Source and essence of inverter OC failure - Database & Sql Blog Articles

Filter 18.432M

The frequency at which OC faults occur in all faults in the frequency converter is perhaps the highest and most frequent. Alarm during start-up, alarm during stop, alarm during operation, even warning when power-on, or even indirectly with other fault codes or phenomena: The inverter has OC type fault!
In the description of the inverter, the description of the OC fault generally has the following explanations: short-circuit on the load side, OC fault when the running current is more than twice; the output module of the inverter is short-circuited; the inverter overcurrent; the load is too large; The deceleration time is too short. In some models, such faults are not marked with OC, and are described by load side short circuit, inverter over load, severe ground fault, etc. This is of course an alias for OC fault.
Some inverters do not inform you of the type of fault. When an OC fault occurs and the opportunity is opened to cause greater danger, it will cause a surface phenomenon similar to the program crash, such as the Invisible P9/G9 series machine. When the module fails, the H00 character appears on the operation panel, and all key operations are rejected. Unknown insiders will think: the program crashes, there is a problem with the CPU board.
Other inverters are more interesting. When other faults cause damage to the running module, or if the module is potentially dangerous when operating in this fault state, that is, in the shutdown state, the OC fault will also be alerted. . For example, the Alpha P2000 type machine, when the main circuit DC voltage detection circuit is damaged, the CPU detects a dangerous power supply voltage, simply can't report the voltage fault, and directly report the OC fault, so that the user does not care about the power supply voltage is too high!
A "Tai'an" N2 power model, which jumps UL or UU when powering up, refuses to operate. Check the signal of the three-phase current transformer. The three signals have serious deviations. At least two of them have been broken. However, check the fault code table of the manual, there is no such two fault codes, guessing that this code is a password that the factory maintenance personnel can crack, which is not enough for outsiders. Is it also an indirect indication of OC failure? Just fix it to get an answer.
I guess the original intention of the inverter circuit designer is this: When the power-on detection module has been broken, or there are factors that endanger the security of the module during operation, even if the module is damaged, the OC failure will be reported in time. The cause is roughly a heavy overcurrent caused by a load side short circuit or an excessive load, or an over current or even a short circuit caused by poor driving or damage to the module itself.
In summary, the essence of the OC failure warning is: the rapid shutdown protection module or the operation has a short circuit danger, the module has been broken! From the perspective of protection, the value of the module in the inverter is like the value of the picture tube in the color TV. It is self-evident; in terms of the danger of forced operation after the OC failure, the person with light failure may be damaged. Module, module is broken and then forced to run, it may cause the device to explode and cause serious personal injury! Therefore, the designer must not consider the module failure first!
Leaving the OC fault of the detection circuit damage false alarm, there is no "fault" in the inverter, only the power supply voltage has a slight unpredictable deviation, or some kind of interference, and the OC fault is also frequently reported. Overhaul will make people scratch their heads.
Most inverters skip the OC signal when the start signal is input. In this case, the module is not damaged, but the drive circuit is abnormal or receives a false alarm signal (but it is not excluded that some models are damaged) Some of them are OC signals when power is on, it may be that the module has been broken, or there are other factors that may endanger the safety of the module. When such factors exist, some of the measures taken by the inverter are: the operation panel can do Other parameters are set to work, but they cannot enter the operation; others simply reject all operations and complete the strike. In the case of running OC signals: there are three possibilities:
1. It is an abnormality in the load: OC fault may be reported during starting, running, and stopping, generally the load is too heavy or the inverter capacity is insufficient;
2. The user's operating parameters of the inverter are improper. For example, if the constant torque load error is set to the secondary deceleration torque load, the acceleration and deceleration time are set improperly, especially for the setting of the large inertia load plus and deceleration time. Or the improper handling of the shutdown method. What's more, it is the misconfiguration of the protection parameters, such as the reduction of the inverter or motor rated current parameters, so that the equipment has frequent over-current alarm shutdown below the rated current, can not be put into operation!
3, the fault of the inverter itself: often caused by the failure of the power supply capacitor of the drive circuit, so that the CPU receives the OC signal with excessive voltage drop of the IGBT tube.
But the three reasons can be summarized as one point: there is a serious overcurrent in the running or stop state or there is a possibility of serious overcurrent, so only the OC signal is reported!
In general, the source of OC failure has the following three aspects:
1. When the running current of the inverter module is too large and reaches 3 times of the rated current, when the tube voltage drop of the IGBT tube rises above 7V, the driver IC returns the overload OC signal to notify the CPU to implement fast shutdown protection;
2. From the three current transformers at the output end of the inverter (two of the low-power models are used), after the sudden rise of the abnormal current is collected, an OC signal is output from the voltage comparator (or the internal circuit of the CPU) to notify CPU, implement fast shutdown protection.
3. The IGBT tube has been or is undergoing short-circuit and open-circuit damage. The "extremely abnormal" tube pressure drop is detected by the driver IC, especially when the tube is open, the tube pressure drop is greater than the protection action threshold greater than 7V.
This is the three sources of inverter OC failure when the fault detection circuit and the drive circuit are normal. The cause is abnormal load over-load such as motor stall on the load side, or the quality defect in the inverter module and the aging of the device.
The causes of OC faults caused by fault detection circuits and drive circuits are also the following three aspects:
1. Three-phase output current detection circuit. When the internal circuit of the current transformer is damaged, so that the fault signal output pin outputs an abnormally high voltage signal, the CPU thinks that the running current is so large that it has reached the short circuit level, and immediately reports the OC signal;
2, the driver IC is damaged, such as J316's 6-pin field effect tube short circuit, the 6-pin voltage is pulled to the low level of the fault detection state, if the CPU does not report the OC signal, it is not the CPU;
3. Although the driver IC is not damaged, the abnormality of the driver circuit causes the module to work abnormally. The driver circuit reports the OC signal at this time, which is not only false alarm, but also very timely and commendable. The power supply of the driver IC is usually a positive and negative dual power supply, and its positive voltage provides the excitation current of the IGBT conduction. Its negative voltage assists the cut-off of the IGTB tube, forcing the charge of the IGBT junction capacitor to be pulled out, making it more reliable and fast. When the capacity of positive voltage filter capacitors (often using 47uF or 100uF capacitors and 330uF for high-power models) is greatly reduced, IGBT tubes are insufficiently energized, even if they run below the rated current, they show a large tube voltage drop. After the detection circuit is processed, the CPU reports an OC fault; the fault manifests at this time is: when the inverter is unloaded or has a very light load, the operation is normal, and the OC fault is reported when the load is slightly loaded.
If the failure of the positive voltage filter capacitor will lead to insufficient excitation of the IGBT tube, and the driver IC will report the OC fault, and the IGBT tube does not have a great danger, then the failure of the negative voltage filter capacitor is much more dangerous. When the arm tube is opened on one phase, the positive voltage of the main circuit is jumped to the C pole of the lower tube. If the negative pressure clamp is insufficient, the instantaneous suction current of the junction capacitance of the tube may cause mis-conduction of the lower arm tube. The consequence is that two common pipes cause a short circuit to the main DC power supply! In this case the module is extremely easy to burst! Whether the positive voltage or negative voltage filter capacitor fails, the inverter may report an OC fault.
The above is the "phenomenon" of OC faults in fault detection and drive circuits, as well as the "hidden phenomenon" of OC faults and the plausible OC phenomenon, which are often not noticed. The following three cases:
1. Repair an Alpha inverter. The 8th pin of the CNN1 terminal is the DC voltage detection signal output pin of the main circuit. It should be about 3.5V when it is normal. When the circuit is damaged, the signal output of 5V or more is equivalent to three-phase. When the AC input voltage reaches 500V or more, the CPU thinks that the operation of the module is compromised. Therefore, the voltage fault is not reported, but the OC is alerted when the power is turned on to draw the user's attention.
2. In the process of repairing the Alpha low-power inverter, it was found that the inverter has a common problem - easy to jump OC fault. Its performance is: more jumps during the start and stop operations, but sometimes also jumps during operation; sometimes it is inexplicably good, can run for a period of time. When it is thought that there is no problem, it starts to frequently jump OC fault; when measuring the U, V, W output voltage with the test pen at no load, it is easy to jump the fault, but after starting the operation after entering the motor, it does not jump, and then one more A while, access to the motor or jump OC failure.
No matter how to find the cause of the fault and carry out the fault detection circuit one by one, it is impossible to find the cause of the fault. There may be some interference in the circuit that is unclear, but the source and cause of the interference are difficult to find. Could it be that the start/stop moment - during the "loading and unloading" of the inverter drive module, caused the fluctuation of the CPU power supply and jumped? Measuring CPU power is 4.98V, very stable, meet the requirements. Later, the 4.98V of the motherboard power supply was occasionally adjusted to 5.02V, and then the start/stop test was performed, and the fault was eliminated! The cause of the fault is actually low for 5V! I see the hidden depth of this fault.
3. When repairing a P9 Inverter machine, the inspection found that: power on, the operation panel displays H.00, all operations are invalid, and the CPU rejects all operations. The overcurrent signals of pins 4 and 6 of the fault signal collection processing circuit U7-HC4044 are all negative voltages, while the static state should be 6V positive voltage. The forward current detection circuit looks forward, and the current signal input amplification U12D is 8, V is 0V, normal; U13D 14 is negative 8V, and the error overcurrent signal is output. Weld the R151 and disconnect the overcurrent fault signal. All the parameters of the operation panel are normal. The cause of the fault is that an overcurrent signal is detected after power-on, so all operations are rejected. The inverter is allowed to rest for a while, and the abnormality can be controlled.
From the above point of view, many circuits and many reasons can cause the inverter to report OC failure, but which circuit has more priority? As far as the fault detection circuit is concerned, is there a warning level for the fault alarm? From a protection point of view, the number of factors will report an OC failure as long as it compromises the security of the module, as stated above. However, in the implementation, some early warning levels can also be seen.
1. The OC signal returned by the driver IC is the first one. For example, the signal reported from the OC signal of the 6th pin of J316, the 11 pin of PC929, and the IPM module is detected. Because the module status is directly detected, as long as the CPU receives the signal, the output of the three-phase trigger pulse is immediately blocked, and the OC signal is reported;
2. The OC signal reported by the three-phase output current transformer. The reporting of this signal has a step process: when there is an overcurrent phenomenon, after a slight overcurrent, after long delay processing and frequency reduction, the current is reported but the OC is not reported. For moderate overcurrent, after a short time delay and other processing are invalid, the current is reported and OC is still not reported. Only when there is a current detection signal with a sharp change and a large amplitude, the OC signal is directly reported without delay;
3, some models of over- and under-voltage detection processing is also similar to the level of alarm detection similar to the current detection: such as the first reported voltage, and accompanied by delay processing. When the extremely high voltage value is detected, the OC is directly reported;
4, Yingwei Teng P9/G9 machine, indirect display OC process, there are also ladder alarm level: power-on detection module or current signal is abnormal, reject all operations; only detect temperature abnormalities, can set parameter values, but can not start and stop operation .
From this point of view, depending on the degree of hazard, the time to report various types of failures also varies. The CPU detects the OC signal by direct shutdown protection or reject operation. The faster the better, no time delay processing; for other fault signals with less harmful degree, there are detection, delay, pre-alarm, alarm shutdown protection and coordination. The frequency is adjusted to make the overcurrent phenomenon disappear and so on. This is the difference in processing between the OC signal and other fault signals.
Therefore, for the protection of the inverter, the warning level of the OC fault signal is red. The highest level of fault protection. Has the highest priority for its unconditional execution.