When various factory equipment uses frequency conversion speed regulation, harmonic disturbance will occur on the power supply side and motor side of the inverter (VFD), which will form 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 working ability of electrical equipment in the electromagnetic environment, and cannot bear the electromagnetic disturbance of other equipment working in the environment.
Noise emission and immunity
EMC is determined by two characteristics related to electrical equipment: noise emission and immunity to disturbances. The specified constraints on noise emission and immunity depend on the environment in which the electrical equipment is used. It is generally divided into the first type of environment (civil environment) and the second type of environment (industrial environment). In a civil environment, i.e. when electrical equipment enters the public power supply system, there are strict regulations on noise emission, but a lower degree of anti-disturbance may be required; Conversely, in industrial environments, electrical equipment has a higher degree of noise immunity but a lower degree of noise emission.
If the electrical equipment is part of the system, it is not necessary to meet any emission and anti-disturbance 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 press high and low frequency disturbances. Among them, high-frequency disturbances mainly include electrostatic discharge (ESD), pulse disturbance, and emission frequencies of electromagnetic fields. Low-frequency turbulence mainly refers to power supply voltage fluctuations, undervoltages, and frequency instability.
VFD and its electromagnetic compatibility
Typically, VFDs are capable of operating in industrial environments where high electromagnetic disturbance (EMI) may be present. In this case, 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 switching wing of the VFD PWM output voltage waveform generates a high-frequency impulse noise current Is through the parasitic capacitor, 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 caused by noise flowing through these two impedances can affect other devices on the same grid, causing disturbance. In addition, low-frequency harmonics can also occur in the rectified part of the VFD, resulting in distortion of the grid voltage. When the short-circuit impedance of the power grid is less than 1%, it is recommended to add a series reactor to press the low-frequency disturbance. The specific harmonic composition is shown in the table below.
With regard to high-frequency disturbance, the high-frequency noise can be suppressed 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 path, and the voltage drop of the high-frequency components will occur in this loop, 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 Is to flow back to the VFD, the shield forms the most efficient 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 agitation 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 pass through the potential and the coupling capacitor into the VFD, and the voltage drop occurs at both ends of the impedance Zi, causing disturbing noise. For this purpose, the most effective method is to block the noise source from the signal cable severely, and it is necessary to ground both ends of the signal cable shielding.
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