Cause analysis of inverter overcurrent and overload

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1. Overcurrent tripping and cause analysis: The overcurrent tripping of frequency converters can be categorized into short-circuit faults, tripping during operation, and tripping during acceleration and deceleration processes. 1.1 Short-circuit fault (1) Fault characteristics a) The first trip may occur during operation, but if the system is restarted after reset, it usually trips as soon as acceleration begins. b) It has a very high inrush current, but most inverters are now capable of triggering protective tripping without being damaged. Due to the very rapid operation of the protection trip, it is difficult to observe the magnitude of its current. (2) Judgment and handling: The first step is to determine whether there is a short circuit. To facilitate judgment, a voltmeter can be connected to the input side before restarting after reset. When restarting, the potentiometer rotates slowly from zero, and at the same time, the voltmeter should be monitored carefully. If the inverter trips immediately as soon as its output frequency starts to rise, and if the pointer of the voltage meter shows signs of returning to \"0\" for a moment, it indicates that there is a short circuit or ground connection at the inverter’s output terminal. In the second step, it is necessary to determine whether there is a short circuit inside the inverter or outside it. At this point, the connections at the output of the inverter should be disconnected, and then the potentiometer should be turned to increase the frequency. If tripping still occurs, it indicates a short circuit inside the inverter ； If it no longer trips, it indicates an external short circuit in the inverter; therefore, the wiring between the inverter and the motor, as well as the motor itself, should be checked. 1.2、Light-load overcurrent: The load is very light, yet an overcurrent trip occurs. This is a phenomenon unique to variable-frequency speed control. In V/F control mode, there is a very serious problem: the instability of the motor’s magnetic circuit system during operation. The basic reason is that during low-frequency operation, in order to drive heavier loads, torque compensation is often required (that is, increasing the U/f ratio, also known as torque enhancement). This causes the degree of saturation in the motor’s magnetic circuit to vary depending on the load level. This overcurrent trip caused by the saturation of the motor magnetic circuit mainly occurs under low-frequency, light-load conditions. Solution: Adjust the U/f ratio repeatedly. 1.3 Overcurrent due to overload (1) Fault symptoms: In some manufacturing machines, the load suddenly increases during operation, or the machine may even get stuck. As a result, the motor’s speed drops significantly because it can no longer handle the load, and the current rises sharply. The overload protection fails to activate in time, leading to an overcurrent trip. (2) Solution: a) First, determine whether there is a fault with the machine itself; if so, repair it. b) If this kind of overload is a phenomenon that often occurs during the production process, first consider whether it is possible to increase the transmission ratio between the motor and the load Increasing the gear ratio appropriately can reduce the braking torque on the motor shaft, preventing the situation where it cannot drive the load. If it is not possible to increase the transmission ratio, then the only option is to consider increasing the capacity of the motor and the inverter. Overcurrent during acceleration or deceleration of 1.4 liters: This is caused by too rapid acceleration or deceleration. The possible measures include the following: (1) Extend the acceleration (deceleration) time. First, determine whether it is permissible to extend the acceleration or deceleration time according to the requirements of the production process; if so, the acceleration (deceleration) time can be increased. (2) Accurate presetting of the acceleration/deceleration self-processing (stall prevention) function: The inverter is equipped with a self-processing (stall prevention) function to handle overcurrents that occur during the acceleration and deceleration processes. When the rising (falling) current exceeds the preset upper limit current, the acceleration (deceleration) will be paused; it will resume only once the current drops below the set value. 2. Overload tripping and cause analysis: When the motor is able to rotate but the operating current exceeds the rated value, this is referred to as an overload. The basic manifestation of overload is that although the current exceeds the rated value, the excess is not significant, and generally no large inrush current is generated. 2.1 Main causes of overload: (1) Excessive mechanical load. The main characteristic of an excessive load is that the motor heats up, and this can be detected by checking the operating current on the display screen. (2) Unbalanced three-phase voltage causes the operating current of one phase to become excessive, leading to overload tripping. Its characteristic is uneven heating of the motor, and this may not be detectable by reading the operating current from the display screen (as the display only shows the current of one phase). (3) Malfunction: A fault occurs in the current detection circuit inside the inverter, causing the detected current signal to be too high, which leads to tripping. 2.2 Inspection methods: (1) Check whether the motor is overheating. If the temperature rise of the motor is not high, first verify whether the preset value of the inverter’s electronic thermal protection function is appropriate. If there is still headroom in the inverter, the preset value of the electronic thermal protection function should be increased. If the temperature rise of the motor is too high, and the overload that occurs is a normal one, it indicates that the motor is under excessive load. At this point, the first thing to consider is whether it is possible to increase the gear ratio appropriately in order to reduce the load on the motor shaft. If it is possible to increase it, increase the gear ratio. If the gear 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 it is not balanced, then check whether the three-phase voltage at the output of the inverter is balanced as well; if it is also unbalanced, the problem lies within the inverter itself. If the voltage at the output of the inverter is balanced, the problem lies in the wiring between the inverter and the motor. It is necessary to check whether all the screws on the terminals have been tightened. If there are contactors or other electrical devices between the inverter and the motor, it is also important to verify that the terminals of those devices are properly secured and that the contacts are in good condition. If the three-phase voltage on the motor side is balanced, it is necessary to know the operating frequency at the time of tripping. If the operating frequency is low and vector control is not being used, then the U/f ratio should be reduced first. If the load can still be driven after this reduction, it indicates that the originally set U/f ratio was too high, resulting in a high peak value for the excitation current; reducing the U/f ratio can help decrease this current ； If the load cannot be driven after the voltage is reduced, then increasing the capacity of the inverter should be considered ； If the frequency converter has vector control functionality, vector control should be used.