude can reach at least 1/πtr, that is, it can reach about 64MHz, and the corresponding signal wavelength is 5m. For a power line carrying a frequency of 60MHz or above, if the length is 1m, since the length of the wire is already comparable to the wavelength of the signal, it can no longer be considered as an ordinary transmission line. During the transmission process of the signal on the line, part of it can still pass through The transmission line enters the equipment under test (conducted emission); part of it escapes from the line and becomes a radiated signal and enters the equipment under test (radiated emission). Therefore, the interference experienced by the device under test is actually a combination of conduction and radiation.
Obviously, the ratio of conduction and radiation will be related to the length of the power line. The shorter the line, the more conductive components, and the smaller the radiation ratio; conversely, the greater the radiation ratio. This is why, under the same conditions, equipment with metal casings is easier to pass the test than equipment with non-metal casings, because equipment with metal casings has stronger resistance to radiation interference.
During the transmission process of EFT interference, part of the interference will escape from the transmission cable, so that the equipment will eventually suffer from the composite interference of conduction and radiation. However, since the amount of conduction accounts for the vast majority and is controllable, for pulse group interference, the most common pulse group interference suppression methods mainly use filtering (filtering of power lines and signal lines) and absorption (using ferrite magnets). ring to absorb).
Among them, the solution using ferrite magnetic ring absorption is very cheap and very effective. The amount of radiation can be minimized by changing the position of the transmission cable. The most effective way is to use filters and ferrite cores at the source of interference and the entrance of the equipment. The former is a thorough treatment of interference sources, while the latter is a door to tightly suppress interference, so that power lines and signal lines that have been processed by filters and ferrite cores no longer contain radiation components.
4. Static interference
The essence of static electricity is charge transfer caused by potential difference. Any substance is composed of atoms, and the basic structures of atoms are protons, neutrons and electrons. Scientists define protons as positively charged, neutrons as uncharged, and electrons as negatively charged. Under normal conditions, the number of protons in an atom is the same as the number of electrons, and the positive and negative charges are balanced, so it appears uncharged to the outside world. However, due to external effects such as friction or in the form of various energies such as kinetic energy, potential energy, thermal energy, chemical energy, etc., the positive and negative charges of the atoms will be unbalanced.
Friction in daily life is essentially a process of constant contact and separation. In some cases, static electricity can be generated without friction, such as induced electrostatic charging, thermoelectric and piezoelectric charging, Helmholtz layer, spray charging, etc. Static electricity can be generated when any two objects of different materials come into contact and then separate. The common method of generating static electricity is friction. The better the insulation of the material, the easier it is to generate static electricity. Because air is also composed of atoms, in layman's terms, static electricity may be generated at any time and anywhere in people's lives. It is almost impossible to completely eliminate static electricity, but there are steps you can take to control static electricity so that it does not cause harm.
In addition, the weather in the north is dry in winter, and the human body is prone to static electricity. When it comes into contact with other people or metal conductors, discharge will occur. People will feel a stinging sensation of electric shock, and sparks can be seen at night. This is the reason why the human body is charged with positive static electricity when chemical fiber clothing rubs against the human body. Common static electricity scenes are shown in Figure 11.
The scope of static electricity hazards is wide. During the storage and transportation of electrostatic hazardous materials, once a combustion or explosion accident occurs due to electrostatic discharge, it is often not only a certain equipment that is damaged, but also a certain place, a certain area, and even the safety of a larger area will be affected. threaten.
In the storage places of electrostatic hazardous materials and the production, use and transportation of electrostatically sensitive materials, conditions that constitute electrostatic hazards are relatively easy to form. Sometimes just a spark can cause a serious disaster. Because the voltage of static electricity is very high, it may cause The chip on the circuit board is burned out, so it is handled by hand.
Prevention of static electricity hazards should be given priority. Grounding static electricity, using anti-static shoes, anti-static clothing, and wrist straps can reduce the hazards of static electricity. When holding a circuit board with your hands, you should first discharge the static electricity from your hands through the metal conductor to prevent the circuit board from being Damage occurs due to static electricity.
5. Radiation interference
Radiated interference, as the name suggests, is the interference caused by the radiation of the frequency converter.
If the cable from the inverter to the motor does not use a shielded wire, it will be the most typical source of radiated interference, because the output of the inverter uses PWM output with a carrier frequency of several K to more than ten K. The schematic diagram of the radiated interference of the inverter is as shown in the figure 12 shown.
Figure 12 Radiated interference of frequency converter
Methods to suppress radiated interference from frequency converters:
In actual engineering projects, users need to purchase frequency converters with integrated filters, use shielded cables, or purchase additional EMC filters.
Installing shielded wires or metal tubes on the motor cables, using EMC mounting plates when installing the inverter, and using special EMC cable connectors on the motor side are all good ways to suppress radiated interference from the inverter motor lines.
3. Grounding of industrial equipment
The primary purpose of grounding is safety. The function of the grounding wire is to connect the conductive part of the metal shell of electrical equipment or electrical cabinets and other live equipment to the earth to form an equipotential body.
The resistance of the safety ground wire is very small, so even if the electrical equipment shell leaks accidentally, the current will flow to the earth through the connecting wire and connect with the earth potential. In this way, the voltage between the electrical box shell and the earth is very low and can be ignored, so When the human body comes into contact with the outer shell of the electrical box, because the voltage is very low, the current passing through the human body is much smaller than the current that would cause electric shock to the human body. The human body will not get electric shock, thereby protecting the safety of personnel.
The working grounding is set up by the operation needs of the power system (such as neutral point grounding). Therefore, under normal circumstances, there will be current flowing through the grounding electrode for a long time, but it is only an unbalanced current of a few amperes to dozens of amperes.
Lightning protection grounding is grounding designed to eliminate the dangerous effects of overvoltage, such as the grounding of lightning rods, lightning protection wires and lightning arresters. Lightning protection grounding will only allow current to flow under the action of lightning impact. The amplitude of the lightning current flowing through the lightning protection grounding electrode can reach tens to hundreds of thousands of amperes, but its duration is very short.
Shielding and grounding is an effective measure to eliminate the harm of electromagnetic fields to the human body, and is also an effective measure to prevent electromagnetic interference. High-frequency technology has been widely used in electric heating, medical treatment, radio broadcasting, communications, television stations, navigation, radar, etc. Under the action of electromagnetic fields, the absorbed radiation energy of the human body will have biological effects and cause harm to the human body, such as slight trembling of fingers. , skin scratches, vision loss, etc. Installing a shielding device on the shell of equipment that generates magnetic fields and grounding the shield can not only reduce the intensity of the electromagnetic field outside the shield, thereby reducing or eliminating the harm of the electromagnetic field to the human body, but also protect the equipment within the shielded grounding body from external electromagnetic fields. interference effects.
To prevent the harmful effects of static electricity and discharge it, anti-static grounding is the most important part of electrostatic protection.
The role of grounding in EMC includes the use of equipotential bodies to achieve a reliable connection with the ground wire, which can maintain equipotentiality over a wide frequency range. In addition, the lowest possible grounding impedance can reduce power supply fault currents and high-frequency currents. It does not pass through the equipment, reducing interference to the equipment.
At present, the domestic grounding system adopts the method of separation of lightning protection grounding, power grounding and digital ground. Such a separate grounding system is the most common in China. However, the high-frequency characteristics of this grounding system are not good enough. The separate grounding system is shown in Figure 13. shown.
Figure 13 Individual grounding system
Users can form a grounding network by grounding power and communication through multiple equipotential bodies, and then connect it to lightning protection grounding through multi-layer composite grounding. Such a grounding system can only be used when the resistance of the grounding system is less than 1 ohm. The grounding system is shown in Figure 14.
Figure 14 Multi-point grounding grounding system
For industrial grounding systems, the smaller the grounding resistance, the better. In systems with a neutral point directly grounded below 1000V, the grounding resistance is less than or equal to 4 ohms, and the repeated grounding resistance is less than or equal to 10 ohms. In systems with ungrounded neutral points with voltages below 1000V, the grounding resistance is generally stipulated to be 4 ohms. However, it is found at many customer sites that the grounding resistance is often less than or equal to 4 ohms. In this case, the grounding should be rectified.
The impedance of cables with a length of 1 meter and different diameters at frequency is shown in Figure 15. It can be seen from the figure that the impedance of the cable is closely related to frequency. At low frequencies, the current impedance of large diameter is low, and the impedance of 35mm2 is 0.5 milliohms. , 1mm2's 2 reaches 18 milliohms, the difference between the two is 36 times, but at high frequencies, such as 100KHz, 35mm2 cable has more than half the resistance of 1mm2, that is to say, two 1mm22 cables in parallel will be better than one 35mm2 The impedance of the cable is still small, which is why a mesh connection is used for grounding.
Figure 15 Impedance table of cables at different frequencies
In a high-frequency environment, the impedance of a conductor mainly depends on its inductance per unit length which is proportional to the unit length, and this inductance plays a decisive role in the impedance of the cable starting from 1 kHz. This means that with only a few meters of conductor length, the impedance of the cable is affected by:
Ø DC or low frequency (LF) several milliohms
Ø About 1 MHz, several ohms
Ø High frequency several hundred ohms (HF) (»100 MHz...)
Ø And the perimeter of the conductor cross-section plays a dominant role (skin effect)
Ø The cross-sectional area of the conductor becomes relatively unimportant
Ø The length of the cable is decisive
After understanding the above principles, you will understand why the shorter the ground cable, the better.
Why can't I use pigtails for shield grounding?
Since at high frequencies, the impedance of the cable is mainly determined by the cable length, twisting the shield into a pig tail will reduce the shielding effect. If the length of the conductor exceeds 1/30 of the signal wavelength, the impedance of the cable becomes "infinite".
The relationship between wavelength and frequency: c=λf (f=c/λ), where, c: wave speed (this is a constant, that is, the speed of light, c is approximately equal to 3*10^8m/s) The unit is meters per second; λ is the wavelength, its unit is meters; f is the frequency of the wave, its unit is hertz Hz.
In the formula:
Ø λ: wavelength unit m.
Ø f: Frequency unit MHz.
Obviously, the grounding impedance of objects of the same length is arranged according to size, Z1>Z2>Z3>Z4, as shown in Figure 16.
Figure 16 Arrangement of impedance of objects of the same length in high-frequency environment
A comparison of several cable shielding grounding methods is shown in Figure 17.
Figure 17 Comparison of cable shielding and grounding
4. Filter and magnetic ring of frequency converter
The built-in or external EMC filter of Schneider inverter ATV320 adopts the same structure. The filter consists of X capacitors between phases, Y capacitors between phases and ground, and common mode chokes. The mode choke used in the filter uses a toroidal ferrite compared to the toroidal transformer, and wires are wound on it to form an inductor.
The purpose of integrating the EMC filter into the frequency converter is to enable the frequency converter to comply with the requirements of the C2 standard of IEC61800-3.
ATV320 product is a 200V single-phase product. When the switching frequency of the inverter is set between 2 and 4kHz, the maximum distance of the motor cable can reach 10 meters. When the switching frequency is set between 4 and 12kHz, when the inverter is set to The maximum motor cable length is 5 meters.
ATV320 product 400V three-phase range (EMC filter), when the switching frequency setting value is between 4 and 12kHz, the maximum cable length from the inverter to the motor is 5 meters.
If an additional EMC filter is installed, the cable length from the inverter to the motor will be longer, and it may also comply with the higher standard C1, while ensuring compliance with the C2 standard.
When wiring the filter, be careful to separate the input and output of the filter to prevent the input of the filter from interfering with the "clean" filter output line and reducing the effect of the filter. The recommended installation diagram of the filter is shown in Figure 18 .
Figure 18 The filter inlet and outlet lines should not be too close
The sine wave filter is to filter the SPWM modulation wave of the frequency converter into an approximately sinusoidal voltage waveform. Since the output of the frequency converter contains high-frequency harmonics, it increases the losses of the power cable and motor; at the same time, extremely high dv/dt will cause several MHz radiation interference; if the motor requires long-term transmission (motor cable exceeds 50 meters), the echo reflection will cause the motor terminal voltage to superimpose, destroy the motor insulation, and cause the motor to burn.
After filtering the output waveform of the frequency converter into a sine wave, the sine wave filter extends the maximum length of the motor cable and reduces the interference of the frequency converter. The sine wave filter can extend the life of the motor, protect the motor insulation, and suppress electromagnetic interference. good.
Anti-interference magnetic ring, also called ferrite magnetic ring, referred to as magnetic ring, is a commonly used anti-interference component in electronic circuits and has a good suppression effect on high-frequency noise.
The more turns of the magnetic ring, the better the effect of suppressing low-frequency interference, but the effect of suppressing high-frequency noise is weak.
In actual use, the number of magnetic ring turns must be adjusted according to the frequency characteristics of the interference current. When the interference signal frequency band is wide, two magnetic rings can be placed on the cable, each with a different number of turns. In this way, one magnetic ring can be used to suppress both high-frequency and low-frequency interference. It is not that the larger the impedance, the better the interference signal suppression effect, because there is a parasitic capacitance on the actual magnetic ring. This parasitic capacitance is connected in parallel with the inductor. However, when encountering high-frequency interference signals, this parasitic capacitance will short-circuit the inductance of the magnetic ring. , thus losing its effect.
When selecting an anti-interference magnetic ring, there are two main factors to consider, namely the impedance characteristics of the magnetic ring and the interference characteristics of the filtered circuit. From the appearance point of view, the preferred choice is to make the magnetic ring as long as possible, as thick as possible, with the smallest inner diameter and as small inductance as possible.
The biggest advantage of using a magnetic ring is that it has no electrical connection with the filtered circuit. The biggest disadvantage is that the magnetic ring is fragile, so it is recommended to use a magnetic ring with a plastic shell and fix it on the filtered power line or control cable.
According to the frequency characteristics of the interference signal, a magnetic ring of nickel-zinc ferrite or manganese-zinc ferrite can be selected. The former has better high-frequency characteristics than the latter. The magnetic permeability of manganese-zinc ferrite ranges from several thousand to tens of thousands, while The value of nickel-zinc ferrite is several hundred to thousands. The higher the magnetic permeability of magnetic ring ferrite, the greater its impedance at low frequencies and the smaller its impedance at high frequencies. Therefore, when suppressing high-frequency interference, it is better to choose Nickel-zinc ferrite, otherwise use manganese-zinc ferrite.
Review Editor: Tang Zihong
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