VFDs frequently encounter various problems during commissioning and operation, with overvoltage being the most common.

When overvoltage occurs, the frequency inverter's overvoltage protection function will activate to prevent damage to its internal circuitry, causing the inverter to stop operating and preventing normal equipment operation.

Therefore, measures must be taken to eliminate overvoltage and prevent malfunctions. Since frequency inverters and motors have different applications and therefore different causes of overvoltage, appropriate countermeasures should be taken based on the specific circumstances.

Overvoltage Generation and Regenerative Braking in Frequency Inverters

Overvoltage in frequency inverters refers to the voltage exceeding the rated voltage due to various reasons, primarily manifested in the DC voltage of the inverter's DC bus.

Under normal operation, the DC voltage of the frequency inverter is the average value after three-phase full-wave rectification. If calculated using a 380V line voltage, the average DC voltage Ud = 1.35Uline = 513V.

When an overvoltage occurs, the energy storage capacitor on the DC bus will be charged. When the voltage rises to approximately 700V (depending on the model), the inverter's overvoltage protection will activate.

There are two main causes of inverter overvoltage: power supply overvoltage and regenerative overvoltage.

Power supply overvoltage refers to the DC bus voltage exceeding its rated value due to excessively high power supply voltage. However, most inverters now have an input voltage that can reach up to 460V, therefore, power supply-related overvoltages are extremely rare.

This article mainly discusses regenerative overvoltage.

The main causes of regenerative overvoltage are as follows: The inverter's deceleration time is set too short when a large GD2 (flywheel torque) load is decelerated; the motor is subjected to external forces (fans, stretchers) or potential energy loads (elevators, cranes) during lowering. Due to these reasons, the actual motor speed is higher than the inverter's commanded speed, meaning the motor rotor speed exceeds the synchronous speed. At this time, the motor's slip is negative, and the direction in which the rotor windings cut the rotating magnetic field is opposite to the direction of the motor's normal operation. The resulting electromagnetic torque is a braking torque that opposes the direction of rotation. Therefore, the motor is actually in a generating state, and the kinetic energy of the load is "regenerated" into electrical energy.

The regenerated energy charges the DC energy storage capacitor of the inverter via the freewheeling diode in the inverter section, causing the DC bus voltage to rise. This is regenerative overvoltage. Because the torque generated during regenerative overvoltage is opposite to the original torque, it is a braking torque; therefore, the process of regenerative overvoltage is also the process of regenerative braking.

In other words, eliminating regenerative energy increases the braking torque. If the regenerative energy is small, since the inverter and motor themselves have a 20% regenerative braking capacity, this portion of electrical energy will be consumed by the inverter and motor. If this energy exceeds the consumption capacity of the inverter and motor, the capacitor in the DC circuit will be overcharged, and the inverter's overvoltage protection function will activate, stopping operation. To avoid this situation, this energy must be disposed of in a timely manner, which also increases the braking torque; this is the purpose of regenerative braking.

Measures to prevent inverter overvoltage: 

Because the causes of overvoltage are different, the countermeasures also differ. For overvoltage phenomena occurring during parking, if there are no specific requirements for parking time or location, the problem can be solved by extending the inverter's deceleration time or using free-stop methods. Free-stop means the inverter disconnects the main switching device, allowing the motor to coast to a stop.

If there are specific requirements for parking time or location, DC braking can be used.

DC braking involves decelerating the motor to a certain frequency, then applying direct current to the stator windings to create a stationary magnetic field.

The rotor windings cut this magnetic field, generating a braking torque that converts the load's kinetic energy into electrical energy, which is then dissipated as heat in the rotor circuit. Therefore, this type of braking is also called regenerative braking. DC braking actually includes both regenerative braking and regenerative braking processes. This braking method is only 30-60% as efficient as regenerative braking, and the braking torque is relatively small. Because dissipating energy in the motor can cause overheating, the braking time should not be too long.

Furthermore, the starting frequency, braking time, and braking voltage of DC braking are all manually set and cannot be automatically adjusted according to the level of regenerative voltage. Therefore, DC braking cannot be used for overvoltages generated during normal operation; it can only be used for braking during stopping.

For overvoltages caused by excessive GD2 (flywheel torque) of the load during deceleration (from high speed to low speed without stopping), the deceleration time can be appropriately extended to resolve the issue. This method also utilizes the principle of regenerative braking; extending the deceleration time simply controls the charging speed of the inverter by the load's regenerative voltage, thus making reasonable use of the inverter's 20% regenerative braking capacity. As for loads where the motor is in a regenerative state due to external forces (including potential energy release), since they are operating in a braking state during normal operation, the excessive regenerative energy cannot be dissipated by the inverter itself, making DC braking or extending the deceleration time impossible.

Compared to DC braking, regenerative braking has a higher braking torque, and the magnitude of the braking torque can be automatically controlled by the inverter's braking unit according to the braking torque required by the load (i.e., the level of regenerative energy). Therefore, regenerative braking is most suitable for providing braking torque to the load during normal operation.

Methods of Variable Frequency Regenerative Braking:

1. Energy Dissipation Type: This method involves connecting a braking resistor in parallel with the DC circuit of the frequency converter. The switching of a power transistor is controlled by detecting the DC bus voltage. When the DC bus voltage rises to approximately 700V, the power transistor conducts, channeling regenerative energy into the resistor, where it is dissipated as heat, thus preventing the DC voltage from rising further. Since the regenerative energy is not utilized, it is considered an energy dissipation type. While also an energy dissipation type, it differs from DC braking in that the energy is dissipated in a braking resistor outside the motor, preventing overheating and allowing for more frequent operation.

2. Parallel DC Bus Absorption Type: This method is suitable for multi-motor drive systems (such as drawing machines). In this system, each motor requires its own frequency converter, and multiple frequency converters share a single grid-side converter. All inverters are connected in parallel to a common DC bus. In this system, one or more motors typically operate normally in braking mode. The motor in braking mode is driven by other motors, generating regenerative energy, which is then absorbed by the motors in motoring mode through the parallel DC bus. If the energy cannot be fully absorbed, it is dissipated through a shared braking resistor. Here, a portion of the regenerated energy is absorbed and utilized, but not fed back into the grid.

3. Energy Feedback Type: In an energy feedback type inverter, the grid-side converter is reversible. When regenerated energy is generated, the reversible converter feeds the regenerated energy back to the grid, ensuring full utilization of the regenerated energy. However, this method requires high power supply stability; a sudden power outage will cause the inverter to fail.