Electromagnetic Compatibility and Electromagnetic Interference Suppression of VFD (Variable-frequency Drive)

1 Introduction

With the miniaturization, multi-function and high performance of VFD (Variable-frequency Drive), especially the full digitalization of control means, the flexibility and adaptability of VFD (Variable-frequency Drive) are constantly increasing. Therefore, in modern industry, VFD (Variable-frequency Drive) is used more and more widely. At present, almost all VFD (Variable-frequency Drive) adopt PWM (pulse width modulation, pwm pulse width modulation technology) control technology.

The motor system driven by pwm frequency conversion converts and controls the electric energy through the power converter, and the performance index of the motor system is greatly improved. However, since the power switching device in the VFD (Variable-frequency Drive) works in the switching state, the high-speed switching action of the device makes the voltage and current jump in a short time, which makes the voltage and current contain rich high-order harmonics. The electromagnetic noise energy of these harmonics will form electromagnetic interference emi through circuit connection or electromagnetic wave space coupling, which will have a great impact on the motor system itself and the surrounding environment [1-4]. In the generated conducted interference, the noise signal frequency ranges from a few khz to tens of mhz, and the interference intensity may far exceed the limit value stipulated by the electromagnetic compatibility standard.

Therefore, designers of modern electronic and electrical systems such as VFD (Variable-frequency Drive) are faced with a problem of "how to ensure that the electronic and electrical systems can achieve the design purpose in the electromagnetic environment where they are located, while not interfering with other surrounding electrical systems. Normal work", that is, the electromagnetic compatibility (electromagnetic compatibility, emc) problem of electronic and electrical systems.

At present, the international community attaches great importance to the issue of electromagnetic compatibility, and several organizations have been established to formulate electromagnetic compatibility standards, such as the cispr standard of the International Radio Special Committee, the series of standards of IEC, and the series of standards of en of the European Union. In China, many organizations such as the National Radio Interference Committee, the Electromagnetic Compatibility Committee of the China Power Supply Society, and the Electromagnetic Compatibility Branch of the IEEE Beijing Branch are also engaged in work related to emc. China's national emc standards have also been formulated and implemented, such as the national standard (gb) and the military standard (gjb), all of which have promoted the research and development of electromagnetic compatibility in China. The electromagnetic interference produced by the pwm variable frequency motor drive system has also been paid more and more attention by people. In order to meet the requirements of electromagnetic compatibility standards, correct design and reasonable use of suppression means can reduce the emission intensity of the system's emi to below the limit of the emc standard, so that electrical equipment and systems can achieve electromagnetic compatibility.

2 Conducted interference mechanism of pwmVFD (Variable-frequency Drive)

The so-called conduction coupling means that the energy of electromagnetic noise is coupled to the disturbed equipment (circuit) in the form of voltage or current in the circuit through metal wires or other components (such as capacitors, inductors and transformers, etc.). Conductive coupling can be divided into direct conductive coupling and common impedance conductive coupling. Direct conductive coupling means that the noise is directly coupled to the harassed equipment (circuit) through actual components such as wires, metal bodies, resistors, capacitors, inductors, and transformers. Common impedance conductive coupling refers to the common ground impedance coupling generated by the noise through the printed circuit board circuit and the chassis ground wire, the public safety ground wire of the equipment, and the common ground impedance in the ground network; the noise passes through the common ground impedance of the AC power supply and the DC power supply. When the source impedance is high, common source impedance coupling occurs.

The switching operation state of the power switching device causes complex mutual coupling among the components in the system, which will form conduction interference. The highest frequency considered for conducted interference is 30mhz, and the corresponding electromagnetic wave wavelength λ in vacuum is 10m. Therefore, for power electronic devices with a size smaller than λ/2π, they belong to the near-field range, and lumped parameter circuits can be used for electromagnetic interference analysis. For the convenience of analysis, the disturbance on the input/output wires of the system can be divided into two parts: common-mode interference and differential-mode interference according to the different transmission and coupling channels of conducted interference. It is generally believed that the common-mode interference is mainly due to the power semiconductor in the system converter. The dv/dt caused by the switching action of the switching device propagates through the stray capacitive coupling of the system to the ground, and the voltage change of one pole will be capacitively coupled to the other pole to generate displacement current. The current generated by the parasitic capacitance does not require a direct electrical connection, or even ground [5]. Its size can be expressed as:

i=cdu/dt

In the formula, c is the equivalent coupling capacitance between the battery interference source and the sensitive equipment.

The differential mode interference is mainly caused by the di/dt caused by the switching of the power semiconductor switching device through the conductor between the input and output lines. Of course, these are only the most essential causes of conducted interference, and different motor systems have different specific causes of conducted interference. In addition, common-mode interference and differential interference can be transformed into each other, and they are not absolutely separated. For example, Figure 1 shows the circuit diagram of the non-essential differential-mode noise caused by the unbalance of the common-mode current transmission channel [6].

Figure 1 The mechanism of non-essential differential mode noise

Figure 2 is a diagram of the electromagnetic interference current flow path of the pwm variable frequency drive motor system, including common-mode interference and differential-mode interference. In pwmVFD (Variable-frequency Drive), in order to ensure that the switch tube will not fail due to overheating during operation, a radiator must be installed on it, and in order to prevent short circuit, the metal shell of the switch tube and the radiator are insulated by heat conduction The medium is isolated from each other, and the radiator is grounded through the chassis, so a large parasitic capacitance is formed between the VFD (Variable-frequency Drive) and the radiator [7,8]. When the inverter is working normally, as the upper and lower switching tubes of each phase bridge arm are turned on in turn, the potential of the midpoint of the bridge arm will undergo a quasi-step change. If you look at this phenomenon from the perspective of emi, then the voltage output by the three bridge arms is the three emi interference sources, and each switching action will charge and discharge the parasitic capacitance between the power switching device and the heat sink, forming a common mode emi current.

Fig. 2 EMI current flow path diagram of pwm variable frequency drive motor system

3 Suppression measures for pwmVFD (Variable-frequency Drive) conducted interference

Since electromagnetic interference must have three elements: electromagnetic interference source, electromagnetic interference propagation path and sensitive equipment, so to suppress the conduction interference of pwm variable frequency drive motor system must also start with three elements, that is, reduce the intensity of interference source, cut off the propagation path and Improve noise immunity of sensitive equipment.

3.1 Emi suppression technology based on reducing the emission intensity of interference sources

From the perspective of reducing the intensity of interference sources, there are three representative methods: changing the circuit topology, improving the control strategy and optimizing the drive circuit.

(1) Change the circuit topology

The idea of improving the circuit topology is mainly to eliminate the common mode voltage output by the converter through a symmetrical structure, and because the switching device