How To Set The Protection Values Of Heavy Load And Light Load Of The Motor? After Reading It, You Will Understand

How to set the protection values of heavy load and light load of the motor? After reading it, you will understand

A few years ago, I saw an engineer solemnly instructing his subordinates to set the overload protection parameters according to the starting load of the three-phase asynchronous motor, and must understand whether this motor is light load start or heavy load start, otherwise something will happen, because the heavy load starting current will be very large.

At that time, I wanted to point out to him that this was a misunderstanding, but I was afraid of offending people, but then I found that many electrical engineers today have this misunderstanding, I really don't know how they learned it in school, of course, they may not be in this major.

In fact, the no-load, light-load, and heavy-duty starting currents of three-phase asynchronous motors are the same, and the overload protection setting value cannot be adjusted arbitrarily. Because the starting torque logic of the motor is like this: Me=Kmdblcos For a motor, the structural constant Km, magnetic flux d, and power factor cosφ are fixed, so the starting current I will not change, but the starting time will be different, the light load start time is short, and the heavy load start time is long.

Friend: The stall current of the motor is fixed, but the curve of the motor from the blockage to the normal standard revolution is different between heavy load start (such as fans and belt conveyors, etc.) and light load start (such as centrifugal water pumps, etc.), that is, the starting time is not the same.

Motor protection should be according to the amount of current of the motor to set the protection value, electronic protection can be according to the load characteristics, set the start time and other protection parameters, thermal relay protection fixed value, if there is test equipment, it is best to use the test equipment off the machine to adjust well, and then install and put into operation, the test site should be the same or similar to the room temperature of the operating place.

As for whether to use step-down starting measures, it depends on the carrying capacity of the power supply system, for example, large-scale petrochemical plants below 180KW use full-voltage start.