After checking the drive circuit of the 22kW Delta VFD Drive and replacing it with a new module, the OC will jump upon startup. The module is newly replaced, and all six drive pulses are working properly. I don’t think it should be. Still checking the measurement, the six negative pressures driving the IC during shutdown are all normal, and the six excitation voltages are also normal after startup. It is necessary to first determine whether the fault is caused by the driver IC or the module.
It is necessary to first test the load carrying capacity of the six drive ICs, that is, to measure the trigger current value of their output. Connect a 15 ohm resistor in series to the output terminal, and then connect a 15 ohm resistor in series to the probe to limit the circuit current to around 0.5A. After the start signal is activated, its current output capacity is measured, and it can still provide a dynamic current of about 150mA even when the original trigger circuit is connected normally. The driving circuit of the V-phase lower arm IGBT tube only outputs about 40mA of current, which obviously cannot meet the excitation requirements of the IGBT tube. The root cause of the OC fault lies in this! There seems to be a misconception about the driving method of IGBT tubes, especially high-power IGBT tubes: IGBT tubes are voltage signal excitation devices, not current type excitation devices. The driving signal only needs to meet the voltage amplitude, without requiring too much current driving capability! I have previously analyzed that even IGBT tubes are essentially current driven devices! The output signals of the driving ICs (PC929 and PC923) of the machine are amplified by a complementary voltage follower and then supplied to the triggering terminals of the module. The push-pull amplifier was originally a pair of field-effect transistors, but due to the lack of the original type of transistor on hand, it has now been replaced with transistor pairs D1899 and B1261. After modification testing, it should be able to meet the excitation requirements. Check the V-phase lower arm circuit. The resistance from pin 11 (pulse output pin) of PC929 to the subsequent power amplifier circuit was originally 100 ohms, but now it has changed to over 100k, causing D1899 to be unable to fully conduct and the output driving current to be too small. After replacing the resistor, the output current is normal. After replacing the power transistor, the base resistance was not measured, resulting in this phenomenon. By the way, I measured the negative current supply capacity of the driving circuit when cutting off negative pressure output. The probe is still connected in series with a 15 ohm resistor, and each circuit is around 30mA. This leads to the conclusion that measuring the output voltage of the driving IC is not as direct and effective as measuring its output current. And it can expose the root cause of the malfunction. When the internal resistance of the circuit output increases due to certain reasons, measuring the driving voltage is often normal, which masks the truth of insufficient driving current.
A 22kW HC1 drive, the fuse of the inverter module power supply series connection is broken, and no other abnormalities are found in the main circuit measurement. After installation, first send the inverter power supply to 24V and jump EOCn, which means overcurrent during acceleration and short circuit on the motor side. Obviously, there is still a malfunction in the module or driver part. It seems that it’s not just about replacing insurance.
Dismantle and recheck the driver circuit board. It was found that there was no positive excitation pulse output in one of the driver circuits. The power amplifier tube (lower tube) of the driver circuit was found to have broken down, and the voltage terminal of the module trigger terminal was continuously embedded on the negative pressure. After replacing the amplifier tube, the pulse circuit is normal. Install the machine, connect to 24V power supply, and power on to trip EfbS, which means the fuse is blown. Remove the 24V power supply and replace the original fuse terminals with light bulbs in series, which will emit strong light upon power transmission. But after removing the trigger terminal during power outage, the individual measurement module was normal; Install the insurance and connect the inverter circuit to a 24V power supply. Start the frequency converter, and when the frequency rises to around 5Hz, the ECOn will still trip. I’m not sure if it’s still a problem with the module or the driver circuit. Recheck the positive and negative voltage and current of the drive output, both are normal. Possible module malfunction. Simply remove all three modules and place them on the workbench for power testing along with the driver board. After powering on, it was detected that the negative pressure on one arm was low, about 2V. Disconnect the trigger terminal, the negative pressure returns to normal value, insert the module trigger terminal, and the negative pressure decreases again. Confirmed that the module was indeed damaged, replaced with a new module, and the fault was repaired.
A ENC EDS1000 11kW Inverter will trip to constant speed overcurrent when accelerating to above 40Hz during operation. But in reality, the operating current is much lower than the rated current, and after switching to other frequency converters, the motor runs normally. Check that the six inverter pulse outputs of the driving circuit are all normal. It is determined that the current transformer circuit detection is abnormal. Check the current detection circuit. The output signal of the current transformer is divided by a 3-ohm resistor and a 30 ohm resistor before being supplied to the motherboard. Suspecting that the current transformer is a non-standard product, an external voltage divider network was connected for adjustment. The partial voltage value may not be accurate enough, causing the current sampling value to be too large and mistakenly skipping the current fault. Or there may be drift in the output value of the internal circuit of the current transformer, which can also cause a false skip current fault.
The simplest method is to adjust the external voltage divider network of the current transformer. Reduce the voltage divider resistance value below it to meet the requirements of the subsequent circuit input voltage range. If conditions permit, the panel current display value can be monitored during operation, and the voltage divider resistance value can be adjusted to match the operating current value with the displayed current value. Often in the maintenance department, it is not possible to connect the frequency converter to the rated load for operation. Therefore, first replace the lower resistor with a 100 Ω potentiometer, and then adjust it to the appropriate position during on-site installation and operation.
A Hitachi L300P75kW Inverter was installed and tested on site after repairing the module fault. When powered on and started, E16.4 or E16.2 jumps. The cause of the fault is a momentary open circuit in the power supply. Stop the machine and measure the three-phase 380V power input. All three phases have 380V and are quite balanced. During operation, when measuring the three-phase output circuit, there is an unstable voltage value in one phase, with fluctuations ranging from 280V to around 350V. The voltage detection circuit of this machine detects the input voltage of T and S phases in the input power supply. When the power grid pollution flash exceeds 15ms, it will protect and shut down. It was determined that the air switch supplying power to the frequency converter had poor contact with one phase, causing the frequency converter to trip E16.4 or E16.2 faults. Upon disassembly and inspection, it was confirmed that a set of contacts had been severely burned out. Repair after replacing the power switch.
This fault is in a stationary state or a low current state, and due to the virtual connection of the air switch, the abnormal input voltage cannot be detected at all. Only visible when turned on. But due to the abnormal detection of the frequency Inverter, it immediately shuts down for protection. Sometimes, if there is no time to detect, the frequency converter has already stopped. So it’s not easy to detect. It took some effort.
Taking over a 15kW WEICHI frequency inverter, it was damaged by lightning strikes. The motherboard and driver board were both struck by lightning, but fortunately, the module and CPU were not damaged.
Inspection:
Control terminal+10V voltage to 0, no output. This voltage is obtained by stabilizing the+15V of the switching power supply through the LM317 (eight pin SMT IC) circuit. At the moment, there is no LM317 SMT IC at hand, so a 100 Ω resistor and a 10V voltage regulator are used as substitutes for repair;
The LF347 chip IC (four operational amplifier integrated circuit) in the voltage detection circuit is damaged, and the LM324 chip is directly used as a substitute. The functions of each pin are consistent;
The SMT transistor for controlling the charging relay is damaged and replaced with a plastic sealed direct insertion transistor D887. All lightning faults have been repaired. The test run is normal.
Repairing the Shanghai Rihong CHRH-415AEE 1.5kW machine, the user reported unstable output and motor jumping. The output module and output module are both normal. After cutting off the power supply to the inverter module, in order to check the quality of the inverter pulse conveying circuit (including the driving circuit), they were installed as a complete machine on the maintenance bench (without the machine cover installed) and powered on for inspection. The operation panel displays normally, but when starting the operation, it jumps E OH means overheating. The thermal signal output terminals of the short-circuit modules T1 and T2 are invalid. Disconnect the thermal signal terminal and connect the original wiring terminal to the potentiometer for voltage regulation. The test is also ineffective. Check the internal circuit diagram of the module. The terminal is only equipped with a thermistor (10k at zero degrees Celsius), which is connected to an external+5V resistor to divide the voltage and directly send the signal to the CPU. According to room temperature, this partial pressure point should be below 2.5V. The measured voltage is 2.3V, and the built-in thermal element and circuit should be normal.
Later, it was discovered by chance that a small square shielding iron sheet was wrapped around the back of the operation panel. When pressing the operation panel, one corner of this iron sheet touched the 41 pin of the CPU, which happened to be the input pin for the overheat signal. Therefore, pressing the button on the operation panel inputs a module overheating signal (disturbance generated) to the CPU, which is truly a coincidence. A piece of cardboard is placed between the operation panel and the motherboard circuit, so that when operating the panel, it no longer jumps OH fault code. The fault was quickly identified as a faulty driver circuit, and the machine was repaired by replacing it with a driver IC.
A. Damage caused by abnormal load Indeed, the protection circuit of the frequency converter is already quite complete. For the protection of expensive inverter modules, various inverter manufacturers have put a lot of effort into their protection circuits, from output current detection to IGBT voltage drop detection in the drive circuit, and strive to implement the fastest overload protection with the fastest strain rate! From voltage detection to current detection, from module temperature detection to phase loss output detection, there has not been a protection circuit for any particular electrical appliance yet. A frequency converter has been focused and invested in this approach. When salespeople talk about the performance of frequency converters, they must also mention the protection function of the frequency converter. They often unconsciously promise users that with the comprehensive protection function of the frequency converter, your motor will not be easily burned. This salesperson doesn’t know that this promise will bring him great passivity!
Does the motor really not burn when using a frequency converter? My answer is: compared to power supply, using a frequency converter makes it easier for the motor to burn, and the motor is prone to burning, making it easier for the inverter module of the frequency converter to be “reimbursed” together. The sensitive overcurrent protection circuit of the frequency converter is at a loss here and has no effect at all. This is a major external cause of damage to the frequency converter module. Listen to me explain the truth behind it. A motor can operate at power frequency, although the operating current is slightly higher than the rated current, there is a certain temperature rise during long-term operation. This is a faulty motor that can indeed run before it burns out. But after connecting to the frequency converter, frequent overloads may occur, making it impossible to operate. It doesn’t matter yet. A motor that can operate in power frequency mode and has been used normally by the user for many years. Please pay attention to the word “many years”. Users may think of saving electricity bills or need to undergo frequency conversion modifications due to process modifications. But after connecting to the frequency converter, there will be frequent OC faults, which is good. The protection has stopped and the module is not damaged. What’s scary is that the frequency converter didn’t immediately trip the OC fault, but was running for no reason – after only three or two days of operation, the module exploded and the motor burned out. The user relied on the salesperson: the quality of the frequency converter you installed is poor, burned my motor, you want to compensate my motor! Prior to this, the motor seemed to have no problems and was running well. The operating current was measured because the load was relatively light, reaching half of the rated current; Tested three-phase power supply, 380V, balanced and stable. It really seems like the damage to the frequency converter, along with the damage to the motor. If I were present, I would be fair and impartial: don’t blame the frequency converter, it’s because your motor is already “critically ill” and suddenly malfunctioned, accompanied by damage to the frequency converter! A motor that has been in operation for many years, due to temperature rise and moisture, the insulation degree of the winding has greatly decreased, and even has obvious insulation defects, which are at the critical point of voltage breakdown. In the case of power supply at power frequency, the input voltage of the motor winding is a three-phase 50Hz sine wave voltage, and the induced voltage generated by the winding is also relatively low. The surge component in the circuit is small, and the decrease in the insulation degree of the motor may only bring about inconspicuous “leakage current”. However, the voltage breakdown phenomenon has not yet occurred between the turns and phases of the winding, and the motor is still “operating normally”. It should be said that as the degree of insulation aging deepens, even under power supply at power frequency, it is believed that in the near future, the motor will eventually burn out due to voltage breakdown between phases or windings caused by insulation aging. But the problem is, it hasn’t burned down yet.
After connecting to the frequency converter, the power supply conditions of the motor become “harsh”: the PWM waveform output by the frequency converter is actually a carrier voltage of several kHz or even more than ten kHz, and various components of harmonic voltage will also be generated in the motor winding power supply circuit. According to the characteristics of the inductor, the faster the change rate of the current flowing through the inductor, the higher the induced voltage of the inductor. The induced voltage of the motor winding has increased compared to the power frequency supply. Insulation defects that cannot be exposed during power supply at power frequency are caused by the inability to withstand the impact of induced voltage under high-frequency carriers, resulting in voltage breakdown between turns or phases of the winding. The sudden short circuit of the motor winding was caused by a phase to turn short circuit in the motor winding. During operation, the module exploded and the motor burned out. In the initial stage of start-up, due to the low output frequency and voltage of the frequency converter, when there is a fault in the load motor, although it causes a large output current, this current is often within the rated value. The current detection circuit acts in a timely manner, and the frequency converter implements a protective shutdown action, so there is no risk of module damage. But if the three-phase output voltage and frequency reach high amplitudes under full speed (or near full speed) operation, if there is voltage breakdown phenomenon in the motor winding, it will instantly form a huge surge current. Before the current detection circuit acts, the inverter module cannot withstand it and will explode and be damaged. From this, it can be seen that protective circuits are not omnipotent, and any protective circuit has its own weaknesses. The frequency converter is unable to effectively protect the motor winding from sudden voltage breakdown during full speed operation. Not only the frequency converter protection circuit, but any motor protector cannot effectively protect against such sudden faults. When such sudden faults occur, it can only be declared that the motor has indeed passed away. This type of fault is a fatal blow to the inverter output module of the frequency converter and cannot be avoided.
The reason for the third type of module damage should not have been a single cause. Hopefully, in the near future, the only reasons for module damage will be the first two. For domestic frequency converters, sometimes it’s just a piece of mouse manure that spoils a pot of soup. Many frequency converters are also quite good, not inferior to foreign products, and they are of good quality and affordable.
Repair an imported Siemens 7.5kW frequency converter due to power supply hiccup fault, with no display on the operation panel. Due to its special installation structure, the machine is surrounded by three circuit boards and a heat dissipation plate in a square shape, with an embedded shell. When repairing, it is necessary to disconnect the circuit board and lay the entire circuit flat on the workbench, such as unfolding a roll of ancient bamboo slips, in order to facilitate maintenance. Moreover, the circuit board is a four layer board, making circuit maintenance difficult.
Starting from the switch power supply circuit, first use the elimination method to cut off the load circuit one by one. If it still cannot vibrate well, it indicates that hiccups are not caused by excessive load. There are no abnormalities in the oscillation and voltage stabilization circuits. Finally, it was found that two 200V voltage stabilizing tubes in the cut-off shunt circuit of the switch tube were damaged due to breakdown. We purchased 110V voltage stabilizing tubes from the market and replaced them with four to repair them. A typical shunt (also known as anti peak voltage absorption) circuit uses a diode connected in series with a resistance capacitance parallel circuit, and then connected in parallel with the primary winding of a switching transformer. The diode connection method is similar to the freewheeling diode connection method of a typical coil circuit. Its function is to quickly release the electrical energy of the primary winding circuit during the period when the switching transistor is approaching cutoff, so that the switching transistor can cut off more quickly. But the circuit consists of two 200V voltage regulators connected in series from the P+end, followed by two thermistors with resistance values of 360k each, connected in series to the drain of the switching tube. The circuit is also connected in parallel to the primary winding. When the switch tube tends to cut off, the sharp decrease in current in the primary winding causes a sharp increase in the back electromotive force of the winding. When it is superimposed with the power supply voltage and exceeds the P+voltage by 400V, this protective circuit breaks down and conducts, releasing this energy back to the power supply. When the back electromotive force energy is small, the current flowing through the two thermistors is small, their temperature rise is also small, their resistance value is large, and the release of energy is also slow. When the back electromotive force energy is large, as the discharge current increases, the resistance temperature rises, the resistance value decreases, and the energy discharge is accelerated. Think about it, this circuit is connected in series with a thermistor, it’s really interesting. Adding a thermistor and a peak voltage absorption circuit with voltage stabilizing diodes to the primary winding of the switch transformer may only be done by Siemens frequency converters. I have also encountered this type of circuit form for the first time.
A 5.5kW Konwo frequency converter sent for repair, the customer said: there is output, but it cannot operate with load, the motor cannot rotate, and the operating frequency cannot be adjusted. Check the main circuit, rectifier and inverter circuits, all of which are normal. Power on, measure the three-phase output voltage without load and it is normal. Connect a 1.1kW no-load motor and start the frequency converter to run. The frequency cannot rise near one or two hertz, and the motor has a pause and produces a creaking sound. No overload or OC fault is reported. Stop and restart, still the same. Disconnect the 550V DC power supply of the inverter module and send another 24V DC low-voltage power supply to check the driving circuit. Check the capacitors and other components of the driving circuit and driving power supply circuit, and they are all normal. The positive and negative pulse currents output by the three arm drive circuit on the inverter output have reached a certain amplitude, and there should be no problem driving the IGBT module; But when measuring the positive and negative pulse currents output by the three arm drive circuit, a module fault is reported. Analyze the reason, as the DC current range of the multimeter is directly short circuited to measure the triggering terminal, the internal resistance of the DC current range of the multimeter is small, which greatly lowers the positive excitation voltage output by the driving circuit, such as below 10V. This voltage cannot trigger the IGBT tube normally and reliably. Therefore, the module fault detection circuit detects the voltage drop of the IGBT tube and reports a fault in the OC module. The fault was actually caused by the measurement method. When the probe was connected in series with a resistance of more than ten ohms and the output current of the drive circuit was measured, the OC fault was not reported. Check the signal output circuit of the current transformer again, and it is also normal. During operation, no fault signal is reported.
I feel like there’s nowhere else to go and I can’t find the cause of the malfunction. Is the problem with the driver, module, current detection, or other circuits? The fault was not detected throughout the afternoon. For a moment, I felt a bit indifferent and worried.
Does the CPU detect abnormal current during startup and take measures to slow down?
Is the current limiting action made by the driving circuit due to abnormal driving or poor module performance? Under low-frequency operation, try to short-circuit the shunt resistors of the U, V, and W output circuits to make the CPU exit the frequency reduction and current limiting action, which is ineffective; Restoring the parameters to their factory values (suspecting that this operating mode may have been manually set) is invalid. Start the frequency converter and observe carefully: after the speed rises to 3Hz, it drops to 0Hz, and repeat this process. The motor stops running. After significantly increasing the acceleration time, it steadily increased to 3Hz and then decreased to 0Hz, indicating that there were no abnormalities in the driving and other circuits. This operating phenomenon should be formed based on the signal emitted by the CPU, which seems to act as a current limiting action based on the current signal. The self deceleration during the starting process is generally due to the following two reasons:
During the startup process, the CPU detects a sharp increase in abnormal current values and performs immediate frequency reduction processing. When the current returns to within normal values, it then increases the frequency for operation;
During the startup process, the CPU detects an abnormal drop in the DC voltage of the main circuit and performs immediate frequency reduction processing. When the voltage of the main circuit returns to within normal values, it then increases the frequency for operation; After the drive and current detection circuits have no issues, maintenance should be carried out from the perspective of voltage. The anomalies caused by voltage can also be divided into two aspects:
Caused by abnormal DC voltage detection circuit in the circuit (drift of reference voltage, variation of sampling resistance, etc.). This signal causes the CPU to mistakenly assume that the voltage is too low, and therefore takes measures to reduce the output frequency to maintain a stable voltage;
The abnormality of the main DC circuit causes a low voltage (loss of capacity of the energy storage capacitor, failure to close the charging short circuit contactor, etc.), which is detected by the detection circuit and causes the CPU to take a frequency reduction action during the startup process. Reinstall and power on the machine, and conduct a motor test. When powered on, no sound of the charging contactor closing was heard. Check that the contactor coil is AC 380V, taken from the R and S power supply incoming terminals. Loose coil lead terminals caused poor contact, and the contactor failed to engage. The large current during startup creates a significant voltage drop on the charging resistor. The sharp drop in the DC voltage of the main circuit is detected by the voltage detection circuit, prompting the CPU to issue a frequency reduction command.
The reason for taking many detours is that the machine only performed frequency reduction treatment when the voltage dropped, and did not report an undervoltage fault. In this case, other models often have reported undervoltage faults. Also due to the reason of no load, during frequency reduction processing, the voltage quickly rises and the frequency continues to rise. Then the voltage drops again, and the frequency converter reduces the frequency processing, allowing the voltage to rise again. This repeated process causes the frequency converter to increase speed, decrease to zero speed, pause and then increase speed again, and then decrease to zero speed. But it does not shut down and does not report any fault signals. It’s a bit funny that such a simple fault should be thoroughly investigated on its normal circuit. Due to its failure to report fault codes, the inspection steps were somewhat bewildered. This article is shared with everyone – when the charging contactor of the Kangwo inverter is in poor contact, it may be adjusted in a frequency reduction manner during the starting process in a light load state, without reporting an undervoltage fault signal and implementing shutdown protection. In the loaded starting state, the DC circuit should have a significant drop and should be able to report an undervoltage fault. The frequency converter is an organic combination of software and hardware circuits, and the above fault phenomena are formed under the automatic control of software programs. If we only rely on the fixed thinking pattern formed by surface phenomena and past experience, without in-depth analysis and detailed observation, we would really treat this simple fault as a difficult one to repair.
A. Strange “fault characters” The user sent a domestically produced frequency converter for repair, which is an Anbang Xin AMB-G9/P9 22kW frequency converter. According to convention, first remove the damaged module and power on to check if the drive circuit is normal; Power on, the operation panel displays OC fault codes; After the short-circuit fault signal returns to the optocoupler, the OC signal will no longer jump. When operating the RUN button on the control panel, the charging relay momentarily disconnects (with a click), the panel indicator light also goes out, and the display screen flashes, displaying a string of “fault characters” that cannot be found in the fault code table. Suspect that there are still other fault signals present. The output terminals of the three-phase output current detection signal are all 0V, normal. For other signals, without surveying the circuit, it is difficult to quickly identify their origins.
Occasionally, when turning off the power and then on again, it is discovered that the so-called “fault characters” mentioned above are actually startup characters! The essence of the malfunction is that there may be a short-circuit load on the load side of the switching power supply, especially in the driving circuit. When the startup signal is activated, the power supply voltage is pulled to an extremely low level, and even the switching power supply will stop vibrating due to this. Except for the charging relay being released due to insufficient suction voltage, the CPU determines that it is re powered on and displays the startup character! In fact, it is a process that is equivalent to turning on power again. Check the driving circuit. There are two power amplification tubes connected in a push-pull form behind the driving IC, which are used to amplify the pulse output by the driving IC and then drive the inverter module. The upper and lower arm driving power amplifier circuits of the U-phase both have a transistor damaged due to module damage and impact. When there is no triggering pulse, a single transistor breakdown short circuit does not form a short circuit to the driving power supply. The arrival of pulse signals caused an instantaneous short circuit to the driving power supply due to the conduction of good and bad transistors, resulting in a momentary shutdown of the switch and power outage. The starting signal was also interrupted due to power outage, and the short circuit state of the power amplifier to the transistor after driving the IC was also relieved due to power outage. Then the CPU determines that the frequency converter is being powered on again, so the operation panel displays the power on character. After dismantling the module, I was eager to power on and check the quality of the drive circuit, but I did not measure and judge the circuit in detail, so I wasted some time on this startup character. After dismantling the module, a thorough inspection should be conducted before powering on the driver board.
B. Fault characters that are also not present in the alarm code table This Anbang Xin frequency converter is also quite unique. After installing the new module, the 530V DC voltage of the DC circuit is not connected first, and a 24V DC power supply is added for testing. After startup, the Br Tr FeiLuRe character jumps again, but can be reset by pressing the reset button; If the 24V power supply is disconnected, this fault still occurs but cannot be reset. Check the fault code for this item, consult the manufacturer, and answer that it is a brake circuit fault, feeling a bit out of place. The external circuit of the terminal is not connected to the brake resistor, and there is no short circuit in the brake components inside the measured terminal. It is possible that after the power supply of the inverter is shut down, the control power supply (the power supply of the switching power supply) may be connected in series, causing the detection circuit to return a fault signal.
After shutting down the 24V power supply, there is still a voltage of about 6V at the inverter power supply terminal. This voltage enters the fault detection circuit through certain links, reaching the alarm level of Br Tr FeiLuRe or acting as a Br Tr FeiLuRe fault signal. Isn’t it? The negative pressure and pulse positive voltage of the six drive circuits are normal, especially with the guarantee of cut-off negative pressure. After the 530V DC voltage is sent to the DC circuit, there is at most an output phase failure, but it is impossible to damage the module. For safety reasons, replace the original 75A quick release fuse with a 2A one and try it directly on. Everything is normal. It can be seen that if the 75A quick release fuse is broken or the IGBT tube of the brake control inside the module is short circuited (which may cause voltage drop in the DC circuit), an alarm signal of Br Tr FeiLuRe may be generated. The source of this signal may be reported to the CPU by the fault detection circuit after detecting an abnormal low voltage in the DC circuit. But is it not appropriate to define it as a fault in the brake circuit? However, the occurrence of “faults” makes it impossible to implement the method of detecting inverter circuits in low-voltage power supply. It also caused some setbacks in the maintenance cost.
