In the background of relatively backward protection function of electrical equipment in the early days, our electricians often only know that there are two major protection functions of short circuit and overload in the ordinary electric control system, and naturally they have a deep understanding of these two protection function actuators fuse and thermal relay. However, with the popularization of more and more electronic industrial control devices, an overcurrent protection function has been added to the two protection functions mentioned above, which makes some colleagues who are not sure what they are doing have no idea! So these three protection functions are all aimed at current monitoring. What are the similarities and differences? This time, I will take the most widely used industrial control device - frequency converter as an example to give you a general understanding of this by revealing the essence of the three protection functions of the device.

It is undoubtedly obvious that the common points of the three protection functions of short circuit, overcurrent and overload are all based on the current in the line or equipment. And they also take the rated current value as the comparison object, as the action basis. In the frequency converter, the sampling of these three protection functions is also almost from the same component - electronic Hall type current transformer (the small power frequency converter is internally divided into milliohm copper pieces).

Next, we will analyze the differences of the three protection functions in detail:

When the overload protection function gives an alarm, it is usually triggered by an abnormally small increase in the rated current of the measured object. The setting value of the protection action is basically 1.05-1.15 times of the rated current. Among the three protection functions, the overload protection action causes the least harm and hardly damages the hardware device. It is a preventive protection function. Therefore, the overload protection action has the lowest priority among the three types of protection.

In short, the overcurrent protection function is an upgraded version of the overload protection function. The frequency converter with over-current protection function setting will clearly indicate that the protection action parameter setting range is 1.5-2 times of rated current. （ http://www.dgdqxx.com/ It can be seen from this that the starting point of overcurrent protection is higher than the overload protection function, so its action priority is higher than the overload protection function! Similarly, due to the large amplitude of abnormal current value when the protection action occurs, the hardware device has been damaged after the over-current protection function in actual work (according to my statistics, the probability of this occurrence is as high as one third)!

Compared with the above two protection functions, the short-circuit protection function is the ultimate manifestation of abnormal current! Once the converter reports a short circuit fault, it usually means that the hardware accessories (except for the converter motherboard software program problems) are nearly 100% damaged! Generally speaking, the action parameter setting range of most inverters for short-circuit protection function is 3-5 times of rated current. Due to its great harmfulness and irreversibility, the priority of short-circuit protection function action is usually the highest in the frequency converter (including other equipment)! It should be noted that when the inverter reports a short circuit fault code, most of them cannot be eliminated through the "reset" operation. In this case, we should completely solve the fault before carrying out subsequent operations, otherwise the consequences will be very serious!

Finally, it should be emphasized that few inverters can maintain the above three protection functions at the same time. In general, nearly half of frequency converters only have overload and short circuit protection functions; However, brand inverters such as Delta, Zhongda and Riye have no short-circuit protection function. Instead, they have overcurrent and overload protection functions. The overcurrent protection function here is almost the same as the short-circuit protection function; In fact, a small number of brand frequency converters have neither overload protection function nor short-circuit protection function, but are replaced by over-current protection function I and over-current protection function II. When using such frequency converters, you should carefully read the accompanying manual, otherwise it is very easy to cause failure of the protection function!