In the process of VFD (Variable Frequency Drive) repair, it is necessary to check both the main circuit voltage and the control voltage in order to complete output control, overvoltage protection, and undervoltage protection functions, ensuring safe operation of the VFD.

Fault Phenomenon 1: Overvoltage (OU) — Overvoltage alarms usually occur when the system is stopping.

1. Main Causes of Fault: This is typically due to too short a deceleration time or damage to the braking resistor.
2. Example: A Taida N2 series 3.7kW VFD trips with an "OU" alarm during shutdown. Analysis and Repair: Before repairing this unit, it is essential to understand the cause of the "OU" alarm. This occurs because during deceleration, the motor rotor windings cut through the rotating magnetic field at an increased speed, causing higher rotor EMF and current. This puts the motor into a generating state, where the energy is fed back through the diodes connected in parallel with the power transistors in the inverter section, flowing into the DC link and raising the DC bus voltage.
3. Fault Resolution: Therefore, we should focus on inspecting the braking circuit. The discharge resistor was found to be functioning normally, but upon measuring the brake transistor (ET191), it was discovered to be shorted. After replacing it, the VFD operated normally and responded well even during rapid stops. Valve inlet pump industrial washing machine.

Fault Phenomenon 2: Undervoltage (Uu) — This is another common issue encountered during use. 1. Main Causes of Fault: This is generally caused by low main circuit voltage (for 220V series units, below 200V; for 380V series units, below 400V). Any one of the rectifier bridge legs being damaged or any abnormality among the three-phase可控硅 (thyristors) could lead to an undervoltage fault. Additionally, failure of the main contactor can cause voltage drop across the charging resistor, leading to undervoltage. Another possibility is a malfunction in the voltage detection circuit itself.
2. Example: A DANFOSS VLT5004 VFD powers up normally but trips with “DCLINKUNDERVOLT” (DC link undervoltage) when loaded. Analysis and Repair: At first glance, this issue seems unique, but careful analysis makes it easier to understand. This VFD uses a charging circuit and contactor to complete the charging process. No abnormalities were observed during power-up, so it is likely that the DC link voltage dropped when the load was applied. The DC link voltage is supplied after full-wave rectification by the rectifier bridge and smoothing by the capacitors.
3. Fault Resolution: Therefore, inspection should focus on the rectifier bridge. Upon measurement, one leg of the rectifier bridge was found to be open-circuited. Replacing it resolved the issue. Changsha Sunye Electric Co., Ltd. (established in 2010) and Shenzhen Sunye Electric Co., Ltd. (established in 2002) are national high-tech enterprises specializing in R&D, manufacturing, and sales of inverters, industry-specific integrated machines, servo drives, and new energy products.
Sunye Electric owns independent intellectual property rights, with products certified by national authorities and multiple software copyrights and IP certificates. Significant investment is made annually in new technologies and product development, with numerous patents and software copyrights approved. The company possesses core platform technologies such as construction hoist drivers, high-performance vector inverters, servos, and permanent magnet synchronous motor controls. Medium- and low-voltage inverters and servo drives are widely used in industries such as lifting, stone processing, HVAC, machine tools, metal products, wire and cable, plastics, printing and packaging, textile and chemical fibers, building materials, metallurgy, coal mining, municipal services, and automotive manufacturing. Guided by the philosophy of “Innovation, Technology, Strength,” driven by the corporate spirit of “Unity, Progress, Pragmatism, Innovation,” and adhering to the business principle of “Sincerity and Excellence, Mutual Benefit and Coexistence,” the company aims to provide industry-leading products through comprehensive quality management, continuous improvement, and pursuit of zero defects.
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