When various factory equipment uses frequency conversion speed regulation, harmonic disturbances will be generated on the power supply side and motor side of the inverter (VFD), which will cause electromagnetic disturbance to the power supply network and other electrical equipment near the VFD. In addition, in order to ensure the long-term reliable operation of the VFD, the wiring of the VFD is very important.
What is EMC?
EMC is the filter. It refers to the outstanding ability of electrical equipment in the electromagnetic environment, and cannot produce unbearable electromagnetic disturbance to other equipment operating in the environment.
Noise emission and immunity
EMC is determined by two characteristics related to electrical equipment: noise emission and immunity. The rules governing noise emission and immunity depend on the environment in which the electrical equipment is used. It is generally divided into the first class environment (civil environment) and the second type of environment (industrial environment). In the civil environment, that is, when electrical equipment enters the public power supply system, there are strict rules for noise emission, but it may require a low degree of anti-nuisance; Conversely, in an industrial environment, electrical equipment has a high degree of noise immunity, but a low degree of noise emission.
If the electrical equipment is part of the system, it is not necessary to meet any emission and nuisance immunity requirements from the outset, but it is necessary for the entire system to meet the relevant electromagnetic compatibility requirements. In general, it is necessary for electrical equipment to have the ability to restrain high-frequency and low-frequency disturbances. In the meantime, high-frequency disturbances mainly include electrostatic discharge (ESD), pulse disturbances, and emission frequencies of electromagnetic fields. Low-frequency disturbance mainly refers to the irregularity of the power supply voltage, undervoltage and frequency instability.
VFD and its electromagnetic compatibility
Typically, VFDs can be operated in industrial environments where high electromagnetic disturbances (EMI) may be present. At the moment, it is a noise transmitter or perhaps a noise receiver.
(1) VFD is a noise emission source
There is a parasitic capacitance Cp inside the motor cable and the motor, so the switch wing of the VFD PWM output voltage waveform generates a high-frequency impulse noise current Is through the parasitic capacitance, making the VFD a noise source. Since the source of the noise current Is is the VFD, it is necessary for it to flow back to the VFD. In the figure, ZE is the ground impedance, and ZN is the impedance between the power cable and the ground. The voltage drop generated by noise flowing through these two impedances can affect other devices on the same grid, causing a nuisance. In addition, the rectified part of the VFD also generates low-frequency harmonics, resulting in grid voltage distortion. When the short-circuit impedance of the grid is less than 1%, it is recommended to add a series reactor to restrain the low-frequency disturbance. The detailed harmonic composition is shown in the table below.
With regard to high-frequency disturbances, high-frequency noise can be restrained if the high-frequency noise current has the correct channel. If an unshielded motor cable is used, the high-frequency noise current Is will flow back to the VFD in an uncertain way, and the voltage drop of the high-frequency components will occur in this circuit, affecting other equipment. In order for the high-frequency noise current to flow back to the VFD along a certain route, it is necessary to use shielded motor cables. It is necessary to connect the cable shield to the VFD housing and the motor housing. When it is necessary for the high-frequency noise current to flow back to the VFD, the shield layer constitutes the best channel.
Although there is no voltage drop on the noise current on the ZE, there will be a voltage drop on the power impedance NZ, which will affect other electrical equipment.
Therefore, a radio nuisance suppression filter should be installed at the input end of the VFD, so that the noise current back to the power supply will be greatly reduced.
(2) VFD as a noise receiver
When the VFD is used as a noise receiver, the high-frequency noise current Is can enter the VFD through the potential and coupling capacitance, creating a voltage drop across the impedance Zi and creating annoying noise. To this end, the best way is to block the noise source from the signal cable, and the shielding of the signal cable is necessary to ground both ends.
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