An article analyzing the overcurrent trip and overload trip of the frequency converter
Overcurrent trip of frequency converter
The overcurrent trip of the frequency converter is divided into short circuit fault, tripping during operation and tripping during speed up and down.
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 increases.
(b) Has a large inrush current, but most inverters are already capable of protective tripping without damage. Since the protection trips very quickly, it is difficult to observe the magnitude of the current.
(2) Judgment and processing
The first step is to determine whether there is a short circuit. In order to facilitate judgment, a voltmeter can be connected to the input side after reset and before restarting. When restarting, the potentiometer will slowly rotate from zero. At the same time, pay attention to the voltmeter. If the output frequency of the inverter trips immediately as soon as it rises, and the pointer of the voltmeter shows signs of returning to "0" instantaneously, it means that the output end of the inverter has been short-circuited or grounded.
The second step is to determine whether there is a short circuit inside the inverter or an external short circuit. At this time, the wiring at the output end of the frequency converter should be disconnected, and then the potentiometer should be rotated to increase the frequency. If it still trips, it means there is a short circuit inside the frequency converter; if it no longer trips, it means there is a short circuit outside the frequency converter, and the slave frequency converter should be checked. the wiring from the controller to the motor, as well as the motor itself.
2. Light load overcurrent: The load is very light, but overcurrent trips: This is a phenomenon unique to variable frequency speed regulation.
In the V/F control mode, there is a very prominent problem: the instability of the motor's magnetic circuit system during operation.
The basic reason is that when operating at low frequency, in order to drive a heavier load, torque compensation (i.e., increasing the U/f ratio, also called torque boost) is often required. As a result, the degree of saturation of the motor's magnetic circuit changes with the load. This kind of overcurrent trip caused by the saturation of the motor's magnetic circuit mainly occurs under low frequency and light load conditions. Solution: Repeatedly adjust the U/f ratio.
3. Heavy load overcurrent:
(1) Fault phenomenon: The load of some production machinery suddenly increases during operation, or even "gets stuck". The speed of the motor drops significantly because it cannot be driven, and the current increases sharply. The overload protection has no time to act, resulting in an overcurrent trip.
(a) First find out whether the machine itself is faulty, and if so, repair the machine.
(b) If this kind of overload is a phenomenon that may often occur during the production process, first consider whether it is possible to increase the transmission ratio between the motor and the load? Appropriately increasing the transmission ratio can reduce the resistance torque on the motor shaft and avoid the situation of immobility. If the transmission ratio cannot be increased, the only option is to consider increasing the capacity of the motor and frequency converter.
4. Overcurrent during acceleration or deceleration: This is caused by excessive acceleration or deceleration. The measures that can be taken are as follows:
(1) Extend the speed-up (deceleration) time. First, understand whether it is allowed to extend the speed-up or deceleration time according to the production process requirements. If allowed, the speed-up (deceleration) time can be extended.
(2) Accurately preset the self-processing (anti-stall) function of rising (lowering) speed. The inverter is equipped with a self-processing (anti-stall) function for overcurrent during the speed rising and falling processes. When the rising (lowering) current exceeds the preset upper limit current, the speed raising (lowering) will be suspended. When the current drops below the set value, the speed rising (lowering) will continue.
Overload trip of frequency converter
The motor is able to rotate, but the operating current exceeds the rated value, which is called overload. The basic reflection of overload is: although the current exceeds the rated value, the extent of the excess is not large, and generally it does not form a large inrush current.
1. Main causes of overload
(1) The mechanical load is too heavy. The main characteristic of overload is the heating of the motor, which can be found by reading the operating current from the display.
(2) The three-phase voltage is unbalanced, causing the operating current of a certain phase to be too large, resulting in overload tripping. This is characterized by uneven heating of the motor, which may not be found when reading the operating current from the display (because the display only displays one phase current).
(3) Malfunction, 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, you should first check whether the electronic thermal protection function of the inverter is reasonably preset. If the inverter still has margin, the preset of the electronic thermal protection function should be relaxed. Setting value. If the temperature rise of the motor is too high and the overload occurs is a normal overload, it means that the motor is overloaded. At this time, the first thing to do is to 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 on the motor side is balanced. If the three-phase voltage on the motor side is unbalanced, then check whether the three-phase voltage on the output end of the inverter is balanced. If it is not balanced either, the problem is inside the inverter. If the voltage at the output end of the frequency converter is balanced, the problem is on the line from the frequency converter to the motor. Check whether the screws at all terminals have been tightened. If there are contactors or other electrical appliances between the frequency converter and the motor, also You should check whether the terminals of the relevant electrical appliances are tightened and whether the contacts are in good condition. If the three-phase voltage on the motor side is balanced, you should know the operating frequency when tripping: If the operating frequency is low and vector control is not used (or no vector control), first reduce the U/f ratio. If the load can still be driven after the reduction, , it means that the original preset U/f ratio is too high and the peak value of the excitation current is too large. The current can be reduced by reducing the U/f ratio; if the load cannot be carried after the reduction, you should consider increasing the frequency converter. Capacity; if the frequency converter has vector control function, vector control mode should be used.
Review Editor: Huang Fei
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