Inverters often encounter a variety of problems during commissioning and use, during which overvoltage is the most common. After the overvoltage occurs, in order to avoid damage to the internal circuit, the overvoltage protection function of the inverter will operate, so that the inverter will operate intermittently, resulting in the equipment can not operate normally.
The so-called overvoltage of the inverter refers to the inverter voltage that exceeds the rated voltage due to various reasons, which is reflected in the DC voltage of the inverter DC bus.
During normal operation, the voltage of the tributary part of the inverter is the uniform value after three-phase full-wave rectification. If the 380V line voltage is calculated, the uniform DC voltage Ud=1.35U line=513V. When an overvoltage occurs, the energy storage capacitor on the DC bus will be charged, and when the voltage rises to about 700V, the inverter will perform an overvoltage protection action (depending on the model).
In fact, there are two main causes of inverter overvoltage: power supply overvoltage and regeneration overvoltage.
1. Power supply overvoltage
Power supply overvoltage refers to the DC bus voltage exceeding the rated value due to the power supply voltage being too high. Nowadays, the input voltage of most frequency converters can reach up to 460V, so overvoltages caused by power supplies are extremely rare.
2. Regeneration overvoltage
The main reasons for the occurrence of regenerative overvoltage are as follows: when the flywheel torque GD2 load is decelerated, the deceleration time of the inverter is set too short; The motor is affected by external forces (e.g. fans, drafters) or under potential loads (e.g. elevators, cranes). Due to these reasons, the actual speed of the motor is higher than the command speed of the inverter, that is, the rotor speed of the motor exceeds the synchronous speed, and the slip rate of the motor is negative, and the direction of the rotor winding cutting the rotating magnetic field is opposite to the condition of the motor, and the electromagnetic torque that occurs is the braking torque that prevents the direction of rotation. So the motor is actually in a power generation situation, and the kinetic energy of the load is "regenerated" into electrical energy.
The regenerative energy is charged by the freewheeling diode of the inverter to the DC energy storage capacitor of the inverter, so that the DC bus voltage rises, which is the so-called regenerative overvoltage. Because the torque that occurs in the process of regenerating the overvoltage is the opposite of the original torque, it is the braking torque, so the process of regenerating the overvoltage is also the process of regenerating the braking.
In other words, regenerative energy is eliminated, which increases braking torque. If the regenerative energy is not large, because the inverter and the motor itself have a regenerative braking ability 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 capacitance of the DC circuit will be overcharged, and the overvoltage protection function of the inverter will operate, so that the operation will be interrupted. In order to avoid this situation, it is necessary to dispose of this part of the energy in time, and also increase the braking torque, which is the purpose of regenerative braking.
In order to avoid the occurrence of problems, it is necessary to adopt useful methods to eliminate overvoltages. However, since the application of the inverter and the motor are different, and the cause of overvoltage is different, the corresponding countermeasures should be adopted according to the specific situation.
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