What is the difference between inverter overload and overcurrent? Overload is a concept of time, which means that the load exceeds the rated load by a certain multiple in a continuous time. The most important concept of overload is continuous time. For example, the overload capacity of a frequency converter is 160% for one minute, that is, there is no problem that the load reaches 1.6 times the rated load for one minute continuously. If the load suddenly becomes smaller in 59 seconds, then the overload alarm will not be triggered. Only after 60 seconds, the overload alarm will be triggered. Overcurrent is a quantitative concept, which refers to how many times the load suddenly exceeds the rated load. The time of overcurrent is very short, and the multiple is very large, usually more than ten or even dozens of times. For example, when the motor is running, the mechanical shaft is suddenly blocked, then the current of the motor will rise rapidly in a short time, leading to overcurrent failure.
Over-current and overload are the most common faults of frequency converters. To distinguish whether the frequency converter is over-current tripping or overload tripping, we must first make clear the difference between them. Generally speaking, overload must also be over-current, but why should the frequency converter separate over-current from overload? There are two main differences: (1) different protection objects Overcurrent is mainly used to protect the frequency converter, while overload is mainly used to protect the motor. Because the capacity of the frequency converter sometimes needs to be increased by one gear or even two gears than the capacity of the motor, in this case, when the motor is overloaded, the frequency converter does not necessarily overcurrent. Overload protection is carried out by the electronic thermal protection function inside the frequency converter. When the electronic thermal protection function is preset, the “current utilization ratio” should be preset accurately, that is, the percentage of the ratio of the rated current of the motor to the rated current of the frequency converter: IM%=IMN*100%I/IM Where, im%-current utilization ratio; IMN—-rated current of motor, a; IN— rated current of frequency converter, a. (2) The change rate of current is different Overload protection occurs in the working process of production machinery, and the change rate of current di/dt is usually small; Overcurrent other than overload is often sudden, and the change rate of current di/dt is often large. (3) Overload protection has inverse time characteristic. Overload protection mainly prevents the motor from overheating, so it has the characteristics of “inverse time limit” similar to thermal relay. That is to say, if it is not much more than the rated current, the allowable running time can be longer, but if it is more, the allowable running time will be shortened. In addition, as the frequency decreases, the heat dissipation of the motor becomes worse. Therefore, under the same overload of 50%, the lower the frequency, the shorter the allowable running time.
Overcurrent trip of frequency converter Over-current tripping of inverter is divided into short-circuit fault, tripping during operation and tripping during acceleration and deceleration, etc. 1, short circuit fault: (1) Fault characteristics (a) The first trip may occur during operation, but if it is restarted after reset, it will often trip as soon as the speed rises. (b) It has a large surge current, but most frequency converters have been able to perform protection tripping without damage. Because the protection trips very quickly, it is difficult to observe its current. (2) Judgment and handling The first step is to judge whether there is a short circuit. In order to facilitate the judgment, a voltmeter can be connected to the input side after reset and before restarting. When restarting, the potentiometer will turn slowly from zero, and at the same time, pay attention to the voltmeter. If the output frequency of the inverter trips as soon as it rises, and the pointer of the voltmeter shows signs of returning to “0″ instantly, it means that the output end of the inverter has been short-circuited or grounded. The second step is to judge whether the inverter is short-circuited internally or externally. At this time, the connection at the output end of the frequency converter should be disconnected, and then the potentiometer should be turned to increase the frequency. If it still trips, it means that the frequency converter is short-circuited; If it doesn’t trip again, it means that there is a short circuit outside the frequency converter. Check the line from the frequency converter to the motor and the motor itself. 2, light load overcurrent load is very light, but overcurrent tripping: This is a unique phenomenon of variable frequency speed regulation. In the V/F control mode, there is a very prominent problem: the instability of the motor magnetic circuit system during operation. The basic reason lies in: When running at low frequency, in order to drive a heavy load, torque compensation is often needed (that is, improving the U/f ratio, also called torque boost). The saturation degree of motor magnetic circuit changes with the load. This over-current trip caused by the saturation of motor magnetic circuit mainly occurs at low frequency and light load. Solution: Adjust the U/f ratio repeatedly. 3, overload overcurrent: (1) Fault phenomenon Some production machines suddenly increase the load during operation, or even “get stuck”. The speed of the motor drops sharply due to the immobility of the belt, the current increases sharply, and the overload protection is too late to act, resulting in overcurrent tripping. (2) Solution (a) First, find out whether the machine itself is faulty, and if it is, repair the machine. (b) If this overload is a common phenomenon in the production process, first consider whether the transmission ratio between the motor and the load can be increased? Appropriately increasing the transmission ratio can reduce the resistance torque on the motor shaft and avoid the situation of belt immobility. If the transmission ratio cannot be increased, the capacity of motor and frequency converter must be increased. 4. Over-current during acceleration or deceleration: This is caused by too fast acceleration or deceleration, and the measures that can be taken are as follows: (1) Extend the acceleration (deceleration) time. First, understand whether it is allowed to extend the acceleration or deceleration time according to the production process requirements. If it is allowed, it can be extended. (2) Accurately predict the acceleration (deceleration) self-treatment (stall prevention) function The inverter has a self-treatment (stall prevention) function for overcurrent during acceleration and deceleration. When the rising (falling) current exceeds the preset upper limit current, the rising (falling) speed will be suspended, and then the rising (falling) speed will continue when the current drops below the set value.
Overload trip of frequency converter The motor can rotate, but the running current exceeds the rated value, which is called overload. The basic reaction of overload is that although the current exceeds the rated value, the magnitude of the excess is not large, and generally it does not form a large impact current. 1, the main reason of overload (1) The mechanical load is too heavy. The main feature of the overload is that the motor generates heat, which can be found by reading the running current on the display screen. (2) The unbalanced three-phase voltage causes the running current of a certain phase to be too large, leading to overload tripping, which is characterized by unbalanced heating of the motor, which may not be found when reading the running current from the display screen (because the display screen only shows one phase current). (3) Misoperation, the current detection part inside the inverter fails, and the detected current signal is too large, resulting in tripping. 2. Inspection method (1) Check whether the motor is hot. If the temperature rise of the motor is not high, first of all, check whether the electronic thermal protection function of the frequency converter is properly preset. If the frequency converter still has a surplus, the preset value of the electronic thermal protection function should be relaxed. If the temperature rise of the motor is too high and the overload is normal, it means that the motor is overloaded. At this time, we should first increase the transmission ratio appropriately to reduce the load on the motor shaft. If it can be increased, increase the transmission ratio. If the transmission ratio cannot be increased, the capacity of the motor should be increased. (2) Check whether the three-phase voltage at the motor side is balanced. If the three-phase voltage at the motor side is unbalanced, check whether the three-phase voltage at the output end of the frequency converter is balanced. If it is also unbalanced, the problem lies inside the frequency converter. If the voltage at the output end of the frequency converter is balanced, the problem lies in the line from the frequency converter to the motor. Check whether the screws of all terminals are tightened. If there are contactors or other electrical appliances between the frequency converter and the motor, check whether the terminals of relevant electrical appliances are tightened and whether the contact conditions of the contacts are good. If the three-phase voltage on the motor side is balanced, you should know the working frequency when tripping: If the working frequency is low and vector control (or no vector control) is used, the U/f ratio should be reduced first. If the load can still be driven after the reduction, it means that the original U/f ratio is too high and the peak value of excitation current is too large, so the current can be reduced by reducing the U/f ratio. If there is no fixed load after reduction, we should consider increasing the capacity of the inverter; If the inverter has vector control function, vector control mode should be adopted.
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