Popular Science Frequency Converter Knowledge: EMC Rules

When various factory equipment adopts frequency conversion speed regulation, harmonic interference will be generated on the power supply side and motor side of the frequency converter (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 good working ability of electrical equipment in the electromagnetic environment, and cannot cause unacceptable electromagnetic interference to other equipment working in the environment.

Noise Emissions and Immunity

EMC is determined by two characteristics related to electrical equipment: noise emission and immunity. The prescribed limits for noise emission and the degree of immunity depend on the environment in which the electrical equipment is used. Generally divided into the first type of 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 regulations on noise emission, but a lower degree of anti-interference may be required; on the contrary, in an industrial environment, the degree of noise immunity of electrical equipment is higher, However, the level of noise emission is low.

If electrical equipment is part of the system, it does not need to meet any emission and anti-interference requirements from the beginning, but the entire system must meet the relevant electromagnetic compatibility requirements. Generally speaking, electrical equipment must have the ability to suppress high-frequency and low-frequency interference. Among them, high-frequency interference mainly includes electrostatic discharge (ESD), pulse interference and emission frequency of electromagnetic field. Low-frequency interference mainly refers to power supply voltage fluctuations, undervoltage and frequency instability.

VFD and its Electromagnetic Compatibility

Typically, VFDs can operate in industrial environments where high electromagnetic interference (EMI) can be present. At this point, it’s a noise transmitter and possibly a noise receiver.

(1) VFD is the source of noise emission

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 pulse 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 must flow back to the VFD. In the figure, ZE is the impedance of the earth, and ZN is the impedance between the power cable and the earth. The voltage drop caused by noise flowing through these two impedances can affect other devices on the same grid, causing interference. In addition, the rectification part of the VFD will also generate low-frequency harmonics, which will cause grid voltage distortion. When the grid short-circuit impedance is less than 1%, it is recommended to add a series reactor to suppress low-frequency interference.

For high-frequency interference, if the high-frequency noise current has the correct channel, the high-frequency noise can be suppressed. If unshielded motor cables are used, the high-frequency noise current Is will flow back to the VFD in an uncertain path, and a voltage drop of high-frequency components will occur in this loop, which will affect other equipment. In order to make the high-frequency noise current flow back to the VFD along a certain route, it is necessary to use shielded motor cables. The cable shield must be connected to the VFD housing and the motor housing. The shield forms the most efficient path when high frequency noise current Is must flow back to the VFD.

Although the noise current will not have a voltage drop on ZE, there will be a voltage drop on the power supply impedance ZN, which will affect other electrical equipment.

Therefore, a radio interference suppression filter should be installed at the input of the VFD, so that the noise current flowing back to the power supply will be greatly reduced.

(2) VFD as 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 the coupling capacitor, and a voltage drop occurs at both ends of the impedance Zi, resulting in interference noise. For this reason, the most effective method is to strictly isolate the noise source from the signal cable, and the shield of the signal cable must be grounded at both ends.