1. Introduction
In the 1980s and 1990s, inverters began to enter the Chinese market and have since gained widespread acceptance. They are now widely used in industries such as metallurgy, textiles, printing, HVAC systems, water supply, and more. Inverters offer significant advantages over traditional speed control methods, including energy efficiency, compact design, and improved system performance. For example, elevators and trams can achieve automatic control, enhancing precision and product quality.
To boost production efficiency and product yield, inverters will play an even larger role in China's future industrial landscape. However, as electronic components, inverters have a theoretical lifespan. In practice, they may experience failures, as shown in Figure 1.
Figure 1: Fault diagram of the inverter
The failure rate of inverters is also influenced by correct usage, maintenance, and environmental conditions. From Figure 1, it’s clear that the repair and maintenance field for inverters has promising growth potential. One common fault is unbalanced output, which we will discuss in detail below.
2. Basic Working Principle of the Inverter
The three-phase AC output (U, V, W) from an inverter directly affects the performance of an asynchronous motor, including speed control and motor longevity. More importantly, it impacts the inverter's own lifespan. After maintenance, ensuring that the U, V, W waveforms meet specifications and that the voltage is balanced is the most fundamental requirement.
Inverters typically consist of a main circuit with power switching devices like IGBTs or GTOs. These components provide voltage regulation and frequency modulation to the motor. The control circuit regulates the output voltage, current, and frequency based on external commands. For applications requiring precise speed control or fast response, feedback signals from the drive circuit are used for closed-loop control.
Additionally, the inverter must include protection circuits against overvoltage, overcurrent, and overheating. These protections also safeguard the motor and transmission system, which directly affect the inverter’s output.
An inverter works in reverse of a rectifier, converting DC power into AC power at the desired frequency. It does this by turning on and off six power switches in the upper and lower bridges at specific intervals, as shown in Figure 2.
Figure 2: Schematic diagram of the inverter
As shown in Figure 2, S1–S6 form a bridge inverter circuit. When these six switches are alternately turned on and off, the phase difference between the U, V, and W outputs is achieved, producing a three-phase AC voltage of 3π. The voltage waveforms of S1–S6 in the driving circuit must be consistent to ensure output voltage balance. Figure 3 shows a typical IGBT gate drive circuit encountered during inverter maintenance.
Figure 3: Typical gate drive circuit
When the gate drive circuit is active, it outputs a positive gate voltage of 15V, which is sufficient to fully saturate the IGBT and minimize on-state losses. It also limits short-circuit current and reduces stress on the device. When the gate voltage is zero, the IGBT turns off, preventing unintended conduction due to dv/dt noise. A negative bias is applied to the gate to reduce turn-off losses. The reverse bias voltage for H-series IGBTs ranges from -5V to 15V.
3. Inverter Output Imbalance and Solutions
During real-world maintenance, unbalanced output (U, V, W) can occur in several ways:
(1) The inverter display shows "MISSMG MOTO PHASE," indicating a missing phase. If the detection circuit is faulty, check the IGBT module and drive circuit. If the IGBT is damaged, replace it along with components like optocouplers, PNP/NPN transistors, electrolytic capacitors, and voltage regulators.
(2) The output voltages of U, V, and W differ by about 100V (e.g., 380V). This could indicate no drive voltage or signal waveform in one of the drive circuits. Measuring between U-V and W-P can help identify the issue.
(3) The DC voltage between U, V, and W-N may show abnormal drive voltage or missing drive signal, causing a phase imbalance. This often results from issues in the drive circuit.
To resolve these problems, check if the drive circuit voltage is normal, if the optocoupler is damaged, or if the electrolytic capacitor is leaking. Using an oscilloscope to measure the six-channel waveforms ensures they meet technical standards, solving the problem effectively.
Another common issue is when the phase difference between the three-phase output exceeds 3%. Although the inverter can 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 diode values, capacitor dryness, or leakage. These imbalances cause slight voltage differences between the phases, which, while not immediately critical, are technically unacceptable.
Our company ensures high-quality repairs by screening and aging components, matching transistor and voltage regulator parameters, and conducting load tests. During testing, the motor runs smoothly, and the sound and performance remain consistent before and after repair.
4. Conclusion
Unbalanced three-phase output is a common inverter fault, but real-world scenarios often involve complex issues. We encourage open communication and continuous improvement to better serve our customers.
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