The frequency converter often encounters various problems during debugging and use, among which the overvoltage phenomenon is the most common. After the overvoltage occurs, in order to prevent the internal circuit from being damaged, the overvoltage protection function of the inverter will act to make the inverter stop running, causing the equipment to fail to work normally.
The so-called overvoltage of the inverter means that the voltage of the inverter exceeds the rated voltage due to various reasons, which is concentrated on the DC voltage of the DC bus of the inverter.
During normal operation, the DC voltage of the inverter is the average value after three-phase full-wave rectification. If calculated with 380V line voltage, the average DC voltage Ud=1.35U line=513V.
When overvoltage occurs, the energy storage capacitor on the DC bus will be charged. When the voltage rises to about 700V, (varies by model) the inverter overvoltage protection will act.
There are two main reasons for the overvoltage of the inverter: power supply overvoltage and regeneration overvoltage.
Power supply overvoltage means that the DC bus voltage exceeds the rated value due to excessive power supply voltage. However, the input voltage of most frequency converters now can reach up to 460V, so the overvoltage caused by the power supply is extremely rare.
The main problem discussed in this article is regeneration overvoltage.
The main reasons for regenerative overvoltage are as follows: when the large GD2 (flywheel torque) load decelerates, the deceleration time of the inverter is set too short; the motor is affected by external force (fan, drafting machine) or potential energy load (elevator, crane). Due to these reasons, the actual speed of the motor is higher than the command speed of the inverter, that is to say, the rotor speed of the motor exceeds the synchronous speed, the slip of the motor is negative at this time, and the direction of the rotor winding cutting the rotating magnetic field is opposite to that of the motor state. , the electromagnetic torque generated by it is the braking torque that hinders the rotation direction. So the motor is actually in a state of generating electricity, and the kinetic energy of the load is "regenerated" into electrical energy. The regenerative energy charges the DC energy storage capacitor of the inverter through the freewheeling diode of the inverter part, so that the DC bus voltage rises, which is the regeneration overvoltage.
The torque generated in the process of regenerative overvoltage is opposite to the original torque, which is braking torque, so the process of regenerative overvoltage is also the process of regenerative braking. In other words, the regenerative energy is eliminated, which increases the braking torque. If the regenerative energy is not large, because the inverter and the motor itself have a regenerative braking capacity of 20%, this part of the electric energy will be consumed by the inverter and the motor. If this part of the energy exceeds the consumption capacity of the inverter and the motor, the capacitor of the DC circuit will be overcharged, and the overvoltage protection function of the inverter will act to stop the operation.
In order to avoid this situation, this part of the energy must be disposed of in time, and the braking torque is also increased, which is the purpose of regenerative braking.
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