Why there is electromagnetic interference _ the cause of electromagnetic interference

Electromagne interference (EMI) is an electronic noise that interferes with cable signals and reduces signal integrity. EMI is usually generated by electromagnetic radiation sources such as motors and machines. Electromagnetic interference is an electromagnetic phenomenon that people have long discovered. It is almost simultaneously discovered with the phenomenon of electromagnetic effects. In 1981, British scientists published an article on "interference", marking the beginning of the study of interference problems. In 1989, the British Post and Telecommunications Department studied the interference problem in communication, and the research on interference problems began to be engineered and industrialized.

We often hear electromagnetic interference, but how does electromagnetic interference come about? In fact, the factors that cause interference are very complicated. There are two modes of interference, namely common mode and differential mode. The “common mode” interference refers to the existence of the line, including the conduction test caused by the field induction, etc. For the secondary and the line, all they feel are common mode interference.

Electromagnetic interference, both conducted and radiated. Conducted interference refers to the coupling (interference) of signals on one electrical network to another electrical network through a conductive medium. Radiated interference means that the source of interference couples (interferes) its signal to another electrical network through the air. In high-speed PCB and system design, high-frequency signal lines, integrated circuit pins, various types of connectors, etc. may become radiation interference sources with antenna characteristics, which can emit electromagnetic waves and affect other systems or other subsystems in the system. normal work. The so-called "interference", electromagnetic compatibility refers to the meaning of the device after the interference is reduced and the interference source to the device.

Electromagnetic interference sources are divided into two categories: natural and artificial. The electromagnetic interference in nature is mainly the atmospheric noise generated by lightning (10MHz) and the atmospheric noise caused by cosmic rays and solar rays (10MHz). The sources of human electromagnetic interference are classified into intentional and unintentional. The former refers to electronic devices that must emit electromagnetic waves, such as FM, AM, TV, broadcast transmitters, and radar and mobile wireless communication devices. The latter refers to those jobs. Electronic equipment that generates unwanted electromagnetic interference signals, such as computer equipment, relays, switches, fluorescent lighting, electric arc welding machines, and ignition devices. Electronic devices that are susceptible to electromagnetic interference include communication receivers (radios, televisions, etc.), radars, navigation devices, computers, etc., especially the power supplies in these electronic devices, which are more sensitive to electromagnetic interference.

In automobiles, electromagnetic interference mainly comes from the following aspects:

(1) During the operation of the electrical system, when the switch of the appliance is turned on or off, the current and voltage of the load change, and the magnetic field changes, high-frequency interference signals are easily generated.

(2) When the inductive load is switched, high-frequency oscillation occurs in the circuit, and the peak voltage of the oscillation can reach about 200V. Especially, the ignition coil and the high-voltage line with poor insulation performance generate high voltage and strong magnetic field. The oscillations excited by any factor will be emitted in the form of electromagnetic waves through wires or the like, which will inevitably cause electromagnetic interference to other electronic devices.

(3) The working systems of the various electronic systems are different, and they interfere with each other in different ways.

(1) Use shielding technology to reduce electromagnetic interference. In order to effectively suppress the radiation and conduction of electromagnetic waves and the noise current caused by higher harmonics, shielded cables must be used for elevator motor cables driven by inverters. The conductance of the shielding layer is at least 1/10 of that of the conductors of each phase conductor core. And the shielding layer should be reliably grounded. It is best to use shielded cables for control cables; double-shielded twisted pairs should be used for transmission lines for analog signals; different analog signal lines should be routed independently and have their own shielding layers. To reduce the coupling between the lines, do not put different analog signals in the same common return line; low-voltage digital signal lines are best to use double-shielded twisted pair or single-shielded twisted pair. Transmission cables for analog signals and digital signals should be shielded and routed separately for short use.

(2) Use grounding technology to eliminate electromagnetic interference. Make sure that all equipment in the elevator control cabinet is well grounded and a thick ground wire. Connect to the power entry ground (PE) or ground bus. It is especially important that any electronic control equipment connected to the frequency converter be shared with the ground and short and thick conductors. At the same time, the ground wire of the motor cable should be grounded directly or connected to the ground terminal (PE) of the inverter. The above grounding resistance value should meet the requirements of relevant standards.

(3) Use wiring technology to improve electromagnetic interference. The motor cable should be independent of other cable routing. At the same time, avoid parallel wiring of the motor cable and other cables in a long distance to reduce the electromagnetic interference caused by the rapid change of the output voltage of the inverter. When the control cable and the power cable cross, try to make They intersect at an angle of 90° and the shield of the motor and control cables must be secured to the mounting plate with suitable clamps.

(4) Use filtering technology to reduce electromagnetic interference. Line reactors are used to reduce the harmonics generated by the frequency converter. They can also be used to increase the impedance of the power supply and help to absorb the surge voltage generated by nearby equipment and the peak voltage of the main power supply. The line reactor is connected in series between the power supply and the inverter power input. When the situation of the main power grid is not known, it is best to add a line reactor. A low-pass filter (the same as FIR) can also be used in the above circuit. The FIR filter should be connected in series between the line reactor and the inverter. For elevator drives operating in noise-sensitive environments, the use of FIR filters can effectively reduce radiated interference from the transmission of the frequency converter.

(5) If the interference of the lighting line, the interference of the motor feedback is too large, and the system power line is disturbed, the communication interference cannot be eliminated through the above various groundings. The magnetic ring can be used to suppress the interference, and the magnetic ring is added in the following order. The communication is back to normal:

1. If the two power cables of the lighting are disconnected at the same time, if the communication returns to normal, please add a magnetic ring on the two lines under the control cabinet, and wind 3 turns (magnetic ring with aperture 20 to 30, thickness 10, length 20 or so) ). If the lighting line is disconnected, there is no effect that the lighting line does not interfere with the communication and will not be processed.

2. Add a magnetic ring from the main board outlet on the communication line C+, C-, and wrap it around. Note that only one lap can be wrapped. After the entanglement, the car communication display will be better, but the effective signal from the car is mostly filtered out, resulting in the car not being registered.

3. Add a magnetic ring winding 2 to 3 turns on the 24V power supply and 0V ground line output from the main board to the car and the call.

4. Wrap a magnetic ring around the three-phase line between the running contactor and the motor. After the magnetic ring is added by the above method, the power, motor and illumination interference in the field can be handled.

(6) Selection of magnetic ring material: According to the frequency characteristics of the interference signal, nickel-zinc ferrite or manganese-zinc ferrite can be selected to select nickel-zinc ferrite or manganese-zinc ferrite. The former has better high-frequency characteristics. the latter. The magnetic permeability of MnZn ferrite is several thousand---tens of thousands, and the nickel-zinc ferrite is several hundred---thousands. The higher the magnetic permeability of the ferrite, the higher the impedance at low frequencies and the lower the impedance at high frequencies. Therefore, in suppressing high-frequency interference, nickel-zinc ferrite should be used; otherwise, manganese-zinc ferrite is used. Or put manganese Zn and nickel zinc ferrite on the same cable at the same time, which can suppress the interference band is wider. Size selection of the magnetic ring: The larger the difference between the inner and outer diameters of the magnetic ring, the larger the longitudinal height, the greater the impedance, but the inner diameter of the magnetic ring must be tightly wrapped to avoid leakage. Installation position of the magnetic ring: The installation position of the magnetic ring should be as close as possible to the interference source, that is, it should be close to the inlet and outlet of the cable.

White IDC Connector

White IDC Connector :IDC connector Automotive Connector is a kind of component that electronic engineering and technical personnel often contact. Its function is very simple: in the circuit is blocked or isolated between the circuit, set up a bridge of communication, so that the current flow, so that the circuit to achieve the intended function. The form and structure of automobile connector are changeable. It is mainly composed of four basic structural components: contact, shell (depending on the variety) and insulator.

White IDC Connector

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