Causes and measures for overvoltage generation of the inverter and prove that regenerative braking is a method to solve the overvoltage phenomenon

The overvoltage of the inverter refers to the voltage of the inverter exceeding the rated voltage due to various reasons, and 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 a line voltage of 380V, the average DC voltage Ud=1.35U line = 513V.
When an overvoltage occurs, the storage capacitor on the DC bus will be charged. When the voltage rises to about 700V, the inverter overvoltage protection action (depending on the model). There are two main causes of overvoltage: power supply overvoltage and regenerative overvoltage. The overvoltage of the power supply means that the DC bus voltage exceeds the rated value because the power supply voltage is too high. Most inverters now have an input voltage of up to 460V, so the overvoltage caused by the power supply is extremely rare.

The main issue discussed in this paper is the regenerative overvoltage. The main reason for generating regenerative overvoltage is 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). For these reasons, the actual motor speed is higher than the command speed of the inverter, that is, the motor rotor speed exceeds the synchronous speed. At this time, the slip 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 is a braking torque that hinders the direction of rotation. Therefore, the motor is actually in a power generation state, and the kinetic energy of the load is "regenerated" into electrical energy.

The regenerative energy charges the inverter DC storage capacitor through the freewheeling diode of the inverter, so that the DC bus voltage rises, which is the regenerative overvoltage. Since the torque generated during the process of regenerating the overvoltage is opposite to the original torque, it is the braking torque, so the process of regenerating the overvoltage is the process of regenerative braking. In other words, the regenerative energy is eliminated and the braking torque is increased. If the regenerative energy is not large, the inverter and the motor itself have 20% regenerative braking capacity, and 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 link will be overcharged, and the overvoltage protection function of the inverter will operate 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.

Since the causes of overvoltages are different, the countermeasures taken are different. For the overvoltage phenomenon generated during the parking process, if there is no special requirement for the parking time or position, it can be solved by extending the deceleration time of the inverter or free parking. The so-called free stop means that the inverter disconnects the main switching device and allows the motor to coast and stop.

If there is a certain requirement for parking time or parking position, DC braking (DC braking) function can be used. The DC braking function is to decelerate the motor to a certain frequency and then input DC power into the stator winding of the motor to form a static magnetic field. The rotor winding of the motor cuts this magnetic field to generate a braking torque, so that the kinetic energy of the load is converted into electrical energy and is consumed in the form of heat in the rotor circuit of the motor. Therefore, this braking is also called energy braking. In the process of DC braking, two processes of regenerative braking and energy braking are actually included. This braking method is only 30-60% efficient for regenerative braking and has a low braking torque. Since the motor is overheated by consuming energy in the motor, the braking time should not be too long. Moreover, the DC braking start frequency, braking time and braking voltage are all manually set and cannot be automatically adjusted according to the level of the regenerative voltage. Therefore, DC braking cannot be used for overvoltage generated during normal operation, and can only be used for Braking when parking.

For deceleration (from high speed to low speed, but not stopping), the overvoltage generated by the excessive GD2 (flywheel torque) of the load can be solved by appropriately extending the deceleration time. In fact, this method also uses the principle of regenerative braking. The deceleration time is only to control the charging speed of the load to the inverter, so that the 20% regenerative braking capability of the inverter itself can be rationally utilized. As for the load that causes the motor to regenerate due to the action of external force (including the potential discharge), since it is normally in the braking state, the regenerative energy is too high to be consumed by the inverter itself, so it is impossible to use DC braking or The method of extending the deceleration time.

Compared with DC braking, regenerative braking has higher braking torque, and the braking torque can be related to the braking torque required by the load (ie, the level of regenerative energy). Automatic control. Regenerative braking is therefore best suited to provide braking torque to the load during normal operation.

1. Energy consumption type:
In this method, a braking resistor is connected in parallel with the DC link of the inverter, and the switching of a power tube is controlled by detecting the DC bus voltage. When the DC bus voltage rises to about 700V, the power tube is turned on, and the regenerative energy is supplied to the resistor to be consumed as heat energy, thereby preventing the DC voltage from rising. Since the regenerative energy is not utilized, it is energy-consuming. The same energy consumption type, it differs from DC braking in that it consumes energy on the braking resistor outside the motor, and the motor does not overheat, so it can work more frequently.

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