The frequency convertor is a power control device that uses frequency conversion technology and microelectronics technology to control the AC motor by changing the frequency of the motor's working power supply. In the industrial field, the interference problems of frequency converters appear more and more serious, and even cause the control system to be unable to be put into use normally. However, the working principle of frequency convertors is destined to generate strong electromagnetic interference.
In various industrial control systems, with the widespread use of power electronic devices such as frequency converters, the electromagnetic interference of the system is becoming more and more serious, and the corresponding anti-interference design technology has become more and more important. The interference of the power frequency converter system can sometimes directly cause damage to the hardware of the system. Sometimes, although it cannot damage the hardware of the system, it often causes the system program of the microprocessor to run out of control, resulting in control failure, resulting in equipment and production accidents. Therefore, how to improve the anti-interference ability and reliability of the system is an important content that cannot be ignored in the development and application of automation devices, and is also one of the keys to the application and promotion of computer control technology. When it comes to the anti-interference problem of the frequency converter, it is necessary to first understand the source and propagation mode of the interference, and then take different measures for these interferences.
The inverter includes a rectifier circuit and an inverter circuit. The input AC power is converted into a DC voltage through the rectifier circuit and the smoothing circuit, and then the DC voltage is converted into pulse voltages of different widths through the inverter. Using this PWM voltage to drive the motor can achieve the purpose of adjusting the torque and speed of the motor. This working principle causes the following three types of electromagnetic interference:
Radio frequency radiated interference comes from the input and output cables of the frequency converter. In the case of the above-mentioned radio frequency conduction emission interference, when there is radio frequency interference current on the input and output cables of the inverter, since the cable is equivalent to an antenna, electromagnetic wave radiation will inevitably be generated, resulting in radiation interference. The PWM voltage transmitted on the output cable of the power frequency converter also contains rich high-frequency components, which will generate electromagnetic wave radiation and cause radiation interference. The characteristic of radiated interference is that when other electronic equipment is close to the frequency converter, the interference phenomenon becomes serious.
The rectifier circuit produces harmonic currents, which create a voltage drop across the impedance of the power supply system, resulting in a distorted voltage waveform. This distorted voltage interferes with many electronic devices (because most electronic devices only work Under sine wave voltage conditions), a common voltage distortion is the flattening of the top of the sine wave. When the harmonic current is constant, the voltage distortion is more serious in the case of weak power supply. The characteristic of this interference is that it will interfere with the equipment using the same power grid, regardless of the distance between the equipment and the inverter.
Since the load voltage is pulse-shaped, the current drawn by the power frequency converter from the power grid is also pulse-shaped. This pulse current contains a large amount of high-frequency components, causing radio frequency interference. The characteristic of this interference is that it will affect the equipment using the same power grid. Interference occurs regardless of the distance between the device and the frequency converter.