What are the self-protection functions of Fuji Electric - inverter?

After years of development, the inverter (VFD) has begun to transition from a purely industrial product to a civilian field. The use of VFD can be said to have gone to the countryside, (constant pressure water supply) penetrated into thousands of households (inverter air conditioning). However, after using it for a period of time, you will find that the failure rate of VFD is much higher than that of general household appliances, and also higher than that of general industrial appliances (PLC, motor), and many faults are accompanied by smoke and explosion. Sometimes the high-voltage system trips. And once it smokes and explodes, the VFD is basically scrapped, and even if it is repaired, the cost is very high.
Now it has entered the market economy, and most of the projects are implemented by engineering companies through design institutes and bidding. As a rule, the indicators of the VFD are explicitly required to have various protection functions. So when the VFD is corrupted, the user can't understand it, and some people even question whether the VFD is fake or not. In fact, VFD has developed to this day, and its protection has been very mature and reliable. But these protections are based on the premise that the inverter itself is not faulty. Once the inverter is short-circuited, or the component fails, it can be a big problem. No one seems to be thinking about who can protect the inverter itself. In China, it is common practice to add an air switch to the entrance of the VFD. What is the effect? Practice has proved that the actual effect is average. When a short circuit occurs inside the VFD, the VFD will burn out (it is more sensitive to open in the air). Even the low-voltage main incoming switch that was not turned on jumped out together with the higher-level high-voltage switch.

It is the integration of VFD's power (high voltage and high current) and electronic (low voltage and low current) devices. There are a lot of power devices in it (rectifier bridges; Electrolytic capacitor; IGBTs) also have a large number of electronic devices (control panel displays). In the event of an internal short circuit (the reasons are detailed below), if there is no fast fusing protection at the inlet, it will inevitably lead to an open circuit or a short circuit in the rectifier bridge. Fortunately, there is no current input to the inverter, and the fault will not continue to expand; Once the rectifier bridge is short-circuited and breaks down, it will inevitably cause a larger short-circuit current. VFDs can burn or even trip, causing immeasurable damage.

Why such a big accident will be caused has to start with the variable frequency drive. When the general inverter is working, the alternating current is converted into direct current through the rectifier bridge, stabilized by DC capacitor, and then converted into alternating current with variable frequency and voltage through the IGBT inverter chopping to drive the motor to rotate.

If the VFD temperature; The VFD is too dusty (the control panel is not dustproof); Due to the aging of the device, the two inverters of the inverter bridge arm are abnormal during the continuous alternating operation, resulting in one device being turned on and the other device not having time to shut down during the alternating conduction process. The "pass-through" of the power devices up and down the same bridge arm causes the positive and negative poles of the DC voltage to be short-circuited. Sometimes, even the direct short circuit of the DC bus inside the inverter is caused by too much dust and some water vapor in the inverter. This short circuit is very scary. Normally, once the VFD detects a fault, it will immediately cut off the output. But if there is no contactor on the inlet side, it is never possible to cut off the input. Everyone knows this, but I probably didn't think it would be big. Once a short circuit is formed, the short circuit current is infinity if the cross-sectional area of the incoming cable is not considered. Under normal circumstances, the inlet side of the VFD is opened for protection. Air switches are usually equipped with a thermomagnetic tripping device. For the protection VFD, the thermal trip is basically useless (the VFD itself is overcurrent and overload protected, and the sensitivity is much higher than that of the open circuit. For example, if the rated current of the motor is 12.2A, the VFD can be set to 12.2, but no brand can be set to 12.2). Magnetic tripping is used for short-circuit protection. The setting value of the normal magnetic tripper is 10 times the rated current of the magnetic tripper. The inherent mechanical time for disconnection is 500-1000ms (i.e. the time it takes for the actuator to disconnect the disconnect contact after a trip is detected). For example, for a 75 kW motor, in general, people will choose a 200 kW open motor. Once the inverter DC is short-circuited, the short-circuit current must be much greater than 2000. After 1 second, the open-circuit motor trips, 400V*2000A*1S=800KW. Many people may not have seen an 800KW motor, and every family has an electricity meter to pay the monthly electricity bill, and the electricity is 1kw at a time. Generally, a family of two people will save a little. It is conceivable that such a large amount of energy is consumed in a year (cooking and cooling, etc.). Instantly flowing into the VFD, it's strange if it doesn't explode. At the same time, it may also cause the power grid to undervoltage and cause the upper level to switch action.

Active measures: to ensure that the electrical room is constant temperature, constant humidity and dust-free (simply put, it is like a home environment, to be precise, it should reach the office environment of public servants at or above the department level in developed cities in China); Clean the inside of the VFD regularly from dust, disassemble, clean and reassemble if necessary. Tighten the power control cable at regular intervals (be sure to designate the comrade in charge of the slow). Unless there are screws with anti-fade pads, they will not tighten over time. There are many examples of VFD damage caused by false connection of power cables. DC electrolytic capacitors are replaced approximately every eight years. When installing an incoming contactor (controlled by an inverter relay), the current input should be cut off immediately after a fault occurs to avoid the expansion of the accident.

Passive measures: Install a fast fuse of appropriate capacity (safety with diode marking) at the incoming line. In general, VFD manufacturers will give a specific model of fast-acting VFD configuration.

What are the self-protection functions of Fuji Electric - inverter?

What are the self-protection functions of Fuji Electric - inverter?

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