How to solve the low voltage tripping problem of frequency converter

The low voltage of the frequency converter mainly refers to the low voltage of the intermediate DC circuit. Generally, the causes of low voltage of the intermediate DC circuit come from two aspects:

1. Low voltage from the power input side

Under normal circumstances, the power supply voltage is 380V, and the allowable error is -15%~10%. After three-phase bridge full-wave rectification, the intermediate DC voltage value is 513V. In individual cases, small voltage fluctuations in the power line voltage will not occur. To cause a low-voltage trip of the frequency converter, the frequency converter will only operate if the effective value of the grid voltage is between 80% and 85% of the rated value and lasts for more than one cycle. The low voltage on the input side of the power supply is mainly due to the fluctuation of the grid voltage or the switching of the main power line, the impact of lightning strikes on the sine wave amplitude of the power supply, the overloading of the transformer of the power plant itself or the unbalanced load, etc.

2. Low voltage from the load side

The main reasons for this are the startup and application of large equipment, line overload or starting of large motors, etc. The frequency converter is composed of two parts: a rectifier and an inverter. Through research on frequency converters, the low voltage of the frequency converter refers to the low voltage of its intermediate DC circuit (that is, the inverter input voltage is too low). General frequency converters have protection functions for overvoltage, voltage loss and instantaneous power outage. The inverter device of the frequency converter is divided into two types: GTR and IGBT. When the inverter device of the frequency converter is GTR, once the voltage is lost or power failure occurs, the control circuit will stop outputting signals to the drive circuit, causing the drive circuit and GTR to stop working, and the motor will will be in a free braking state. When the inverter device is IGBT, the inverter will be allowed to continue working for a short time td after voltage loss or power outage. If voltage loss or power outage lasts for totd, the inverter will stop running due to self-protection. Generally, td is between 15 and 25ms, while the power "power sloshing" time to is generally more than a few seconds. The inverter will self-protect and stop running, causing the motor to stop running. Therefore, solving the problem of low-voltage tripping of the frequency converter cannot start from the inherent time td and voltage loss time to of the frequency converter, but must start from the amplitude that can withstand the voltage reduction.

Solution to low voltage trip of frequency converter

To solve the problem of low voltage tripping of the frequency converter, two key points must be mastered:

I    Choose an inverter with IGBT inverter device;

II   To choose an inverter that can still work normally under conditions of significant voltage loss.

The motor is able to rotate, but the operating current exceeds the rated value, which is called overload. The basic reflection of overload is: although the current exceeds the rated value, the extent of the excess is not large, and generally it does not form a large inrush current.

Causes and inspection methods of inverter overload tripping

I  Main causes of inverter overload

1. Malfunction, the current detection part inside the inverter fails, and the detected current signal is too large, resulting in tripping.

2. The mechanical load is too heavy. The main characteristic of overload is the heating of the motor, which can be found by reading the operating current from the display.

3. The three-phase voltage is unbalanced, causing the operating current of a certain phase to be too large, resulting in overload tripping. This is characterized by uneven heating of the motor, which may not be found when reading the operating current from the display (because the display only displays one phase). current).

II. How to check whether the frequency converter is overloaded

1. Check whether the three-phase voltage on the motor side is balanced. If the three-phase voltage on the motor side is unbalanced, then check whether the three-phase voltage on the output end of the inverter is balanced. If it is not balanced, the problem is inside the inverter.

If the voltage at the output end of the frequency converter is balanced, the problem is on the line from the frequency converter to the motor. Check whether the screws at all terminals have been tightened. If there are contactors or other electrical appliances between the frequency converter and the motor, also You should check whether the terminals of the relevant electrical appliances are tightened and whether the contacts are in good condition.

If the three-phase voltage on the motor side is balanced, you should know the operating frequency when tripping: If the operating frequency is low and vector control is not used (or no vector control), first reduce the U/f ratio. If the load can still be driven after the reduction, , it means that the original preset U/f ratio is too high and the peak value of the excitation current is too large. The current can be reduced by reducing the U/f ratio;

If the load cannot be carried after lowering, you should consider increasing the capacity of the frequency converter; if the frequency converter has a vector control function, the vector control method should be used.

2. Check whether the motor is hot. If the temperature rise of the motor is not high, you should first check whether the preset of the electronic thermal protection function of the inverter is reasonable. If the inverter still has margin, the preset of the electronic thermal protection function should be relaxed. value.

If the temperature rise of the motor is too high and the overload occurs is a normal overload, it means that the motor is overloaded. At this time, the first thing to do is to increase the transmission ratio appropriately to reduce the load on the motor shaft. If it can be increased, increase the transmission ratio. If the transmission ratio cannot be increased, the capacity of the motor should be increased.