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The AC speed control system referred to today mainly denotes a frequency conversion speed control system for AC motors using electronic power converters. Due to its advantages over direct current drives, variable frequency speed control systems are often chosen as the preferred drive solution in many applications. Modern variable frequency speed control systems use 16-bit or 32-bit microcontrollers as their control core, enabling fully digital control. Their speed regulation performance is roughly comparable to that of direct current drives; however, maintaining such systems is more complex than maintaining direct current drives. In the event of a failure, it is difficult for ordinary electrical technicians in enterprises to handle it. Here, we analyze common faults in variable frequency drives, examining the causes of these faults and the methods for dealing with them. I. Parameter setting-related faults In the use of common frequency converters, it is very important that their parameters are set correctly in order to ensure they can meet the requirements of the drive system; incorrect parameter settings can cause the frequency converter to not function properly. 1. Parameter Settings For common frequency converters, the manufacturer usually sets a default value for each parameter at the time of production; these parameters are known as factory values. With these parameter values, the user can operate it normally via the panel, but panel operation does not meet the requirements of most drive systems. Therefore, before using the inverter correctly, the user should set its parameters based on the following aspects: (1) Confirm the motor parameters – the inverter requires information such as the motor’s power, current, voltage, speed, and maximum frequency, which can be obtained directly from the motor’s nameplate. (2) The control method adopted by the frequency converter, namely speed control, torque control, PID control, or other methods. After adopting a control method, static or dynamic identification is generally required depending on the control accuracy. (3) Set the startup mode of the inverter. Generally, inverters are set to start via the panel at the time of manufacture, but users can choose the appropriate startup method based on actual conditions, such as through the panel, external terminals, or communication methods. (4) Regarding the selection of the input signal, there are generally various ways to set the frequency of an inverter: via the panel, externally, through external voltage or current, or via communication. Of course, the frequency of an inverter can be set using one or more of these methods. After setting the above parameters correctly, the frequency converter can function properly. To achieve better control performance, the relevant parameters must be modified according to the actual conditions. 2. Handling of parameter setting-related faults Once a parameter setting-related fault occurs, the inverter cannot operate properly; generally, the parameters can be modified according to the manual. If the above methods don’t work, it’s best to restore all parameters to their factory settings and then reconfigure them following the steps mentioned above. The method for restoring parameters varies depending on the frequency converter model of each company. II. Overvoltage-related faults The overvoltage in inverters is primarily manifested in the voltage of the branches of the DC bus. Under normal conditions, the DC voltage of the inverter is the average value after three-phase full-wave rectification. If calculated using a line voltage of 380V, the average DC voltage Ud = 1.35 × U_line = 513V. When an overvoltage occurs, the energy storage capacitor of the DC bus is charged; when the voltage rises to around 760 V, the inverter’s overvoltage protection mechanism activates. Therefore, inverters all have a normal operating voltage range; when the voltage exceeds this range, the inverter is likely to be damaged. There are two common types of overvoltage. 1. Excessive input AC voltage This condition occurs when the input voltage exceeds the normal range; it usually happens during holidays when the load is light, causing the voltage to rise or fall and leading to circuit faults. In such cases, it is best to disconnect the power supply and conduct inspections and repairs. 2. Overvoltage during power generation This situation occurs quite frequently; it happens mainly when the synchronous speed of the motor is higher than its actual speed, causing the motor to operate in power-generation mode. Since the frequency converter does not have a braking unit, two scenarios can lead to this fault. (1) When the inverter drives a load with high inertia, the deceleration time is set to be relatively short. During the deceleration process, the speed output by the inverter is high, while the load slows down more slowly due to its own resistance. As a result, the speed of the motor driven by the load becomes higher than the speed corresponding to the frequency output by the inverter; the motor then operates in a power-generation mode. Since the inverter does not have an energy feedback unit, the voltage in its DC circuit rises above the protective threshold, leading to a fault. This type of fault often occurs in the drying section of paper machines. To address this issue, a regenerative braking unit can be added, or the inverter’s parameters can be modified to increase the deceleration time. Functions added to the regenerative braking unit include energy-consuming type, parallel DC bus absorption type, and energy feedback type. In the energy-consuming type, a braking resistor is connected in parallel to the DC circuit of the inverter, and the on/off state of the power transistor is controlled by detecting the DC bus voltage. The parallel DC bus absorption type is used in multi-motor drive systems, where one or several motors are often operating in a power generation mode, generating regenerative energy; this energy is absorbed by the motors that are in electric drive mode through the parallel bus. The grid-side converter of an energy-feedback inverter is reversible; when regenerative energy is generated, the reversible converter feeds this energy back into the power grid. (2) This fault can also occur when multiple electric actuators drive the same load, mainly due to a lack of load distribution. Taking two motors driving a load as an example, when the actual speed of one motor is greater than the synchronous speed of the other motor, the motor with the higher speed acts as the prime mover, while the motor with the lower speed operates in generator mode, which can lead to faults. In paper machines, this often occurs in the press section and wire section, and load distribution control is required during handling. The characteristics of the inverter, which is part of the drive speed chain of the paper machine, can be adjusted to be more flexible. III. Overcurrent faults Overcurrent faults can be classified into accelerating, decelerating, and constant-speed overcurrents. It may be caused by factors such as too short acceleration and deceleration times of the inverter, sudden changes in the load, uneven load distribution, or output short circuits. At this time, it is generally possible to address the issue by extending the acceleration and deceleration times, reducing sudden load changes, adding energy-consuming braking elements, designing load distribution, and inspecting the circuit. If an overcurrent fault persists even after the load is disconnected, it indicates that the inverter circuit of the frequency converter is short-circuited, and the frequency converter needs to be replaced. IV. Overload faults Overload faults include frequency conversion overload and electric machine overload. It may be caused by reasons such as an excessively short acceleration time, excessive DC braking force, too low grid voltage, or an overloaded load. This can generally be achieved by extending the acceleration time, extending the braking time, and checking the grid voltage. The load is too heavy; the selected motor and frequency converter are not capable of driving it, or it may be due to poor mechanical lubrication. In the former case, a high-power motor and inverter must be replaced ； In the latter case, the production machinery needs to be overhauled. V. Other faults 1. Under-voltage This indicates a problem with the power input section of the inverter; it needs to be checked before it can operate. 2. Excessively high temperature If the motor is equipped with a temperature monitoring device, check its cooling conditions ； The inverter’s temperature is too high; check its ventilation. 3. Other situations Such as hardware failures or communication issues; you can contact the supplier. This post was last edited by 771207 on 2008-6-13 at 20:08.]