While inverter damage caused by abnormal loads is undeniable, inverter protection circuits are already quite sophisticated. Inverter manufacturers have invested heavily in protecting their expensive inverter modules, from output current detection to IGBT voltage drop detection in the drive circuit, striving for the fastest possible overload protection. From voltage detection to current detection, from module temperature detection to phase loss detection, no other electrical appliance's protection circuit is as focused and dedicated. Inverter sales staff, when discussing inverter performance, invariably mention its protection features, often unwittingly promising users: "With an inverter, its comprehensive protection functions will prevent your motor from burning out easily." This salesperson is unaware that this promise will bring them significant trouble!
Will motors really not burn out when using a frequency converter? My answer is: compared to mains power, motors are actually more prone to burning out when using a frequency converter. And this increased motor burnout also easily leads to the failure of the frequency converter's inverter module. The frequency converter's sensitive overcurrent protection circuit is completely ineffective in this situation. This is a major external cause of frequency converter module damage. Let me explain why.
A motor can run at mains frequency, although the operating current is slightly higher than the rated current, and there will be some temperature rise after prolonged operation. This is a faulty motor; it could indeed run before burning out. However, after connecting it to a frequency converter, frequent overloads occurred, eventually causing it to stop running. This is not a major issue yet.
A motor, which has been running normally at its base frequency for many years (note the word "many years"), is being upgraded to a frequency converter to save on electricity costs or for process improvements. However, after connecting the frequency converter, it frequently trips the overcurrent (OC) fault, which is good because the protection system shuts down the motor and the module itself isn't damaged. The scary part is that the frequency converter doesn't immediately trip the OC fault; instead, it runs inexplicably—after only two or three days, the module explodes, and the motor burns out. The user then blames the salesperson: "The frequency converter you installed is of poor quality; it burned my motor, and you have to compensate me for it!"
Before this, the motor seemed to be working perfectly fine, and the operating current was measured at only half of the rated current due to the light load. The three-phase power supply was measured at 380V, which was balanced and stable. It really seems like the inverter failed, causing damage to the motor as well.
The motor, after years of operation, has experienced a significant reduction in winding insulation due to factors such as temperature rise and moisture absorption, even exhibiting obvious insulation defects and teetering on the brink of voltage breakdown. Under mains frequency power supply, the motor windings are input with a three-phase 50Hz sinusoidal voltage, resulting in low induced voltage and minimal surge components in the line. The reduced insulation may only cause a negligible leakage current, but voltage breakdown between turns and phases has not yet occurred, and the motor continues to operate normally. It should be said that with further insulation aging, even under mains frequency power supply, this motor will eventually burn out in the near future due to phase-to-phase or winding voltage breakdown caused by insulation aging. However, the problem is that it hasn't burned out yet.
After connecting the frequency converter, the power supply conditions for the motor become "deteriorated": the PWM waveform output by the frequency converter is actually a carrier voltage of several kHz or even tens of kHz, which will generate various harmonic voltage components in the motor winding power supply circuit. As can be seen from the characteristics of inductance, the faster the rate of change of the current flowing through the inductor, the higher the induced voltage of the inductor. The induced voltage of the motor winding is higher than that under mains frequency power supply. Insulation defects that are not exposed under mains frequency power supply cannot withstand the impact of the induced voltage under high-frequency carrier, thus causing voltage breakdown between turns or phases of the winding. Short circuits between phases or turns in the motor winding cause sudden short circuits in the motor winding, resulting in the module exploding and the motor burning out during operation.
During the initial startup phase of a frequency converter, because the output frequency and voltage are both within a relatively low amplitude, a fault in the load motor will cause a large output current, but this current is usually within the rated value. The current detection circuit will activate in time, and the frequency converter will implement protective shutdown, preventing the module from being destroyed. However, if the inverter is running at full speed (or near full speed), the three-phase output voltage and frequency will both reach a high amplitude. If there is a voltage breakdown in the motor windings, a huge surge current will be generated instantaneously. The inverter module will be unable to withstand this surge current and will be destroyed before the current detection circuit activates.
This shows that protection circuits are not omnipotent; every protection circuit has its weaknesses. Inverters are powerless against sudden voltage breakdowns in motor windings during full-speed operation, and therefore cannot provide effective protection. Furthermore, not only inverter protection circuits, but no motor protector can effectively protect against such sudden faults. When such a sudden fault occurs, it can only be concluded that the motor has indeed reached the end of its service life.
Such faults are a fatal blow to the inverter output module of the frequency converter, and there is no escaping it.
Other faults caused by power supply or load issues, such as overvoltage, undervoltage, heavy load, or even overcurrent caused by stalled rotor, can effectively protect the module under normal conditions of the inverter's protection circuit, greatly reducing the probability of module damage. These will not be discussed further here.
Module damage caused by faulty circuitry in the inverter itself
1. A faulty drive circuit can cause level one damage to the module.
As can be seen from the power supply method of the drive circuit, it is generally powered by two power supplies, positive and negative. +15V provides the excitation voltage for the IGBT transistors, enabling them to turn on. -5V provides the cutoff voltage for the IGBT transistors, ensuring reliable and rapid cutoff. When the +15V voltage is insufficient or lost, the corresponding IGBT transistor cannot turn on. If the module fault detection circuit of the drive circuit can also detect the IGBT transistors, then as soon as the inverter receives the running signal, the module fault detection circuit will report an OC signal, and the inverter will implement a protective shutdown action, posing almost no harm to the module.
However, if the -5V cutoff voltage is insufficient or lost (similar to a three-phase rectifier bridge, we can consider the inverter output circuit as an inverter bridge, with IGBTs forming three upper and three lower arms, such as the IGBTs in the U-phase upper and lower arms), when any phase's upper (lower) arm is energized and turns on, the corresponding lower (upper) arm IGBT will, due to the loss of cutoff voltage, experience charging of the gate-emitter junction capacitance by the collector-gate junction capacitance of the IGBT, leading to mis-conduction of the transistor. This results in both transistors short-circuiting the DC power supply! The consequence is: the entire module will explode!
The loss of cutoff negative voltage can be caused by several factors: a damaged driver IC; a damaged lower transistor in the power drive stage (usually composed of two complementary voltage follower power amplifiers); a poor connection in the trigger terminal leads; or a faulty negative power supply branch in the drive circuit or a failed power supply filter capacitor. If any of these issues occur, it will be a fatal blow to the module, and the damage will be irreversible.
2. A faulty pulse transmission path will also pose a threat to the module.
The six PWM inverter pulses output by the CPU typically pass through six inverting (or non-inverting) buffers before being sent to the input pins of the driver IC. From the CPU to the driver IC, and then to the trigger terminals of the inverter module, if any one of the six signals is interrupted—
(1) The inverter may report an OC (overhead) fault. The voltage drop of the IGBTs in the lower three arms of the inverter bridge during conduction is detected and processed by the module fault detection circuit. However, in a small number of inverters, the voltage drop detection circuit for the IGBTs in the upper three arms is present, while in most inverters, it is omitted. If the IGBT that loses its excitation pulse happens to have a voltage drop detection circuit, the detection circuit will report an OC fault after the excitation pulse is lost, and the inverter will shut down for protection.
(2) The inverter may experience phase misalignment. The IGBT tube that loses its excitation pulse is the one without a voltage drop detection circuit. Only the negative cutoff voltage exists to reliably cut it off. The bridge arm of that phase only has a half-wave output, causing the inverter to operate in phase misalignment. As a result, a DC component is generated in the motor windings, which also forms a large surge current, causing the module to be damaged by the impact! However, the probability of damage is lower than that of the first reason.
If this pulse transmission path is constantly interrupted, even if the module fault circuit cannot function, the current detection circuits such as the current transformer can still function and provide protection. However, the problem arises when this transmission path is intermittently open or closed due to faults such as poor contact, or even random interruptions. The current detection circuit will not react in time, causing the inverter to produce an "intermittent phase bias" output, resulting in a large inrush current that damages the module. In this output state, the motor will "jump" and make a "clunking" sound, with a significant increase in heat generation and losses, making it very easy to be damaged.
3. Failure or malfunction of the current detection circuit and module temperature detection circuit will prevent the module from effectively protecting against overcurrent and overheating, thus causing damage to the module.
4. When the capacity of the energy storage capacitor in the main DC circuit decreases or fails, the pulsating component of the DC circuit voltage increases. This is not obvious when the inverter is started under no-load or no-load conditions, but during the start-up under load, the circuit voltage surges and the inverter module explodes and is damaged. The protection circuit is also helpless in the face of this.
For frequency inverters that have been in operation for many years, the capacity of the DC circuit's energy storage capacitor should not be overlooked after a module failure. Complete capacitor loss is rare, but if it does occur, it will undoubtedly damage the inverter module during load startup!
A small number of domestically produced frequency inverters are of poor quality and use substandard materials, making their modules extremely prone to damage.
This reflects a weakness in the national character, a kind of national itch. It's true that competition in the frequency converter market has intensified in recent years, and profit margins are increasingly narrowing. However, competitiveness can be improved through technological advancements and increased productivity. Using methods like selling old products as new, substandard materials, and cutting corners by reducing module capacity to increase market share is unwise and purely a short-sighted, short-term strategy.
1. Poor quality and shoddy manufacturing increase the failure rate of the inverter's fault protection circuit. As the inverter module is not effectively protected by the protection circuit, the probability of module damage increases.
2. The capacity of the inverter module should generally be at least 2.5 times the rated current to ensure long-term safe operation. For example, for a 30kW frequency converter with a rated current of 60A, a 150A to 200A module should be selected. A 100A module would be too small.
However, some manufacturers dare to use 100A modules for installation! Even worse, some use old or substandard modules. These types of frequency converters not only easily damage the modules during operation, but the modules also frequently explode during startup! The staff installing these frequency converters on-site are terrified and use a wooden stick to press the start button on the control panel from a distance.
If a module with a small capacity is forced to operate under strain, and the module is overloaded, the protection circuit becomes practically useless (protection is based on the inverter's rated power capacity instead of the module's actual capacity). It would be abnormal if the module didn't frequently explode.
These machines seemed to be very popular when they first hit the market because of their low price, but it didn't last long before the manufacturers had no choice but to go bankrupt.
This third reason for module failure shouldn't even be a cause. Hopefully, in the near future, only the first two reasons will remain as causes for module failure.
