Les fournisseurs d'unités de retour d'énergie rappellent que les convertisseurs de fréquence rencontrent fréquemment divers problèmes lors de leur mise en service et de leur utilisation, parmi lesquels la surtension est la plus courante. En cas de surtension, afin de prévenir tout dommage au circuit interne, la fonction de protection contre les surtensions du convertisseur de fréquence s'active, entraînant son arrêt et, par conséquent, un dysfonctionnement de l'équipement.
Il est donc impératif de prendre des mesures pour éliminer les surtensions et prévenir les défauts. Les causes de surtension variant selon les applications des convertisseurs de fréquence et des moteurs, des mesures adaptées doivent être mises en œuvre en fonction de chaque situation.
Génération de surtension dans le convertisseur de fréquence et freinage régénératif
The so-called overvoltage of a frequency converter refers to the situation where the voltage of the frequency converter exceeds the rated voltage due to various reasons, which is mainly manifested in the DC voltage of the frequency converter's DC bus.
During normal operation, the DC voltage of the frequency converter is the average value after three-phase full wave rectification. If calculated based on a 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 around 700V (depending on the model), the overvoltage protection of the frequency converter will be activated.
There are two main reasons for overvoltage in frequency converters: power overvoltage and regenerative overvoltage.
Power overvoltage refers to the situation where the DC bus voltage exceeds the rated value due to excessive power supply voltage. Nowadays, the input voltage of most frequency converters can reach up to 460V, so overvoltage caused by power supply is extremely rare.
The main issue discussed in this article is the regeneration of overvoltage.
The main reasons for generating regenerative overvoltage are as follows: when the load of GD2 (flywheel torque) decelerates, the deceleration time set by the frequency converter is too short;
The motor is subjected to external forces (such as fans and stretching machines) or potential loads (such as elevators and cranes) when lowered. Due to these reasons, the actual speed of the motor is higher than the commanded speed of the frequency converter, which means that the rotor speed of the motor exceeds the synchronous speed. At this time, the slip rate of the motor is negative, 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 direction of rotation. So the electric motor is actually in a generating state, and the kinetic energy of the load is' regenerated 'into electrical energy.
The regenerative energy is charged to the DC energy storage capacitor of the inverter through the freewheeling diode of the inverter, causing the DC bus voltage to rise, which is called regenerative overvoltage. The torque generated during the process of regenerating overvoltage is opposite to the original torque, which is the braking torque. Therefore, the process of regenerating overvoltage is also the process of regenerative braking.
In other words, eliminating regenerative energy increases braking torque. If the regenerative energy is not large, the inverter and motor themselves have a regenerative braking capacity of 20, and this part of the electrical energy will be consumed by the inverter and motor. If this energy exceeds the consumption capacity of the frequency converter and motor, the capacitor of the DC circuit will be overcharged, and the overvoltage protection function of the frequency converter will be activated, causing the operation to stop. To avoid this situation, it is necessary to dispose of this energy in a timely manner, while also increasing the braking torque, which is the purpose of regenerative braking.
Measures to prevent overvoltage of frequency converters
Due to different causes of overvoltage, the measures taken are also different. For the overvoltage phenomenon generated during parking, if there are no special requirements for parking time or location, the method of extending the deceleration time of the frequency converter or free parking can be used to solve it. The so-called free parking refers to the frequency converter disconnecting the main switch device, allowing the motor to slide freely and stop.
If there are certain requirements for parking time or parking location, DC braking function can be used.
The DC braking function is to slow down the motor to a certain frequency, and then apply DC power to the stator winding of the motor to form a static magnetic field.
The motor rotor winding cuts this magnetic field and generates a braking torque, which converts the kinetic energy of the load into electrical energy and consumes it in the form of heat in the motor rotor circuit. Therefore, this type of braking is also known as energy consuming braking. The process of DC braking actually includes two processes: regenerative braking and energy consumption braking. This braking method has an efficiency of only 30-60% of regenerative braking, and the braking torque is relatively small. Due to the fact that consuming energy into the motor can cause overheating, the braking time should not be too long.
Moreover, the starting frequency, braking time, and braking voltage of DC braking are all manually set and cannot be automatically adjusted based on the level of regenerative voltage. Therefore, DC braking cannot be used for overvoltage generated during normal operation and can only be used for braking during parking.
For the overvoltage caused by the excessive GD2 (flywheel torque) of the load during deceleration (from high speed to low speed without stopping), the method of extending the deceleration time appropriately can be adopted to solve it. In fact, this method also utilizes the principle of regenerative braking. Extending the deceleration time only controls the charging speed of the inverter by the regenerative voltage of the load, so as to make reasonable use of the regenerative braking capacity of the inverter itself. As for the loads that cause the motor to be in a regenerative state due to external forces (including potential energy release), since they operate normally in a braking state, the regenerative energy is too high to be consumed by the frequency converter itself. Therefore, it is impossible to use DC braking or extend the deceleration time.
Compared with DC braking, regenerative braking has a higher braking torque, and the magnitude of the braking torque can be automatically controlled by the braking unit of the frequency converter according to the required braking torque of 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.
Method of frequency conversion regenerative braking:
1. Energy consuming type:
This method involves paralleling a braking resistor in the DC circuit of a frequency converter, and controlling the on/off of a power transistor by detecting the DC bus voltage. When the DC bus voltage rises to around 700V, the power transistor conducts, passing the regenerated energy into the resistor and consuming it in the form of thermal energy, thereby preventing the rise of DC voltage. Due to the inability to utilize regenerated energy, it belongs to the energy consumption type. As an energy consuming type, its difference from DC braking is that it consumes energy on the braking resistor outside the motor, so the motor will not overheat and can work more frequently.
2. Parallel DC bus absorption type:
Suitable for multi motor drive systems (such as stretching machines), in which each motor requires a frequency converter, multiple frequency converters share a grid side converter, and all inverters are connected in parallel to a common DC bus. In this system, there is often one or several motors working normally in the braking state. The motor in the braking state is dragged by other motors to generate regenerative energy, which is then absorbed by the motor in the electric state through a parallel DC bus. If it cannot be fully absorbed, it will be consumed through a shared braking resistor. The regenerated energy here is partially absorbed and utilized, but not fed back into the power grid.
3. Energy feedback type:
Le convertisseur côté réseau de l'onduleur à récupération d'énergie est réversible. Lorsqu'une énergie est produite par récupération d'énergie, le convertisseur réversible la réinjecte dans le réseau, permettant ainsi son utilisation optimale. Cependant, cette méthode exige une grande stabilité de l'alimentation électrique ; en cas de coupure de courant soudaine, une inversion de polarité peut se produire.