Once the power lines are connected to the inverter’s output terminals (U, V, W), one of the converter circuits may activate due to a signal, leading to a dangerous short circuit. This can cause serious damage to the inverter components. Therefore, during the design phase of the inverter's main circuit, it is crucial to ensure that no power can accidentally enter the output side.
The inverter operates by first converting AC power into DC using a rectifier bridge, and then inverting it back into three-phase AC power through an inverter circuit. If the power line is mistakenly connected to the output (U, V, W) of the inverter, a signal may trigger one of the switching transistors, creating a direct short. This can quickly lead to the failure of the inverter module, potentially causing permanent damage.
To avoid such issues, especially for new users, it is essential to carefully follow the wiring instructions and never connect power without proper understanding. Improper connections can result in the inverter being destroyed, which can be both costly and dangerous. 
Below are the potential consequences of incorrectly connecting the input and output of a voltage-source high-voltage inverter.
**Consequences**
When the high-voltage input is incorrectly connected to the inverter’s isolation switch, and the user switch is closed, the power from the high-voltage bus will be directly routed to the inverter’s output. At this point, the capacitors inside each unit are uncharged, and their maximum allowable voltage drop is less than ±18V/DC. They cannot withstand high voltages. In the case of a misconnection, the peak voltage on each phase could reach as high as ±8164V—far exceeding the safe limits of IGBTs, thyristors, and capacitors. As a result, each phase cell becomes a conductor, forming a current path with no resistance. This leads to an enormous short-circuit current, which can cause the freewheeling diode in the IGBT module to explode, damaging the IGBT itself and other parallel components. The heat generated by this current can also melt cables and joints, causing further damage. Additionally, the sudden surge may trip the user’s circuit breaker, halting the short-circuit but leaving all units already damaged.
Moreover, when the inverter is connected to a high-voltage source due to reversed input and output, the phase-shifting transformer at the inverter’s input side becomes connected to the motor. This prevents normal power delivery to the unit, and the control system loses its power supply. If the user doesn’t detect the issue, the main control system won’t alert them, and the computer won’t record the incident or its timing.
**Preventive Measures**
1) After installation and commissioning, it is important to clearly distinguish and label the input and output cables.
2) When replacing cables, make sure to identify and mark the incoming and outgoing lines, and perform tests before reinstallation.
3) Any cable replacement or testing work should be done by at least two people, with mutual checks and, if needed, verification by an electrical engineer.
4) Always double-check the wiring labels on the inverter. Typically, L1, L2, and L3 indicate the input, while U, V, and W represent the output. Taking these precautions can significantly reduce the risk of dangerous misconnections.
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Shenzhen Unitronic Power System Co., Ltd , https://www.unitronicpower.com