Overvoltage primarily manifests in the DC voltage of the inverter's DC bus. Under normal operating conditions, the inverter's DC voltage is the average value after three-phase full-wave rectification. If we calculate using a line voltage of 380V, the average DC voltage Ud = 1.35U_line = 513V. When an overvoltage occurs, the energy storage capacitor on the DC bus will be charged. When the voltage rises to approximately 800V, the inverter's protection system will activate.
Variable frequency drive schematic diagram
I. Overvoltage Analysis on the Power Input Side
Overvoltage mainly manifests in the DC voltage of the inverter's DC bus (point P1 in the schematic diagram). During normal operation, the inverter's DC voltage is the average value after three-phase full-wave rectification. If the line voltage is 380V, the average DC voltage Ud = 1.35U_line = 513V. When an overvoltage occurs, the energy storage capacitor on the DC bus will be charged. When the voltage rises to approximately 800V, the inverter's protection system will activate. Overvoltages on the power input side are generally caused by lightning strikes or overvoltages formed when the compensation capacitor is closed or opened. Their main characteristics are a large voltage change rate (dv/dt) and a large amplitude.
II. Analysis of Internal Overvoltage in Frequency Converter
If both the input and load sides of the inverter show problems, and the overvoltage fault persists even after disconnecting the load side, then an internal inverter fault can be identified. Voltage detection signals are typically acquired from the 530V DC voltage at the PN terminal of the intermediate DC circuit. Therefore, the most direct detection method is to measure the voltage of the intermediate DC bus using a DC voltmeter. A low DC bus voltage may indicate a low power input voltage or an open circuit in the rectifier circuit; a high DC bus voltage may indicate a high power input voltage or motor energy feedback. If the inverter reports an overvoltage when the DC bus voltage is normal, the first thing to consider is whether the circuit is damaged when using a voltmeter.
III. Overvoltage Analysis Caused by Load Side
The causes of overvoltage on the load side are complex, mainly due to the inverter's deceleration time being set too short during load deceleration; the motor being affected by external forces; or the potential energy load being released. Because of these reasons, the actual motor speed is higher than the inverter's commanded speed. At this time, the motor slip is negative, and the magnetic torque opposes the rotating braking torque. The motor is in a generating state, and the current is fed back to the inverter's DC energy storage capacitor through the inverter's six freewheeling diodes (see schematic diagram VD7, VD8, VD9, VD10, VD11, VD12), charging it and increasing its DC bus voltage. If the inverter does not take measures to dissipate this energy, it will cause the voltage of the capacitor in the intermediate DC circuit to rise. Tripping will occur when the limit is reached.
IV. Overvoltage Fault Handling Methods
1. Overvoltage handling methods caused by the power input side: In cases where there is a possibility of impulse overvoltage, overvoltage caused by lightning, or overvoltage formed when the compensation capacitor is closed or opened on the power input side, surge absorption devices or series reactors can be connected in parallel on the input side to solve the problem.
Variable frequency drive reactor
Inverter surge protection
2. Overvoltage handling methods caused by the load input side
(1) Increased deceleration time of variable frequency parameters or adoption of free stop.
If overvoltage occurs during shutdown, and the process does not specify a shutdown time or location, the deceleration time can be appropriately increased. The overvoltage during shutdown occurs because after the frequency converter issues the deceleration command, the actual motor speed is higher than the synchronous speed determined by the frequency converter. This causes reverse feedback through the freewheeling diode, leading to a voltage increase. Therefore, simply adjusting the frequency converter parameters is sufficient.
(2) DC braking stop
DC braking is primarily used for stopping conditions. It cannot be used if DC overvoltage occurs during constant-speed motor operation. DC braking involves applying a direct current to the motor, creating a stationary magnetic field. The motor rotor windings cut this magnetic field, generating braking torque and converting the load's kinetic energy into electrical energy. During this process, the motor overheats, and the braking time should not be too long. Because the voltage, frequency, and time are manually set during braking, it can only be used for stopping.
DC braking circuit diagram (this diagram does not include frequency converter)
(3) Add braking resistor to frequency converter
If an overvoltage occurs in a motor operating at constant speed, a braking resistor will be used. The principle is that when the inverter experiences overvoltage, the energy storage capacitor charges. When the inverter reaches its critical protection value, it will activate its protection mechanism. Adding a braking resistor activates the braking unit's circuit, releasing energy onto the resistor as heat, thus ensuring the inverter operates normally.
