In modern industrial control, the electric drive system using frequency converter to control motor has the advantages of remarkable energy-saving effect, convenient adjustment and control, simple maintenance, networked centralized remote control, and control system with PLC. These advantages of frequency converter make it more and more widely used in the field of industrial automatic control. This paper analyzes the fault problems in the application of frequency converter, and introduces the treatment methods.

1. Some problems in the application of frequency converter

1.1 harmonic problems

The switching devices in the main circuit of the frequency converter will produce harmonics in the process of making and breaking the circuit. The lower harmonics usually have a great impact on the motor load, causing torque ripple; The higher harmonic will increase the leakage current of the output cable of the frequency converter and make the motor output insufficient. Harmonic interference will also lead to the misoperation of relay protection devices, making the measurement of electrical instruments inaccurate and even unable to work normally.

1.2 noise and vibration problems

Using frequency converter for speed regulation will produce noise and vibration, because the output waveform of frequency converter contains high-order harmonic components. With the change of operating frequency, the fundamental component and higher harmonic component change in a wide range, which is likely to resonate with the natural mechanical vibration frequency of the motor, and this resonance is the source of noise and vibration.

1.3 heating problem

The converter generates heat due to internal loss during operation, which accounts for 98% of the main circuit and about 2% of the control circuit. At the same time, in summer, the ambient temperature is too high, which makes the temperature of the frequency converter rise, and the temperature can be as high as 80~90 ℃. Because the frequency converter is an electronic device, which contains electronic devices and electrolytic capacitors, too high temperature is easy to cause component failure, so that the LCD screen data cannot be displayed, and the frequency converter protection action often occurs.

Therefore, it is necessary to suppress the harmonic output of the frequency converter within the allowable range, eliminate or weaken the noise and vibration, and heat the frequency converter to prolong the service life of the frequency converter.

2 Analysis and treatment of some problems in the application of frequency converter

2.1 handling of harmonic problems

The treatment of harmonic problem is to cut off the propagation path of interference and suppress the high-order harmonics on the interference source.

The transmission ways of cutting off interference are:

1) Cut off the way of common ground wire to transmit interference. The grounding of power wire and control wire should be separated, that is, the grounding terminal of power device should be connected to the ground wire, and the grounding terminal of control device should be connected to the metal shell of the device panel.

2) The separation of the wire wiring of the signal line away from the interference source current is effective to eliminate this interference, that is, separate the high-voltage cable, power cable, control cable from the instrument cable and computer cable.

The ways to suppress high-order harmonics on interference sources are:

1) Increase the internal impedance of the power supply of the frequency converter. Generally, the internal impedance of the power supply equipment can buffer the reactive power of the DC filter capacitor of the frequency converter. The larger the internal impedance is, the smaller the harmonic content is. This internal impedance is the short-circuit impedance of the transformer. Therefore, when choosing the power supply of frequency converter, it is best to choose the transformer with large short-circuit impedance.

2) Install filter. Add LC type passive filter in front of the frequency converter to filter out high-order harmonics, usually 5 and 7 harmonics.

3) Install reactor install line reactor in front of the frequency converter to suppress the overvoltage at the power side.

4) Setting active filter active filter automatically generates a current with the same amplitude and opposite phase as harmonic current, so it can effectively absorb harmonic current.

2.2 treatment of noise and vibration problems

1) When the low-order harmonic component in the output of the frequency converter resonates with the natural mechanical frequency of the rotor, the noise increases; When the high-order harmonic component in the output of the frequency converter resonates with the iron core, housing, bearing frame, etc. near their respective natural frequencies, the noise increases.

The noise, especially the harsh noise, produced by the drive motor of the frequency converter is related to the switching frequency of PWM control, especially in the low-frequency region. To solve this problem, the AC reactor is usually connected at the output side of the frequency converter. If the electromagnetic torque has margin, the u/f can be set smaller to stabilize and reduce the noise.

2) When the frequency converter works, the magnetic field caused by the high-order harmonic in the output waveform produces electromagnetic force on many mechanical parts. When the frequency of the force is close to or coincides with the natural frequency of these mechanical parts, resonance will occur. The major influence on vibration is the lower harmonic component, which has a great influence in PAM mode and square wave PWM mode. However, when SPWM is used, the low-order harmonic component is small, and the influence is also small.

The method to reduce or eliminate vibration is to connect an AC reactor at the output side of the frequency converter to absorb the high-order harmonic current components in the output current of the frequency converter. When using PAM mode or square wave PWM mode frequency converter, you can use SPWM mode frequency converter to reduce pulsating torque, which can weaken or eliminate vibration and prevent mechanical parts from being damaged by vibration.

2.3 treatment of heating problems

The operating ambient temperature of general frequency converter is generally required to be -l0 ℃ ~ +50 ℃. In order to ensure the reliable operation of the frequency converter and extend the service life of the frequency converter, it is necessary to heat the frequency converter. In winter, the built-in fan of the frequency converter can be used to take away the heat inside the frequency converter box; The temperature itself is 40 ℃ in summer. Using the internal heat taken away by the built-in fan of the frequency converter can only raise the temperature in the room and the frequency converter box. At this time, the best way is to use the window or hit several evenly and appropriately on the upper and lower walls of the machine distribution room next to the frequency converter box φ 500mm hole, and ensure that there is a certain space around the frequency converter in the control cabinet to maintain good natural ventilation. If this doesn't work, you can turn on the fan, or install an exhaust fan and air duct at the hole to forcibly extract the heat generated by the frequency converter out of the room. Finally, air conditioning can be used to forcibly cool the space environment where the frequency converter is installed.