Admittedly, the protection circuits of frequency converters are already quite sophisticated. For the protection of the expensive inverter modules, all frequency converter manufacturers have put considerable 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 response speed!
From voltage detection to current detection, from module temperature detection to phase loss output detection, no other electrical appliance's protection circuit is as focused and dedicated as that of a frequency converter. And frequency converter salespeople, when discussing the performance of frequency converters, invariably mention their protection functions, often unwittingly promising users: "With a frequency converter, its comprehensive protection features will prevent your motor from burning out." 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 supply, motors are actually more prone to burning out when using a frequency converter. And this increased risk of motor burnout also makes the frequency converter's inverter module more likely to fail. 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.
Damage caused by abnormal load
A motor can run at mains frequency, although the operating current is slightly higher than the rated current, and there is a certain 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, it experiences frequent overloads, eventually stopping it from running. This isn't a major issue, though.
A motor has been running normally at the mains frequency for many years—note the word "many years." The user wants to save on electricity costs or needs to upgrade to a frequency converter due to process modifications. However, after connecting the frequency converter, it frequently trips the OC (overhead ignition) fault. This is normal; the protection shuts down, and the module is not damaged.
The scary thing was that the inverter didn't immediately trip to an overcurrent fault; instead, it ran for no apparent reason—only two or three days before the module exploded and the motor burned out. The user blamed the salesperson: "The inverter you installed is of poor quality; it burned my motor, you have to compensate me for it!"
Before this, the motor seemed to be working perfectly, running smoothly. Measuring the operating current showed it only reached half of its rated current due to the light load; the three-phase power supply was measured at 380V, perfectly balanced and stable. It really seems like the inverter failed, causing damage to the motor in the process.
If I were there, I would say this to my master: Don't blame the inverter; your motor was already "terminally ill," and its sudden malfunction damaged the inverter in the process!
After years of operation, the insulation of a motor windings has significantly decreased due to factors such as temperature rise and moisture absorption, sometimes 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 lower induced voltage and smaller surge components in the circuit. 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 as the insulation ages further, even under power frequency conditions, the motor will eventually burn out in the near future due to voltage breakdown between phases or windings caused by insulation aging. But the problem is, it hasn't burned out yet.
After the inverter is connected, the power supply conditions of the motor become "bad": the PWM waveform output by the inverter is actually a carrier voltage of several kHz or even tens of kHz, and various harmonic voltages will be generated in the power supply circuit of the motor winding.
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 windings is higher than that under mains frequency power supply (Public Account: Pump Manager). Insulation defects that are not exposed under mains frequency power supply cannot withstand the impact of induced voltage under high-frequency carrier waves, thus causing voltage breakdown between turns or phases of the windings. Short circuits between phases or between turns in the motor windings cause sudden short circuits in the motor windings. During operation—the module explodes, and the motor burns out.
During the initial startup phase of the inverter, since the output frequency and voltage are both within a low amplitude range, a fault in the load motor may cause a large output current, but this current is usually within the rated value. The current detection circuit will activate in time, and the inverter will implement a protective shutdown action, so there is no risk of the module being destroyed.
However, if the three-phase output voltage and frequency reach high amplitudes when operating at full speed (or near full speed), a voltage breakdown in the motor windings will instantly generate a huge surge current. The inverter module will be unable to withstand the surge and will explode before the current detection circuit can activate.
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's safety provided the inverter's protection circuit is functioning properly, greatly reducing the probability of module damage. These will not be discussed further here.
Damage to the module caused by a faulty circuit 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. The +15V voltage provides the excitation voltage for the IGBT transistor to turn it on. The -5V provides the cutoff voltage for the IGBT transistor to reliably and quickly turn it off.
When the +15V voltage is insufficient or lost, the corresponding IGBT cannot be turned on. If the module fault detection circuit of the drive circuit can also detect the IGBT, then as soon as the inverter puts on the running signal, the module fault detection circuit can report an OC signal, and the inverter will implement a protection shutdown action, which is almost harmless 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 across the IGBTs in the lower three arms of the inverter bridge is detected and processed by the module fault detection circuit. While some inverters have voltage drop detection for the IGBTs in the upper three arms, most omit this detection circuit. 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 frequency converter may experience phase imbalance operation. The IGBT tube that loses the excitation pulse is the one without a tube voltage drop detection circuit. Only the presence of a cutoff negative voltage can reliably cut it off. The bridge arm of that phase only has a half-wave output, causing the frequency converter to operate in phase imbalance. The consequence is that a DC component is generated in the motor winding, which also forms a large surge current (Public Account: Pump Manager), thus 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 always open, even if the module fault circuit cannot function, the current detection circuit such as the current transformer can still function and provide protection. However, the problem is that this transmission path may be intermittent due to faults such as poor contact, or even random interruptions. The current detection circuit may not be able to react in time, causing the inverter to output "intermittent phase bias", resulting in a large inrush current and damaging the module.
In this output state, the motor will "jump" and make a "clunking" sound, and the heat generation and losses will increase significantly, making it 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 operating for many years, the capacity of the DC circuit's energy storage capacitor cannot be ignored after a module failure. Complete capacitor loss is rare, but if it does occur, it will undoubtedly damage the inverter module during load startup!
Inferior quality, shoddy workmanship
Some domestically produced frequency converters are of poor quality and use substandard materials, making their modules extremely prone to damage.
Indeed, competition in the frequency converter market has intensified in recent years, and profit margins for frequency converters have become increasingly narrow. However, the competitiveness of products can be improved through technological advancements and increased productivity.
Using old materials to pass off as new, inferior materials to pass off as superior, and cutting corners by reducing module capacity to increase market share is unwise and a short-sighted approach.
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, a 30kW inverter with a rated current of 60A should use a 150A to 200A module. A 100A module is too small. However, some manufacturers dare to use 100A modules! Even worse, some use old or substandard modules. Such inverters are not only prone to module damage during operation, but the modules often explode during startup! The workers installing these inverters on-site are terrified, using a wooden stick to press the start button on the control panel from a distance.
If a module with a small capacity can barely operate, 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 came out due to 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.
For domestically produced frequency converters, sometimes a single bad apple spoils the whole bunch. Many frequency converters are actually quite good, comparable to foreign products, and offer excellent quality at a lower price.
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