Inverters often encounter various problems during commissioning and use, among which overvoltage is the most common.
When an overvoltage occurs, the inverter's overvoltage protection function will activate to prevent damage to its internal circuitry, causing the inverter to stop operating and preventing the equipment from functioning properly. Therefore, measures must be taken to eliminate the overvoltage and prevent the fault from occurring. Since inverters and motors have different applications and the causes of overvoltages differ, appropriate countermeasures should be taken based on the specific situation.
Overvoltage generation and regenerative braking
Overvoltage in a frequency converter refers to a 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 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 around 700V (depending on the model), the inverter's overvoltage protection will activate. There are two main causes of 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 modern inverters have an input voltage that can reach up to 460V, therefore, power supply-related overvoltages are extremely rare.
This article primarily 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 decelerates; 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; that is, 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.
Regenerative energy charges the inverter's DC energy storage capacitor via the inverter's freewheeling diodes, 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 regenerative overvoltage process is also a regenerative braking process. 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 energy will be consumed by the inverter and motor. If this energy exceeds the inverter and motor's consumption capacity, the DC circuit capacitor will be overcharged, triggering the inverter's overvoltage protection function and stopping operation. To avoid this situation, this energy must be disposed of promptly, which also increases the braking torque; this is the purpose of regenerative braking.
1. Methods for handling overvoltage in frequency converters
(1) For the breaking overvoltage of the phase-shifting transformer of the frequency converter, the overvoltage absorption circuit is composed of RC absorption network and zinc oxide surge arrester, which achieves good results.
(2) For overvoltage generated by transformer switching on under load, a switch with good periodic performance can be selected (switches will become out of period after long-term operation); a good RC absorption circuit or active suppressor technology can be adopted; a transformer with electrostatic shielding measures can also effectively suppress switching overvoltage. However, the difficulty in making electrostatic shielding layers for high-power transformers is quite high.
(3) Regarding the overvoltage generated by the commutation of the rectifier element, the following points should be noted: the reverse withstand voltage of the rectifier element must be sufficient, and the absorption circuit and freewheeling circuit must be properly designed. Otherwise, the rectifier device may be damaged by the overvoltage.
(4) Since the overvoltage during inverter operation is mainly generated when the transformer is switched on and off, the overvoltage of the inverter should be suppressed starting from the transformer. This can be achieved by:
① Increasing the transformer's magnetizing inductance and capacitance to ground, and increasing the magnetizing inductance reduces the no-load current, will increase the cost of the transformer.
②Increase the transformer's capacitance to ground: In principle, this is easy to analyze, but in reality, due to the limitations of the transformer's structure and materials, it is unlikely to make a transformer with any insulation method or high insulation level. Therefore, it is also quite difficult to significantly increase the transformer's capacitance to ground C.
