Abstract: This paper mainly introduces the structure of the inverter control circuit and its anti-interference, and analyzes several common inverter faults. Keywords: Inverter, Control Circuit, Fault Analysis 1 Introduction With the increasingly widespread application of inverters in industrial production, understanding the structure of inverters, the electrical characteristics of main components, the function of some commonly used parameters, and common faults is becoming increasingly important. 2 Inverter Control Circuit The loop that provides control signals to the main circuit supplying power (voltage and frequency adjustable) to the asynchronous motor is called the control circuit, as shown in Figure 1. The control circuit consists of the following circuits: frequency and voltage calculation circuit, main circuit voltage and current detection circuit, motor speed detection circuit, drive circuit that amplifies the control signals from the calculation circuit, and inverter and motor protection circuits. Figure 1 Within the dotted line in Figure 1, the absence of a speed detection circuit indicates open-loop control. Adding a speed detection circuit to the control circuit, i.e., adding speed commands, allows for more precise closed-loop control of the asynchronous motor speed. 1) The operational circuit compares and calculates external speed and torque commands with the current and voltage signals from the detection circuit to determine the inverter's output voltage and frequency. 2) The voltage and current detection circuit is isolated from the main circuit potential to detect voltage and current. 3) The drive circuit drives the main circuit devices; it is isolated from the control circuit to turn the main circuit devices on and off. 4) I/O Input/Output Circuit: To improve human-machine interaction, the inverter has multiple input signals (such as running, multi-speed operation, etc.) and various internal parameter output signals (such as current, frequency, protection action drive, etc.). 5) Speed ​​Detection Circuit: The speed signal is sent to the arithmetic circuit via a speed detector (TG, PLG, etc.) mounted on the asynchronous motor shaft. Based on instructions and calculations, the motor can operate at the commanded speed. 6) Protection Circuit: Detects the voltage and current of the main circuit. When overload or overvoltage occurs, to prevent damage to the inverter and asynchronous motor, the inverter stops working or suppresses the voltage and current values. The protection circuit in the inverter control circuit can be divided into two types: inverter protection and asynchronous motor protection. The protection functions are as follows: (1) Inverter Protection ① Instantaneous Overcurrent Protection When the current flowing through the inverter devices reaches an abnormal value (exceeding the allowable value) due to short circuits on the load side of the inverter current, the inverter operation is stopped instantaneously, and the current is cut off. When the output current of the converter reaches an abnormal value, the inverter operation is also stopped. ② Overload Protection When the inverter output current exceeds the rated value and continues to flow for more than the specified time, the operation must be stopped to prevent damage to the inverter devices, wires, etc. Appropriate protection requires inverse time characteristics, using thermal relays or electronic thermal protection (using electronic circuits). Overload is caused by excessive load GD2 (inertia) or motor stall due to excessive load. ③ Regenerative Overvoltage Protection When the inverter motor decelerates rapidly, the regenerative power DC circuit voltage will rise, sometimes exceeding the allowable value. Overvoltage can be prevented by stopping the inverter operation or stopping rapid deceleration. ④ Instantaneous power outage protection: For instantaneous power outages within a few milliseconds, the control circuit works normally. However, if the instantaneous power outage lasts for more than 10 milliseconds, not only will the control circuit malfunction, but the main circuit will also be unable to supply power. Therefore, the inverter will stop operating after detection. ⑤ Grounding overcurrent protection: When the inverter load is grounded, grounding overcurrent protection is sometimes required to protect the inverter. However, to ensure personal safety, a leakage circuit breaker needs to be installed. ⑥ Cooling fan malfunction: If there is a cooling fan device, the temperature inside the device will rise when the fan malfunctions. Therefore, a fan thermal relay or a device heat sink temperature sensor is used to detect the malfunction and stop the inverter. In cases where the temperature rise is very small and does not hinder operation, it can be omitted. (2) Protection of asynchronous motors: ① Overload protection: The overload detection device is shared with the inverter protection. However, when considering overheating during low-speed operation, a temperature detector is embedded in the asynchronous motor, or the electronic thermal protection installed in the inverter is used to detect overheating. When the operation is frequent, it is possible to reduce the motor load, increase the capacity of the motor and inverter, etc. ② Over-speed protection: When the output frequency of the inverter or the speed of the asynchronous motor exceeds the specified value, the inverter stops operating. (3) Other protections: ① Prevent stall overcurrent: When the asynchronous motor tracks slowly during rapid acceleration, the overcurrent protection circuit will activate, and the operation cannot continue (stall). Therefore, control should be performed before the load current decreases to suppress the frequency rise or make the frequency drop. The same control is sometimes performed for overcurrent during constant speed operation. ② Prevent stall regenerative overvoltage: The regenerative energy generated during deceleration causes the DC voltage of the main circuit to rise. In order to prevent the regenerative overvoltage circuit protection from activating, control should be performed before the DC voltage drops to suppress the frequency drop and prevent the inability to operate (stall). 3 Anti-interference measures for inverter control circuit: Due to the nonlinearity of the main circuit (switching operation), the inverter itself is a source of harmonic interference, while its peripheral control circuit is a low-energy, weak-signal circuit, which is easily affected by interference from other devices, causing the inverter itself and peripheral equipment to malfunction. Therefore, anti-interference measures must be taken for the control circuit when the inverter is installed and used. 1) Basic Control Circuit of Frequency Converter The basic circuits for signal exchange with the outside world are of two types: analog and digital. ① 4-20mA current signal circuit (analog); 1-5V/0-5V voltage signal circuit (analog). ② Switching signal circuit, including the frequency converter's start/stop commands, forward/reverse commands, etc. (digital). External control command signals are introduced into the frequency converter through the above basic circuits. Simultaneously, interference sources also generate interference potentials in their circuits, intruding into the frequency converter via the control cable. 2) Basic Types of Interference and Anti-interference Measures ① Electrostatic Coupling Interference: This refers to the electrostatic capacitive coupling between the control cable and surrounding electrical circuits, generating a potential in the cable. Measures: Increase the distance from the interference source cable; when it reaches more than 40 times the conductor diameter, the interference level becomes less noticeable. Install a shielding conductor between the two cables and then ground the shielding conductor. ② Electrostatic Induction Interference: This refers to the potential induced in the cable by changes in magnetic flux generated by surrounding electrical circuits. The magnitude of the interference depends on the magnitude of the magnetic flux generated by the interference source cable, the closed-loop area formed by the control cable, and the relative angle between the interference source cable and the control cable. Measures: ① Control cables are generally laid separately from main circuit cables or other power cables, with a separation distance typically greater than 30cm (minimum 10cm). If separation is difficult, the control cable can be laid through an iron pipe. ② Control conductors are twisted together; the smaller the twisting gap and the shorter the route, the better the anti-interference effect. ③ Electromagnetic wave interference: This refers to the control cable acting as an antenna, generating potential in the cable due to external electromagnetic waves. Measures: Same as described in 1 and 2. If necessary, the inverter should be placed in an iron box for electromagnetic wave shielding, and the shielding iron box should be grounded. ④ Poor contact interference: This refers to interference generated in the cable due to poor contact at the electrical contacts and relay contacts of the inverter control cable, resulting in changes in resistance. Measures: For poor relay contact, use parallel contacts, gold-plated contact relays, or sealed relays. Cable connection points should be tightened and reinforced regularly. ⑤ Power line conducted interference: This refers to the potential directly generated in the power system by other equipment when various electrical devices receive power from the same power system. Measures: The inverter's control power supply is supplied by a separate system, and a line filter is installed on the input side of the control power supply; an insulation transformer is installed, and it is shielded and grounded. ⑥ Grounding interference: refers to the machine body grounding and signal grounding. Various unexpected interferences can be induced by weak voltage and current loops and any unreasonable grounding. For example, setting more than two grounding points will generate a potential difference at the grounding points, causing interference. Measures: The speed setting control cable is grounded at one point, and the grounding wire is not used as a signal path. The cable grounding is carried out on the inverter side, using a dedicated grounding terminal, not shared with other grounding terminals, and the resistance of the grounding terminal lead point is minimized, generally not exceeding 100d. 3) Other precautions ① The control cabinet with the inverter should be kept as far away as possible from large-capacity transformers and motors. Its control cable lines should also avoid these devices with large leakage flux. ② Weak voltage and current control cables should not be close to circuit breakers and contactors that are prone to arcing. ③ It is recommended to use 1.25mm×2 or 2mm×2 shielded stranded insulated cables for the control cables. ④ The shielding of the shielded cable must be continuous to the same length as the cable conductor. When the cable is connected in the terminal box, the shielding terminals must be interconnected. 4. Common Inverter Fault Analysis 1) Inverter Charging and Starting Circuit Faults General-purpose frequency converters are typically voltage-type converters, operating in an AC-DC-AC mode. This means the input is an AC power supply, which is rectified by a three-phase rectifier bridge to become a DC voltage. This DC voltage is then converted by a three-phase bridge inverter circuit into a three-phase AC power supply with adjustable voltage and frequency, which is then output to the load. When the frequency converter is first powered on, the charging current is very large due to the large capacitance of the smoothing capacitor on the DC side. A starting resistor is usually used to limit the charging current. Two common frequency converter starting circuits are shown in Figure 1. After charging is complete, the control circuit short-circuits the resistor through relay contacts or a thyristor. A starting circuit fault typically manifests as a burnt-out starting resistor, and the frequency converter alarm displays a DC bus voltage fault. Generally, designers choose smaller starting resistors (10–50Ω, 10–50W) to reduce the frequency converter's size. Frequent switching of the inverter's AC input power, poor contact of the bypass contactor, or increased on-resistance of the bypass thyristor can all cause the starting resistor to burn out. In such cases, a replacement resistor of the same specification should be purchased, and the cause of the burnt-out resistor must be identified. If the fault is caused by the switching of the input power frequency, this phenomenon must be eliminated before the inverter can be put into use; if the fault is caused by the bypass relay contacts or the bypass thyristor, these components must be replaced. Figure 2 2) The inverter has no fault display but cannot run at high speed. One of our frequency converters is functioning normally, but it cannot be adjusted to high-speed operation. Inspection revealed no faults in the converter; parameter settings were correct, and the speed control input signal was normal. Upon power-on testing, the DC bus voltage was only around 450V, while the normal range is 580-600V. Further testing on the input side revealed a missing phase. The fault was caused by poor contact in one phase of an air switch on the input side. Why doesn't the frequency converter alarm for a missing input phase and can still operate at low frequencies? In fact, frequency converters can still operate with a missing input phase; most frequency converters have a lower limit for bus voltage. The DC bus voltage is 400V, meaning the inverter only reports a low DC bus voltage fault when it drops below 400V. With two-phase input, the DC bus voltage is 380 * 1.2 = 452V > 400V. When the inverter is not running, the DC voltage can reach normal values ​​due to the smoothing capacitor. Newer inverters use PWM control technology, with voltage and frequency regulation handled by the inverter bridge. Therefore, they can still operate normally even with a single input phase loss in the low-frequency range. However, because of the low input and output voltages, the asynchronous motor torque is low, and the frequency cannot be increased. 3) Inverter displays overcurrent When this fault occurs, first check if the acceleration time parameter is too short and the torque increase parameter is too large, then check if the load is too heavy. If none of these phenomena are present, disconnect the current transformer on the output side and the Hall current detection point on the DC side, reset and run to see if overcurrent occurs. If it does, it is very likely that the 1PM module is faulty, because the 1PM module contains overvoltage, overcurrent, undervoltage, overload, overheat, phase loss, and short circuit protection functions. These fault signals are transmitted to the microcontroller via the output Fn pin of the module control pin. After receiving the fault information, the microcontroller blocks the pulse output and displays the fault information on the panel. Generally, replacing the 1PM module is the solution. 4) Inverter displays overvoltage fault Overvoltage faults in frequency converters typically occur during thunderstorms. Lightning strikes the converter's power supply, causing the DC-side voltage detector to trip. In this case, simply disconnecting the converter's power for about 1 minute and then reconnecting it will usually reset it. Another scenario is when the frequency converter is driving a large inertia load, leading to overvoltage. This is because the converter's deceleration and stopping is regenerative braking. During stopping, the converter's output frequency decreases linearly, while the load motor's frequency is higher than the converter's output frequency. The load motor is in a generating state, converting mechanical energy into electrical energy, which is absorbed by the smoothing capacitor on the DC side of the frequency converter. When this energy is large enough, a so-called "pump-up phenomenon" occurs, causing the voltage on the DC side of the frequency converter to exceed the maximum voltage of the DC bus and trip. For this fault, one solution is to either increase the deceleration time parameter, increase the braking resistor, or add a braking unit; another is to set the frequency converter's stopping mode to free stop. 5) Motor overheating, frequency converter displays overload. If this type of fault occurs in a frequency converter that is already in operation, the load condition must be checked. If this type of fault occurs in a newly installed frequency converter, it is likely due to improper V/F curve settings or problems with motor parameter settings. For example, a newly installed frequency converter drives a variable frequency motor with rated parameters of 220V/50Hz, while the frequency converter is set to 380V/50Hz at the factory. Because the installer did not set the frequency converter's V/F parameters correctly, the rotor of the motor becomes magnetically saturated after running for a period of time, causing the motor speed to decrease, overheat, and become overloaded. Therefore, this parameter must be set correctly before using a new frequency converter. Additionally, when using the sensorless vector control mode of the frequency converter, incorrect settings of the rated voltage, current, and capacity of the load motor can also lead to motor thermal overload. Another situation is that setting the frequency converter's carrier rate too high can also cause motor overheating and overload. Finally, electrical designers often design frequency converters to operate at low frequencies without considering the reduced heat dissipation of motors operating at low frequencies, leading to overheating and overload after a period of operation. For this type, a cooling device must be installed. 5. Conclusion Although new frequency converters manufactured using advanced processes and components have very high reliability when used as asynchronous motor drives, improper use or accidental events can still cause damage. To use frequency converters effectively in production, familiarity with their structural principles and understanding of common faults is particularly important for technicians.