Currently, frequency converters are increasingly widely used in various fields such as chemical engineering, power, metallurgy, and civil applications. Their use is no longer limited to electrical technicians. For instrumentation and automation personnel serving production sites, understanding frequency converters, mastering their basic principles, and handling common faults is particularly important in actual production. Furthermore, it is a tool for improving one's own automation system capabilities. I. A Brief Introduction to Frequency Converters A frequency converter is a device that converts industrial frequency power (50Hz or 60Hz) into AC power of various frequencies to achieve variable speed operation of motors. 1. Basic Structure of a Frequency Converter 2. Functions of Each Circuit a. Control Circuit The control circuit controls the main circuit. It transmits signals to the rectifier, intermediate circuit, and inverter, while also receiving signals from these parts. All frequency converters use signals from the control circuit to switch the semiconductor devices of the inverter; this is a common feature of all frequency converters. b. Rectifier The rectifier is connected to a single-phase or three-phase AC power supply, generating a pulsating DC voltage. The rectifier circuit converts AC power into DC power (AC-DC conversion). c. Intermediate Circuit: The DC intermediate circuit smooths and filters the output of the rectifier circuit. It converts the rectified voltage into DC current; stabilizes and smooths the pulsating DC voltage for use by the inverter; and converts the fixed DC voltage after rectification into a variable AC voltage. d. Inverter: The inverter generates the frequency of the motor voltage, and the inverter circuit converts the DC power back into AC power (DC-AC conversion). II. Application of Frequency Converters in Production (Taking Fuji G7 Frequency Converter as an Example) 1. Control Principle of Frequency Converter (See Figure 2): The frequency conversion speed control device circuit consists of air switch QF2, AC contactor KM1, and frequency converter U1. The operation of U1 is controlled by the changeover switch button S1 and start switch button S2 installed on the electrical control cabinet panel; or the start and stop buttons installed on the explosion-proof operating column on site; and the start and stop buttons of the DCS control system. When starting U1, QF1 and QF2, as well as QF12 on the control circuit (see Figure 3) must be closed first: (1) PTC1 on the motor is energized for motor over-temperature protection; (2) The start switch button S2 on the electrical control cabinet panel is in the start position. When the air switch QF2 is energized, its linkage normally open contact closes, so that the AC contactor KM1 is energized; then the normally open contact of KM1 closes, and the frequency converter is in the energized state; (3) At this time, press the start button on the DCS system screen or the start button on the explosion-proof operating column on site, then K1 is energized, and similarly, the normally open contact of K1 closes; in this way, the frequency converter is in the running state, and at the same time, the normally open contact of K1 closes to self-protect the DCS start button or the start button on site. 2. Frequency converter frequency regulation circuit (see Figure 2 and Figure 4) (1) QF11 is closed, and through AC-DC power conversion, 24V power is provided to the "voltage U/current I" and "current I/current I" converters respectively. The DCS system screen displays a 0-100% signal. The control system outputs a 4-20mA current signal through the analog output card FM151, and the "current I/current I" converter converts it to provide the inverter with an appropriate current signal as the input of the inverter's analog input terminal (AM, AC). (2) After internal conversion, the output signal of the inverter's analog output terminal (FM, AC) is converted into a corresponding 4-20mA current signal through the "voltage U/current I" converter; the DCS control system analog input card FM148A displays the inverter's operating frequency percentage (%) on the DCS system screen, and the corresponding frequency value can be calculated. At present, the configuration software of the DCS system is very powerful. Through DCS program configuration, the operating frequency of the inverter can be directly displayed on the screen. 3. Inverter application extension For production safety, a bypass contactor KM2 is usually added to the inverter circuit; if KM1 or the inverter itself fails, the motor can still operate normally. Alternatively, a 10V power supply (A1, A3) and a 4-20mA current signal (A1, A2) can be provided through the external frequency setting terminal of the inverter. The frequency change of the inverter can be controlled by the change in the magnitude of the voltage or current signal. III. Analysis and Handling of Common Inverter Faults The causes of inverter faults are nothing more than external interference and internal faults. External factors include external electromagnetic interference, harsh installation environment, abnormal power supply conditions such as phase loss, low voltage, and power outage, as well as induced lightning strikes. 1. Overcurrent Fault (OC) Overcurrent faults can be divided into acceleration, deceleration, and constant speed overcurrent. This may be caused by the inverter's acceleration/deceleration time being too short, sudden load changes, uneven load distribution, output short circuits, etc. In this case, it can generally be resolved by extending the acceleration/deceleration time, reducing sudden load changes, adding external energy-consuming braking components, designing load distribution, and checking the circuit. If the inverter still has an overcurrent fault after disconnecting the load, it indicates that the inverter circuit is faulty and the inverter needs to be replaced. If these phenomena are not observed, it may be a false alarm. Press the reset button and restart to see if the overcurrent phenomenon still occurs. 2. Overload Protection (OL) Overload faults include inverter overload and motor overload. It may be caused by reasons such as too short acceleration time, excessive DC braking, too low grid voltage, or excessive load. It can generally be resolved by extending the acceleration time, extending the braking time, and checking the grid voltage. If the load is too heavy, the selected motor and inverter may not be able to drive the load, or it may be caused by poor mechanical lubrication. If it is the former, a higher power motor and inverter must be replaced; if it is the latter, the production machinery must be repaired. In addition, you can check whether the motor temperature is normal and whether the three-phase voltage is balanced: if it is unbalanced, check the inverter output; if it is balanced, consider whether the inverter's U/f curve setting is incorrect or the motor parameter setting is wrong. 3. Overheat Protection (OH) The only solution is ventilation. 4. Overvoltage Fault (OU) The overvoltage of the inverter is mainly manifested in the DC bus branch voltage. 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 power supply for about 1 minute and then reconnecting it will usually reset the converter. Another scenario is when the frequency converter is driving a large inertial load, leading to overvoltage. For this type of fault, one solution is to increase the deceleration time parameter, increase the braking resistor, or add a braking unit. Another solution is to set the frequency converter's stopping mode to free stop. 5. Other faults related to parameter settings: Once a parameter setting fault occurs, the frequency converter will not operate normally. Parameters can usually be modified according to the instruction manual. If this doesn't work, it's best to restore all parameters to factory settings. The method for restoring parameters varies depending on the company. Some frequency converters have a one-key recovery function, while others require step-by-step resetting. Generally, more advanced frequency converters offer more convenient and faster parameter restoration functions. Other faults include undervoltage (LU), overheating, hardware failure, and communication failure. IV. Conclusion Inverters have a wide range of applications. Although the failure rate of inverters in actual use is very low, to effectively utilize and use inverters in production, it is essential to understand their structure and principles, and be familiar with common faults. This is particularly beneficial for technical personnel, especially those in instrumentation and automation, and will be helpful in the design and application of automation systems. Only by rationally and effectively configuring the inverter system while meeting process requirements can equipment achieve greater efficiency.