Maintenance process of two Delta frequency converters VFD-A

Taking over two inverters of the same model (Delta) brand, the inspection showed that the inverter output module was damaged and the driving circuit was severely damaged: the driving integrated circuit T250V or exploded, or the output end was short circuited to the power supply ground, the filtering capacitor sprayed liquid, the voltage regulator tube was broken or opened, the resistance was open or increased, and the circuit board was carbonized and damaged. Continuing the inspection, it was found that one arm of the three-phase rectifier bridge of one inverter was broken, and the charging current limiting resistor was damaged The charging resistor is short circuited and the relay contacts are stuck, resulting in severe damage. It was found that several of the signal introduction resistors on the input side of the driving integrated circuit were also in an open circuit state. The other end of this resistor was connected to the CPU trigger pulse output end, and the CPU was also subjected to strong electrical shocks. If the CPU control board is damaged again, then these two frequency converters have little repair value.

1. After drawing the main circuit and driver circuit, conduct a comprehensive inspection, scrape off the carbonized part of the circuit board with a small knife, and remove all damaged components. There is no short circuit phenomenon in the main circuit during measurement. After power transmission inspection, the display is normal, indicating that the switch power supply and control part are basically normal. Use an oscilloscope to measure six driver inputs (trigger signals from the CPU), with a peak voltage of 1.5V (measured by a multimeter at 0.6V) and a carrier frequency of 10kHz that varies with frequency and pulse width. From this, I finally felt relieved. It seems that except for some damage to the inverter and drive circuits, all other circuits are normal, and the impact resistance of the CPU’s three-phase pulse output terminal is really good. Start purchasing parts immediately and prepare for comprehensive repairs.
2. Replace all damaged parts of the driver circuit (more than 30 components), and test the static DC voltage of each pin of the driver integrated circuit when powered on, all of which are normal; Use an oscilloscope to measure the output waveforms of each integrated circuit within the normal range, and then weld the inverter output module.
3. Power on inspection, using a multimeter to measure the AC range, found three-phase imbalance phenomenon. Switching to DC 500V range for measurement, there is no DC component between V and W, but there is DC voltage between U, V and U and W! Regardless of frequency and voltage, there should be no direct current component included. During the experiment of attaching three star shaped connected light bulbs at the output end, the flickering phenomenon was observed to be too obvious. Based on experience, when the frequency is set above 20 Hz, there should be no noticeable flicker, but gradually becomes more pronounced below 15 Hz; Adjusting to around 30 Hz, there is still flickering phenomenon. Based on the above detection, it is determined that one of the positive and negative half wave voltages outputted by the U-phase has no output!
4. Hurry up and stop the power supply. Upon inspection, it was found that the voltage regulator diode DD11 in the EU circuit triggered power supply was damaged. Due to the weak bonding of the original SMD component after replacing it with a regular component, it was accidentally desoldered during the installation of the inverter module, resulting in the triggering end of the upper transistor in the U phase being forced to be at a low level – negative pressure. The upper transistor is always in the cut-off state, which means that only the negative half wave output of the lower transistor is conductive in this phase, resulting in a DC component in the output! Weld DD11, test the machine with power on, measure the three-phase output balance, and ensure that the DC component is zero. Connect it to a 5.5kW submersible pump for testing, and both startup and operation are normal. As a result, the first frequency converter was successfully repaired.
   When repairing the second machine, the cleaning steps of the first machine were repeated, and finally the inverter module was welded. After connecting three light bulbs and powering them on, first adjust the output frequency to a few hertz, and then momentarily short circuit the control terminals DCM and FWD (forward starting control). The ears heard a “snap” sound, but the heart only let out a bitter cry. It was clear that the newly replaced MG25Q6ES42 inverter output module had exploded and damaged in an instant!
   Remember to test the output waveform of the six drive circuits before welding the inverter module, and it is completely normal. There should be no problem. We also soldered all the parallel resistors triggered by the inverter module at the output end and measured them with a meter to confirm good soldering. A check, oh my! The three trigger signal introduction resistors soldered to the damaged EU, EV, and EW terminals on the front of the circuit board have all been soldered. However, the GX and GY terminals on the back of the circuit board are located on the back and have been soldered to the inverter module. The two output pulse introduction resistors (one of which was originally 100 Ω and two parallel resistors were replaced by a 0.5W51 Ω resistor during repair) forgot to be soldered, resulting in the instantaneous and painless reverse output module Regret didn’t even have time to explode and damage! It is truly regrettable that an expensive inverter module that can cost hundreds or even thousands of yuan suddenly breaks down.
   In the case of not connecting the trigger circuit – in the case of triggering resistance open circuit damage – when one arm of the trigger terminal of the inverter output module is suspended, the reckless input of the operating signal can cause damage to the inverter module in the blink of an eye. It is strictly prohibited to open a trigger input terminal in the starting state, otherwise it will cause serious consequences of module damage! During the repair process, before conducting the power test, it is necessary to check whether the trigger terminal leads are securely connected. For faults that damage the inverter module when starting with power on, first and thoroughly check the module drive circuit!
   But what is the mechanism of its damage? From the fault phenomenon, it can be seen that the inverter module is damaged by short-circuit breakdown and explosion. The cause of the short circuit is not overvoltage breakdown, but rather overcurrent damage. But when three 15 watt light bulbs are connected to the load, they are unloaded (in fact, even if they are completely unloaded, there will be short-circuit damage), and there will be no back electromotive force entering, so there is no dangerous overvoltage in the entire circuit. So how does overcurrent damage occur? The analysis is as follows: When the inverter circuit is working normally, six trigger pulses are used to control the opening and closing of six IGBT tubes in a certain order, converting the DC power supply into three-phase alternating voltage output. Each phase output is alternately conducted and cut off by two pipes, forming the positive and negative half waves of that phase. There is a certain time interval during the handover of two pipes, also known as a certain dead zone time, which means that it is not allowed to have two pipes conducting simultaneously during any given time period. The simultaneous conduction of the upper and lower tubes will inevitably lead to a short circuit to the power supply, and the consequence is the explosion and damage of the inverter module! This kind of damage is not directly related to the external load, and even when unloaded, it will still be damaged as usual. In the above example, the triggering end of the upper tube is suspended, and the negative bias voltage required for the tube to cut off is zero. When the lower tube is triggered to conduct, it is equivalent to instantaneously short circuiting the emitter of the upper tube to ground. At this time, the upper tube generates a charging current through the positive pole of the power supply to form a capacitor between the collector gate and the input capacitor between the gate and emitter. When the triggering resistor is not open circuit, this charging current is absorbed by a sufficiently large negative bias voltage, Cannot touch the upper tube. But at this point, due to the disappearance of negative bias voltage, this charging current forms a positive gate bias voltage, which is sufficient to make the upper tube conductive. The common connection of the upper and lower tubes causes a direct short circuit in the power supply, and of course, a “snap” sound will be heard. Similarly, when the trigger of the lower transistor introduces a resistance circuit breaker, the conduction of the upper transistor is equivalent to introducing a high voltage into the collector of the lower transistor, and it will also instantaneously generate a charging current that charges the collector gate capacitor and gate emitter capacitor through the collector. When the trigger resistor is not open, this charging current is absorbed by a sufficiently large negative bias voltage and cannot touch the lower transistor. But at this point, due to the disappearance of negative bias voltage, this charging current forms a positive gate bias voltage of the lower transistor, which also forms a situation where the two transistors are connected together and short-circuit the DC power supply.

In the article on repairing the Sanken IHF frequency converter, I mentioned that the failure of the DC circuit energy storage filter capacitor is the second level killer that causes damage to the inverter module. The suspension of the trigger terminal of the inverter module should be a much more powerful first level killer! The similarity between the two is that the damage is extremely high, and the protective circuit often cannot act in time. The difference between the two is that: 1. The former is due to capacitor failure, and the harmonic of the DC circuit causes overvoltage breakdown damage to the inverter module, while the latter is due to the disappearance of the cut-off negative bias of the transistor, causing overcurrent short circuit damage to the power supply caused by the two transistors being connected together. 2. The damage of the former still has a gradual process, and it is damaged during startup or operation. If the load is very light or empty, it will not cause damage; The latter is simply non process damage, manifested as the instantaneous failure of the inverter module upon receiving the starting signal, whether it is loaded or unloaded! So the harm of the latter is particularly severe, especially when it easily occurs during the process of fault repair. A slight mistake can lead to the complete abandonment of previous achievements and regret!
Guarantee measures after repair and before startup: First, cut off the power supply of the inverter module. 1. Measure the static DC voltage on the input and output sides of the drive integrated circuit to ensure it is in a normal state; 2. Measure the output pulse waveform of the six channel drive, observe while adjusting the frequency, and ensure that the amplitude and frequency are equal; 3. First, connect the power supply of the inverter module to a low DC voltage. If the+24V voltage supplied by the parallel power supply is turned off, conduct a start stop test to check the balance of the three-phase output and the presence of DC components. In this step, if there is an abnormality in the driving circuit, the fault has already been exposed; 4. If there is no low-voltage DC voltage condition, a 15-40W light bulb can be connected in series in the inverter power supply circuit before starting the test. This light bulb not only serves as a current output current indicator, but more importantly, when the poor driving circuit causes an output short circuit, the voltage drop drops on the light bulb, and the limiting effect of the light bulb resistance protects the module from damage. The light bulb may also act as a fuse, and the blown filament also protects the inverter module. 5. After detecting that the no-load output is normal, remove the series connected light bulb and restore the power supply to the inverter module. Finally, check the connection wire of the trigger terminal and check the tightness of the screws; 7. Complete machine assembly with motor test.
When installing on site, identify the cause of the previous damage, adjust relevant parameters based on the electrical, mechanical, and temperature environment on site, or add accessories. If there is a capacitor compensation cabinet on site and the frequency converter is installed densely, resulting in severe power pollution and high power harmonics, a three-phase reactor can be installed on the input side of the power supply to avoid further damage in a short period of time; If it is found that the load inertia is large and fast parking is necessary, and the frequency converter is prone to overvoltage damage, the user should be required to install a brake unit and brake resistor before putting it into operation. It should be noted that the damage to some frequency converters is mostly caused by improper user use and adjustment. If these harmful factors are not eliminated, the repaired frequency converter is likely to be damaged again in a short period of time, causing unnecessary losses to users and maintenance personnel.