The inverter has no short-circuit protection for the reason – Reynolds

Short-circuit maintenance is a function of equipment connected between the power supply and the load, which can effectively disconnect the short-circuit point from the power supply when the load side is short-circuited. If an equipment has a short-circuit maintenance function, it must have a certain short-circuit breaking ability; A device with short-circuit breaking ability means that it has short-circuit maintenance function.

　　 The inverter has no short-circuit protection for the reason – Reynolds

The inverter has no short-circuit protection for the reason – Reynolds

Take the commonly used air switch as an example. When the short-circuit current is greater than the short-circuit breaking current of the air switch, the air switch must have started maintenance, but its capacity is too small to break the short-circuit current and will eventually be damaged. Why is its breaking capacity less than the actual short-circuit current? One is the selection of air switches, the other is the capacity of the power grid, because the short-circuit current in the circuit can be calculated according to the circuit parameters, and the power supply capacity and line impedance are related, which is also to be considered in planning and selection.

Since the inverter (Variable-frequency Drive: hereinafter referred to as VFD) will be directly damaged after a short circuit, there is no short-circuit maintenance function. It is damaged, indicating that there is a problem in some part of the system.

There is no short-circuit maintenance on the output side of the VFD, because the output power equipment of the VFD is now an IGBT power module. When the VFD or load output is short-circuited, the IGBT itself has the function of anti-short-circuit, and the lock does not output current. Maintain power equipment from short-circuit currents. On the other hand, if short-circuit maintenance is added, there will be a problem with the output of the VFD. First of all, the short-circuit current is very fast. If the IGBT can't lock itself, it will be finished before it maintains itself. It's useless to add maintenance, because it's too fast to stop. Secondly, it is important to note that the load must not be rolled over during IGBT operation, otherwise the IGBT will break down due to high dv/dt.

Thanks to this, IGBTs are able to resist short circuits on their own. In other words, it is not afraid of short circuits in the load. If the output power of the VFD explodes, it is not caused by the short circuit itself. There must be other reasons, such as the sudden open circuit of the load during operation or overload (IGBT is afraid of overload and short circuit).

First, during VFD operation, the load is suddenly disconnected (open circuit), and the module is blown up at this moment. All this is due to the fact that the dv/dt is too high at the moment (especially when the load is heavy). At this point, the condition of the sub-line is like a knife switch with a load*. Although the output power module of the VFD has RC resistor-capacitance absorption function, the excessive dynamic voltage does not prevent the breakdown of the IGBT. Therefore, the bombing module does not comment at the moment.

First of all, the thermal overload blasting module, which is the output power module of VFD, has been planned for heat dissipation and temperature rise, including the cross-section of heat dissipation, the flow rate of air or the flow rate of cooling medium, etc. If the overload index is running, the longer the time, the heavier the heat accumulation in the pipe. If the temperature rises beyond the promised temperature, it is the same as blowing up the module.

Remember that as early as the mid-to-late 90s of the 20th century, Xi'an Power Electronics Technology Research Institute had an academic exchange on IGBT power equipment in the past. At that time, domestic IGBT was in the research and development stage, and foreign products were widely used, while domestic products were not available. At that time, domestic SCR equipment was already properly sophisticated, and all kinds of power were available, but IGBT did not have it. I remember when an engineer gave a speech saying that when they were doing experiments, IGBTs were not afraid of short circuits. There is no output at the time of the short circuit, so it turns off quickly. IGBT is particularly afraid of heat, and it is easy to fry if the heat dissipation is not good. Therefore, I have a deep memory of the content of that exchange activity. I also know what the weaknesses of IGBT are.

When working with components, you should be aware of their temper and equipment. Only in this way will you be able to scientifically, rationally and correctly select and equip your planned system, and make it work safely and reliably. Therefore, most inverter manufacturers know that adding output and switch to VFD or maintenance function is self-defeating, and it is a load operation, which will undoubtedly be a fatal mistake; If it is a "small horse-drawn cart", it is lucky to cut corners and want to save some money. The potential risks are there, and they are very serious.

The inverter has no short-circuit protection for the reason – Reynolds

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