How to Fix DC Bus Overvoltage in Inverters

Overvoltage Fault Analysis in Inverter Voltage Detection

The inverter is the core component of modern electric drive systems, enabling various motor speed control functions and operational requirements. During normal operation, to ensure system safety and stability, the inverter continuously monitors key operating parameters—such as voltage, current, temperature, and frequency—to guarantee proper equipment function. This article provides a brief analysis of overvoltage-related faults in the inverter’s voltage detection circuitry.

Inverter overvoltage typically refers to the DC bus voltage exceeding a safe threshold, posing a risk to internal components and triggering a protective shutdown. Under normal conditions, the DC bus voltage is the average value after three-phase full-wave rectification and filtering. For a 380V AC input, the theoretical DC bus voltage is:
Ud = 380V × 1.414 ≈ 537V.

During an overvoltage event, the main DC bus capacitor charges and stores energy, causing the bus voltage to rise. When the voltage approaches the capacitor’s rated voltage (around 800V), the inverter activates overvoltage protection and shuts down. Failure to do so may degrade performance or cause permanent damage. Generally, inverter overvoltage can be attributed to two main causes: power supply issues and load-related feedback.

1. Excessively High Input AC Voltage

If the input AC supply voltage exceeds the allowable range—due to grid voltage surges, transformer faults, faulty cabling, or overvoltage from diesel generators—overvoltage can occur. In such cases, it is recommended to disconnect the power supply, inspect and rectify the issue, and only restart the inverter once the input voltage returns to normal.

2. Regenerative Energy from the Load

This is common with high-inertia loads, where the motor’s synchronous speed exceeds the actual output speed of the inverter. The motor then operates in generator mode, feeding electrical energy back into the inverter and causing the DC bus voltage to rise beyond safe limits, resulting in an overvoltage fault. This issue can be addressed through the following measures:

(1) Extend Deceleration Time

Overvoltage in high-inertia systems often results from too-short deceleration settings. During rapid deceleration, mechanical inertia keeps the motor spinning, causing its synchronous speed to exceed the inverter’s output frequency. This drives the motor into regenerative mode. By extending the deceleration time, the inverter reduces its output frequency more gradually, ensuring the motor’s synchronous speed remains below the inverter’s output speed, thus preventing regeneration.

(2) Enable Overvoltage Stall Prevention (Overvoltage Stall Inhibition)

Since overvoltage often occurs due to excessively rapid frequency reduction, this function monitors the DC bus voltage. If the voltage rises to a preset threshold, the inverter automatically slows the rate of frequency decrease, maintaining the output speed above the motor’s synchronous speed to prevent regeneration.

(3) Use Dynamic Braking (Resistor Braking)

Activate the dynamic braking function to dissipate excess regenerative energy through a braking resistor. This prevents the DC bus voltage from rising beyond safe levels.

(4) Additional Solutions

• Install a regenerative feedback unit to return excess energy back to the power grid.

• Use a common DC bus configuration, connecting the DC buses of two or more inverters in parallel. Excess energy from a regenerating inverter can then be absorbed by other inverters driving motors in motoring mode, helping to stabilize the DC bus voltage.