1. Introduction
In the 1980s and 1990s, inverters began to enter the Chinese market and have since gained widespread acceptance. They are now commonly used in various industries such as metallurgy, textiles, printing, HVAC systems, and water supply. The main advantage of inverters lies in their ability to provide efficient speed control for electric drives compared to traditional methods. This includes energy savings, compact design, and improved system performance, especially in applications like elevators and trams where automatic control is essential. As a result, inverters are expected to play an even greater role in China’s industrial development in the future.
Since inverters are electronic devices, they have a theoretical lifespan. However, in real-world applications, failures can occur. The failure probability is illustrated in Figure 1.
Figure 1: Inverter Fault Diagram
The failure rate of inverters is also influenced by correct usage, maintenance, and environmental conditions. From Figure 1, it's clear that the field of inverter repair holds promising opportunities. One common fault is unbalanced output from the inverter, which we will discuss in detail below.
2. Basic Working Principle of Inverters
The quality of the three-phase AC output (U, V, W) and its voltage balance directly affects the performance of asynchronous motors, motor lifespan, and the overall reliability of the inverter. After maintenance, ensuring that the U, V, W outputs meet the required standards is crucial.
Inverters typically consist of power switching components such as IGBTs or GTOs, which convert DC power into AC power with variable frequency. The control circuit regulates the voltage, current, and frequency based on external commands. For precise speed control or fast response applications, feedback signals from the drive system are used for closed-loop control. Additionally, protection circuits are necessary to prevent overvoltage, overcurrent, and overheating, safeguarding both the inverter and the connected motor.
An inverter works opposite to a rectifier. It converts DC power into AC power at the desired frequency by turning on and off six power switches (S1–S6) in a specific sequence, as shown in Figure 2.
Figure 2: Inverter Schematic
In this configuration, S1 through S6 form a bridge inverter. When these switches are alternately activated, a balanced three-phase AC voltage is generated. The gate drive circuit plays a critical role in maintaining the balance of the output voltage. A typical IGBT gate drive circuit is shown in Figure 3.
Figure 3: Typical IGBT Gate Drive Circuit
When the gate drive circuit is active, it provides a +15V gate voltage, which ensures the IGBT is fully saturated, minimizing conduction losses. During turn-off, a -5V bias is applied to reduce switching losses. Proper gate drive is essential for stable inverter operation.
3. Inverter Output Imbalance and Solutions
During maintenance, output imbalance between U, V, and W can be categorized into different cases:
(1) If the inverter display shows “MISSMG MOTO PHASE,†it indicates a missing phase. In such cases, the IGBT module and drive circuit should be checked. Replacing damaged components like optocouplers, transistors, capacitors, and voltage regulators usually resolves the issue.
(2) A voltage difference of about 100V between U, V, and W (e.g., 380V output) may indicate a faulty drive circuit. Checking the drive signal and voltage levels across S1–S6 can help identify the problem.
(3) If the DC voltage between U, V, and W-N is abnormal, it could mean a problem with the drive circuit. This can lead to imbalances in the phase voltages. Testing the drive signals with an oscilloscope and replacing faulty components is a common solution.
Another issue is when the phase difference between U, V, and W exceeds 3%. Although the inverter may still operate, it cannot handle heavy loads for long periods. This is often due to asymmetry in the drive circuits, such as differences in transistor parameters, Zener diodes, or capacitor leakage. To ensure reliable performance, we conduct thorough testing, including load tests and motor sound checks, to guarantee balanced and stable output after repairs.
4. Conclusion
Unbalanced three-phase output is a common inverter fault, but it can present complex challenges in practice. By understanding the root causes and applying proper diagnostic techniques, most issues can be resolved. We encourage open communication among professionals and continue to strive for excellence in our services to better meet customer needs.
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