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What is the reason for the damage of inverter output module?

Jan 16, 2021

What is the reason for the damage of inverter output module?

1. Damage caused by abnormal load


    It is true that the protection circuit of the inverter is quite complete. For the protection of expensive inverter modules, various inverter manufacturers have done enough work in their protection circuits, from output current detection to IGBT tube voltage drop detection of the drive circuit, and strive to implement rapid Overload protection! From voltage detection to current detection, from module temperature detection to lack of phase output detection, etc., there is no protection circuit for any kind of electrical appliances. Like a frequency converter, it must be focused and invested. The sales staff of the frequency converter, when mentioning the performance of the frequency converter, must also mention the protection function of the frequency converter, often unconsciously promise to the user: with the up converter, its comprehensive protection function, your motor is not easy Burned. The salesperson did not know that this promise would bring him great passivity!


    With an up-converter, will the motor really not burn? My answer is: Compared with the industrial frequency power supply, the motor is easier to burn with the up-converter, and the easy burn of the motor makes the inverter module of the inverter easy to be "reimbursed". The sensitive over-current protection circuit of the frequency converter is just at a loss here and can't do anything. This is a major external cause of damage to the inverter module. Listen to me tell the whole story.


    A motor can run under the power frequency state, although the operating current is slightly larger than the rated current, there will be a certain temperature rise in long-term operation. This is a diseased motor that can indeed run before it burns. However, after the inverter is connected, it will be overloaded frequently, and even unable to run. It doesn't matter.


    A motor can run under the power frequency state. The user has used it normally for many years. Please pay attention to the word "years". The user thinks of saving electricity bills, or because of technological transformation, the need for frequency conversion transformation. But after the inverter is connected, the frequency jump OC fault, this is good, the protection stops, and the module is not broken. The frightening thing is that the inverter does not immediately jump to the OC fault, but is in operation for no reason-after only three or two days of operation, the module blows up and the motor burns out. The user relied on the salesperson: The inverter you installed is of poor quality and burned my motor. You have to pay for my motor!


    Before that, the motor seemed to be really no problem. It ran well. The running current was measured. Because the load was lighter, it reached half of the rated current; the three-phase power supply, 380V, was very balanced and stable. It really looks like the inverter is damaged, and the motor is damaged.


    Frequency converter


    If I were there, I would be fair: I don't blame the inverter, but your motor is already "deadly ill", it suddenly broke out, and the inverter was damaged!


    For motors that have been in operation for many years, the insulation of the windings has been greatly reduced due to the temperature rise and dampness of the motor, and even has obvious insulation defects, which is at the critical point of voltage breakdown. In the case of power frequency power supply, the input of the motor windings is a three-phase 50Hz sine wave voltage, the induced voltage generated by the windings is also low, the surge component in the line is small, and the degree of insulation of the motor is reduced. The inconspicuous "leakage current", but the voltage breakdown between the turns and phases of the winding has not yet occurred, and the motor is still "normally running". It should be said that with the further deepening of insulation aging, even in the case of power frequency power supply, I believe that in the near future, the motor will eventually burn out due to the voltage breakdown between phases or windings due to insulation aging. But the problem is that it is not burned now.


    After the inverter is connected, the power supply conditions of the motor have become "bad": the PWM waveform output by the inverter is actually a carrier voltage of several kHz or even more than ten kHz, and various components will be generated in the power supply loop of the motor winding. Harmonic voltage. It can be known from the characteristics of the inductor that the faster the change speed of the current flowing through the inductor, the higher the induced voltage of the inductor. The induced voltage of the motor windings is higher than when the power frequency is supplied. Insulation defects that cannot be exposed during power frequency power supply are not resistant to the impact of the induced voltage under the high-frequency carrier, so the voltage breakdown between winding turns or phases occurs. The phase-to-phase and inter-turn short circuits of the motor windings caused a sudden short circuit of the motor windings. During operation-the module was blown up and the motor burned.


    In the initial stage of the inverter starting, because the output frequency and voltage are both within a relatively low amplitude, when the load motor has a fault, although a large output current is caused, this current is often within the rated value, and the current detection circuit operates in time. The inverter implements a protective shutdown action, and the module is not likely to blow up. But if running at full speed (or close to full speed), the three-phase output voltage and frequency both reach a relatively high amplitude. At this time, if there is a voltage breakdown in the motor winding, a huge surge current will be formed instantly. Before the current detection circuit operates, the inverter module cannot withstand it and bursts and is damaged.


    It can be seen from this that the protection circuit is not a panacea, and any protection circuit has its "soft underbelly". The frequency converter is powerless to prevent the sudden voltage breakdown of the motor windings during full-speed operation and cannot effectively protect it. Not only the inverter protection circuit, any motor protector, can not implement effective protection for such sudden failures. When such a sudden failure occurs, it can only be declared: the motor is indeed "dying."


    This type of fault is a fatal blow to the inverter output module of the inverter, and there is no escape.


    Other causes caused by power supply or load, such as over-voltage, under-voltage, heavy load, and even over-current caused by stalling, can effectively protect the safety of the module under the premise that the protection circuit of the inverter is normal. The probability of damage will be greatly reduced. Not much discussion here.


    2. Module damage caused by bad circuit of the inverter itself


    1. Poor drive circuit will cause a primary hazard to the module. It can be seen from the power supply mode of the drive circuit, which is generally powered by positive and negative power supplies. The +15V voltage provides the excitation voltage of the IGBT tube to turn it on. -5V provides the cut-off voltage of the IGBT tube to make it cut off reliably and quickly. When the +15V voltage is insufficient or missing, the corresponding IGBT tube cannot be turned on. If the module fault detection circuit of the drive circuit can also detect the IGBT tube, the OC signal can be reported by the module fault detection circuit as soon as the inverter is put into operation. The inverter implements protective shutdown action, which is almost harmless to the module.


    And in case the -5V cut-off negative voltage is insufficient or lost (like a three-phase rectifier bridge, we can first regard the inverter output circuit as an inverter bridge, and then three upper bridge arms and three lower bridge arms are formed by IGBT tubes. Bridge arms, such as U-phase upper bridge arm and U-phase lower bridge arm IGBT tube.) When the upper (lower) bridge arm of any phase is activated and turned on, the corresponding lower (upper) bridge arm IGBT tube is The loss of cut-off negative pressure forms the charging of the gate-emitter capacitance by the collector-gate junction capacitance of the IGBT tube, which leads to the misconduction of the tube, and the two tubes share a short circuit to the DC power supply! The consequence is: the modules are all blown up!


    The loss of cut-off negative pressure is caused by damage to the driver IC; it may also be caused by the damage of the down tube of the power driver stage (usually composed of two-stage complementary voltage follower power amplifier) after the driver IC; the trigger terminal lead connection Bad; then the negative power supply branch of the drive circuit is bad or the power filter capacitor fails. Once one of the above phenomena occurs, it will be a fatal blow to the module! It is irreparable.


    2. Poor pulse transmission path will also pose a threat to the module


    The 6-channel PWM inverter pulse output by the CPU is often sent to the input pin of the drive IC through a six-phase inverting (in-phase) buffer, from the CPU to the drive IC, and then to the trigger terminal of the inverter module. There is a break-


    (1) The inverter may report an OC fault. In the lower three-arm IGBT tube of the inverter bridge, the tube voltage drop during turn-on is detected and processed by the module fault detection circuit, while the upper three-arm IGBT tube has tube voltage drop detection in a small part of the inverter. In most inverters, the tube pressure drop detection circuit is omitted. When the IGBT tube that loses the excitation pulse happens to have a tube voltage drop detection circuit, after the excitation pulse is lost, the detection circuit will report an OC fault and the inverter will stop protection;


(2) The inverter may run in partial phase.

The IGBT tube that lost the excitation pulse is exactly the tube without the tube pressure drop detection circuit, and only the cut-off negative pressure exists, which can make it cut off reliably. This phase bridge arm has only half-wave output, which causes the inverter to run in a biased phase. The consequence is that DC components are generated in the motor windings, and a large surge current is formed, which causes the module to be damaged by impact! But the probability of damage is lower than the first reason.


    If the pulse transmission path of this road is always broken, even if the module fault circuit cannot function, the current detection circuits such as transformers can play a role, and it can also play a protective role, but it is afraid that this transmission path will be due to poor contact. The current detection circuit is inexplicable and too late to respond, causing the inverter to cause "intermittent biased" output, forming a large inrush current and damaging the module.


    In this output state, the motor will "jump" and make a "cackling" sound. The heat generation and loss will increase greatly, and it will be easily damaged.


    3. The current detection circuit and the module temperature detection circuit fail or malfunction, which can not effectively protect the module from overcurrent and overheat, thus causing damage to the module.


    4. After the capacity of the energy storage capacitor of the main DC circuit drops or loses capacity, the pulsating component of the DC circuit voltage increases. After the inverter is started, it is not obvious at no-load and no-load conditions, but during the starting process with load, the circuit The voltage surge is surging, the inverter module bursts and is damaged, and the protection circuit is at a loss as to what to do.


    For the inverter that has been running for many years, after the module is damaged, the inspection of the energy storage capacitor capacity of the DC loop cannot be ignored. The complete loss of capacitance of the capacitor is rarely encountered, but once it is encountered, it will cause damage to the inverter module during the start-up process with load, which is also certain!


    3. A small number of domestic frequency converters with inferior quality and cutting corners are easily damaged.


    This is a manifestation of the bad roots of the people, the itch of the nation. It is true that the competition in the inverter market has become increasingly fierce in recent years, and the profit margin of inverters has become narrower and narrower. However, the competitiveness of their products can be improved through technological advancement and productivity improvement. However, it is unwise to increase your market share by using the old to replace the new, the shoddy, and to reduce the module capacity and cut corners to increase your market share. It is purely a short-sighted act.


    1. Inferior quality and refining and indiscriminate manufacturing have increased the failure rate of the inverter fault protection circuit, and the inverter module cannot be effectively protected by the protection circuit, thereby increasing the probability of module damage.


    2. The capacity of the inverter module should generally be more than 2.5 times the rated current to ensure long-term safe operation. For example, a 30kW inverter with a rated current of 60A, the module should be 150A to 200A. Using 100A is too small. But some manufacturers dare to install with 100A modules! What's more, there are old modules and defective modules. This type of inverter is not only easy to damage the module during operation, but also the module often bursts during the startup process! The staff who installed this type of inverter on site were all scared and used a wooden stick to press the start button of the operation panel from a distance.


    A module with a small capacity must be able to run barely, the module is overloaded, and the protection circuit is formed as a dummy (protected according to the marked power capacity of the inverter instead of the actual capacity value of the module), and the module will not be frequently blown up , It's really abnormal.


    This kind of machine, because of its low price, seems to be very popular at the beginning, but it will not take long for the manufacturer to go bankrupt.


    The third reason for module damage should not have been a cause, but I hope that in the near future, only the first two reasons remain for the module damage.