Discussion on unbalanced output of the hottest fre

2022-10-04
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Discussion on unbalanced output of frequency converter

1 introduction

in the 1980s and 1990s, the frequency converter just entered the Chinese market and has been widely accepted at present, and is used in metallurgy, textile, printing and dyeing, printing, cigarette machine production lines, buildings, water supply and other fields. Because the frequency converter does have many advantages over the traditional speed regulation in terms of electric drive speed regulation, such as energy saving, miniaturization of equipment, improvement of comfort, such as elevators, trams, etc., it can realize automatic control, improve control accuracy, improve product quality, improve production efficiency and product qualification rate, and the frequency converter will be increasingly widely used in our Congress in the future

because the frequency converter is an electronic product, it has a design service life in theory. In practical application, it sometimes alarms for failure, and its failure probability is shown in Figure 1

in practical application, the failure rate of frequency converter is also related to correct use, maintenance and use environment. It is not difficult to see the prospect of frequency converter maintenance from Figure 1. Unbalanced output of frequency converter is a common and typical fault of frequency converter, which is discussed here for the discussion of the majority of peers

2 basic working principle of inverter of frequency converter

the quality of three-phase (U, V, w) AC output frequency waveform and the degree of voltage balance of frequency converter directly affect the speed regulation operation state of asynchronous motor and the service life of motor, more importantly, it affects the service life of frequency converter. For a repaired frequency converter, the waveform of u, V, w three-phase AC output meets the requirements and voltage balance is the most basic

usually, the frequency converter mainly includes: the main circuit IGBT, or GTO and other power switching devices constitute the inverter to provide the asynchronous motor with the power supply for voltage and frequency regulation. The output voltage or current and frequency of this power supply are controlled by the control command of the control loop, and the control command is obtained by the external operation command calculation, For occasions requiring precise control speed or fast response, the calculation should also include the signals detected by the main circuit of the frequency converter and the transmission system for closed-loop control. The composition of the protection circuit should not only prevent the faults caused by the overvoltage protection, overcurrent protection and overheating protection of the main circuit of the frequency converter, but also protect the asynchronous motor and transmission system, etc. Therefore, the inverter faults that directly affect the main circuits of u, V and W output are of great importance

the inverter is opposite to the rectifier. The inverter converts the DC power into the AC power of the desired frequency, and turns on and off the six power switching devices of the upper bridge and the lower bridge at the determined time. As shown in Figure 2

Figure 2 Schematic diagram of inverter

in Figure 2, S1 ~ S6 form a bridge inverter circuit. These six switches can be turned on and off alternately to obtain a three-phase AC voltage with a phase difference of 2/3 π on the three phases of u, V and W at the output. It can be seen that the voltage waveform of S1 ~ S6 in the drive circuit is consistent with that when it is on and off, which is particularly important for the output voltage balance. Figure 3 shows a typical IGBT grid drive circuit encountered in the maintenance of the frequency converter

Figure 3 typical grid drive circuit

when the grid drive circuit is turned on, a positive grid voltage of 15V is output. This value is enough to fully saturate the IGBT and minimize its on state loss. At the same time, it also limits the short-circuit current and the power stress it brings. According to a report recently released by coherent market insights (Seattle). When the grid voltage is zero, the IGBT is in the off state in order to ensure that the IGBT remains off when dv/dt noise occurs in the collector emitter voltage. It is necessary to apply an off bias voltage on the grid, and the use of reverse bias voltage can also reduce the off loss. The reverse bias voltage of H series IGBT is within the range of -5v ~ 15V

3 unbalanced output of frequency converter and Countermeasures

in the actual maintenance, the oil used by U6 and experimental machine varies according to the climate of the region, and the imbalance of V and W output can be divided into three situations:

(1) the output phase of the frequency converter display (missmg moto phase) is missing. If the fault of the detection circuit is eliminated, directly check the IGBT module and the drive circuit, and the conclusion is that the IGBT module is damaged, and the drive circuit is also faulty. The problem can be basically solved by replacing the IGBT module and the upper components of the driving circuit, such as optocoupler, PNP, NPN, a pair of driving transistors, electrolytic capacitors, regulators, etc

(2) the output U of the frequency converter has a difference of about 100V between V and W in infrastructure construction, such as highway construction (output 380V is an example). In the driving circuit, there is no driving voltage and driving signal waveform in one of the S1 ~ S6 driving circuits. By measuring the output terminals u, V, w-p

(3) the DC voltage between u, V and W-N can be found that the driving voltage is abnormal or there is no driving signal waveform, which leads to the phase difference caused by the abnormal operation of one phase of u, V and W

the solution is to check whether the driving circuit voltage is normal, whether the optocoupler is broken, whether the electrolytic capacitor leaks liquid, etc. If the 6-channel waveform measured by oscilloscope meets the technical requirements, the problem can be solved

another phenomenon is that the difference between the three-phase output AC voltages of the frequency converter u, V and W is greater than 3%. Although it can be used, it cannot be used for a long time and with large loads. This is mainly caused by the asymmetry of the main devices between the driving circuits S1 ~ S6, such as the technical parameters of the transistor, the parameters of the voltage stabilizer, the liquid dry, liquid leakage and leakage of the capacitor, etc. the loss of the devices on the six way driving circuit makes a certain difference in their parameters, resulting in a small potential difference between the output u, V and W of the frequency converter. Although the above situation can be used, it is technically unacceptable. Our company pursues excellence. Through screening and aging of various devices, such as the consistency and matching of transistor technical parameters and regulator technical parameters, we can ensure that the driving signal in the driving circuit meets the technical requirements, ensure that the IGBT module is saturated, and the consistency of conduction time is guaranteed by the quality of the devices. When the repaired frequency converter is doing the load test, the motor sound is light when the motor is running, The three-phase current of the motor with the same power and the same power load before and after repair is much smaller

4 conclusion

the three-phase imbalance of the output of the frequency converter is a typical common fault of the frequency converter, but various complex problems may be encountered in practice. We hope that bio based packaging materials will become an important choice. We hope you can communicate together. At the same time, we also hope to better serve our customers. (end)

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