Analysis of VFD (Variable-frequency Drive) Purchasing Behavior: VFD (Variable-frequency Drive) Self-interference and Anti-interference Problems

In a survey on the purchase behavior analysis of Chinese domestic low-voltage VFD (Variable-frequency Drive) users, we clearly found that the biggest problem affecting users' purchase of VFD (Variable-frequency Drive) is the VFD (Variable-frequency Drive) itself. The problem of interference and anti-interference is still the main problem that users have when using VFD (Variable-frequency Drive). However, compared with 2012, the ratio of this segment has dropped slightly. This phenomenon shows that China has made great improvements in the interference and anti-interference problems of VFD (Variable-frequency Drive) in the past two years. Because many domestic VFD (Variable-frequency Drive) manufacturers have been aware of this concern of customers, and have taken positive responses, they have made great efforts and improvements in interference and anti-interference issues.

The efforts made by domestic VFD (Variable-frequency Drive) companies in these areas include:

Analyze the source of VFD (Variable-frequency Drive) disturbance

The first is the disturbance from the external grid.

(1) Interference of thyristor commutation equipment to VFD (Variable-frequency Drive)

(2) Interference of power compensation capacitor to VFD (Variable-frequency Drive)

The second is the external interference of VFD (Variable-frequency Drive) itself.

(1) Input current waveform VFD (Variable-frequency Drive) input is diode rectifier and capacitor filter circuit. Obviously, only when the line voltage UL of the power supply is greater than the DC voltage UD across the capacitor can there be a charging current in the rectifier bridge. Therefore, the charging current always appears near the amplitude value of the supply voltage in the form of a discontinuous shock wave. It has strong higher harmonic components. The relevant data shows that the harmonic components of the 5th harmonic and the 7th harmonic in the input current are the largest, which are 80% and 70% of the 50HZ fundamental wave respectively.

(2) Waveforms of output voltage and current Most of the inverter bridges of VFD (Variable-frequency Drive) use SPWM modulation, and the output voltage is a series of rectangular waveforms whose duty cycle is sinusoidally distributed; Due to the inductive nature of the windings, the current in the stator is very close to a sine wave. However, the harmonic components equal to the carrier frequency are still larger.

Anti-jamming Countermeasures of Frequency Conversion Speed Control System

According to the basic principle of electromagnetics, the formation of electromagnetic interference (EMI) must have three elements: the source of electromagnetic interference, the path of electromagnetic interference, and the system sensitive to electromagnetic interference. To prevent interference, hardware anti-jamming and software anti-jamming can be used. Among them, hardware anti-interference is the most basic and important anti-interference measure of the application measure system. Generally, interference is suppressed from the two aspects of resistance and prevention. The general principle is to suppress and eliminate the interference source, cut off the coupling channel of the interference to the system, Reduce the susceptibility of the system to interfering signals. Specific measures can be used in engineering, such as isolation, filtering, shielding, grounding and other methods.