How to solve the problem of low voltage tripping in frequency inverter
2024-06-04
1. Low voltage from power input side
Under normal circumstances, the power supply voltage is 380V, with an allowable error of -15%~10%. After three-phase bridge full wave rectification, the intermediate DC voltage value is 513V. In some cases, small voltage fluctuations in the power line voltage will not cause low voltage tripping of the frequency converter. Only when the effective value of the grid voltage is between 80%~85% of the rated value and lasts for more than one cycle, will the frequency converter operate. The low voltage on the input side of the power supply is mainly due to fluctuations in the grid voltage or switching of the main power line, lightning strikes affecting the amplitude of the sine wave of the power supply, overloading or load imbalance of the transformer in the power plant itself.
Under normal circumstances, the power supply voltage is 380V, with an allowable error of -15%~10%. After three-phase bridge full wave rectification, the intermediate DC voltage value is 513V. In some cases, small voltage fluctuations in the power line voltage will not cause low voltage tripping of the frequency converter. Only when the effective value of the grid voltage is between 80%~85% of the rated value and lasts for more than one cycle, will the frequency converter operate. The low voltage on the input side of the power supply is mainly due to fluctuations in the grid voltage or switching of the main power line, lightning strikes affecting the amplitude of the sine wave of the power supply, overloading or load imbalance of the transformer in the power plant itself.
2. Low voltage from load side
The main reasons for this are the start-up and application of large equipment, line overload, or start-up of large motors. A frequency converter consists of a rectifier and an inverter. Through research on frequency converters, low voltage refers to the low voltage in the intermediate DC circuit (i.e. the input voltage of the inverter is too low). General frequency converters have protection functions for overvoltage, voltage loss, and instantaneous power outage. The inverter components of the frequency converter are divided into GTR and IGBT. When the inverter components of the frequency converter are GTR, once there is a voltage loss or power outage, the control circuit will stop outputting signals to the drive circuit, causing both the drive circuit and GTR to stop working, and the motor will be in a free braking state. When the inverter device is an IGBT, it will allow the inverter to continue working for a short period of time td after voltage loss or power outage. If the voltage loss or power outage time totd, the inverter self-protection will stop running. Generally, the TD is between 15-25ms, and the power supply "shaking" time is usually a few seconds or more. The frequency converter will self-protection stop running, causing the motor to stop running. Therefore, solving the problem of low voltage tripping in frequency converters cannot start from the inherent time td and voltage loss time to, but must start from the amplitude that can withstand voltage reduction.
The main reasons for this are the start-up and application of large equipment, line overload, or start-up of large motors. A frequency converter consists of a rectifier and an inverter. Through research on frequency converters, low voltage refers to the low voltage in the intermediate DC circuit (i.e. the input voltage of the inverter is too low). General frequency converters have protection functions for overvoltage, voltage loss, and instantaneous power outage. The inverter components of the frequency converter are divided into GTR and IGBT. When the inverter components of the frequency converter are GTR, once there is a voltage loss or power outage, the control circuit will stop outputting signals to the drive circuit, causing both the drive circuit and GTR to stop working, and the motor will be in a free braking state. When the inverter device is an IGBT, it will allow the inverter to continue working for a short period of time td after voltage loss or power outage. If the voltage loss or power outage time totd, the inverter self-protection will stop running. Generally, the TD is between 15-25ms, and the power supply "shaking" time is usually a few seconds or more. The frequency converter will self-protection stop running, causing the motor to stop running. Therefore, solving the problem of low voltage tripping in frequency converters cannot start from the inherent time td and voltage loss time to, but must start from the amplitude that can withstand voltage reduction.
Solution to low voltage tripping of frequency converter
To solve the problem of low voltage tripping in frequency converters, two key points need to be mastered:
One is to choose a frequency converter with IGBT inverter components;
The second is to choose a frequency converter that can still operate normally under conditions of significant voltage loss.
An electric motor that can rotate but operates with a current exceeding its rated value is called an overload. The basic reflection of overload is that although the current exceeds the rated value, the magnitude of the excess is not significant, and generally does not form a significant surge current.
To solve the problem of low voltage tripping in frequency converters, two key points need to be mastered:
One is to choose a frequency converter with IGBT inverter components;
The second is to choose a frequency converter that can still operate normally under conditions of significant voltage loss.
An electric motor that can rotate but operates with a current exceeding its rated value is called an overload. The basic reflection of overload is that although the current exceeds the rated value, the magnitude of the excess is not significant, and generally does not form a significant surge current.
Reasons and inspection methods for overload tripping of frequency converters
1、 The main reasons for frequency converter overload
1). Misoperation caused a malfunction in the internal current detection part of the frequency converter, resulting in a detected current signal that was too high and causing a trip.
2). The main characteristic of excessive mechanical load is the heating of the motor, which can be detected by reading the operating current on the display screen.
3). Unbalanced three-phase voltage causes excessive operating current in a certain phase, leading to overload tripping. Its characteristic is uneven heating of the motor, which may not be detected when reading the operating current from the display screen (as the display screen only shows one phase current).
1、 The main reasons for frequency converter overload
1). Misoperation caused a malfunction in the internal current detection part of the frequency converter, resulting in a detected current signal that was too high and causing a trip.
2). The main characteristic of excessive mechanical load is the heating of the motor, which can be detected by reading the operating current on the display screen.
3). Unbalanced three-phase voltage causes excessive operating current in a certain phase, leading to overload tripping. Its characteristic is uneven heating of the motor, which may not be detected when reading the operating current from the display screen (as the display screen only shows one phase current).
2、 Method for checking whether the frequency converter is overloaded
1). Check if the three-phase voltage on the motor side is balanced. If the three-phase voltage on the motor side is unbalanced, check if the three-phase voltage at the output end of the frequency converter is also balanced. If it is also unbalanced, the problem lies inside the frequency converter.
If the voltage at the output end of the frequency converter is balanced, the problem lies in the wiring from the frequency converter to the motor. All terminal screws should be checked to ensure they are tightened. If there are contactors or other electrical appliances between the frequency converter and the motor, the terminals of the relevant electrical appliances should also be checked to ensure they are tightened and the contact condition of the contacts is good.
If the three-phase voltage on the motor side is balanced, the operating frequency at the time of tripping should be understood. If the operating frequency is low and vector control (or no vector control) is not used, the U/f ratio should be reduced first. If the reduction can still drive the load, it indicates that the preset U/f ratio is too high and the peak excitation current is too high. The current can be reduced by reducing the U/f ratio;
If the load cannot be carried after reduction, it should be considered to increase the capacity of the frequency converter; If the frequency converter has vector control function, vector control method should be adopted.
2). Check if the motor is heating up. If the temperature rise of the motor is not high, first check whether the electronic thermal protection function of the frequency converter is pre-set reasonably. If there is still room for the frequency converter, the pre-set value of the electronic thermal protection function should be relaxed.
If the temperature rise of the motor is too high and the overload occurs is considered normal overload, it indicates that the motor is overloaded. At this point, the first thing to consider is whether it is possible to increase the transmission ratio appropriately to reduce the load on the motor shaft. If possible, increase the transmission ratio. If the transmission ratio cannot be increased, the capacity of the motor should be increased.
1). Check if the three-phase voltage on the motor side is balanced. If the three-phase voltage on the motor side is unbalanced, check if the three-phase voltage at the output end of the frequency converter is also balanced. If it is also unbalanced, the problem lies inside the frequency converter.
If the voltage at the output end of the frequency converter is balanced, the problem lies in the wiring from the frequency converter to the motor. All terminal screws should be checked to ensure they are tightened. If there are contactors or other electrical appliances between the frequency converter and the motor, the terminals of the relevant electrical appliances should also be checked to ensure they are tightened and the contact condition of the contacts is good.
If the three-phase voltage on the motor side is balanced, the operating frequency at the time of tripping should be understood. If the operating frequency is low and vector control (or no vector control) is not used, the U/f ratio should be reduced first. If the reduction can still drive the load, it indicates that the preset U/f ratio is too high and the peak excitation current is too high. The current can be reduced by reducing the U/f ratio;
If the load cannot be carried after reduction, it should be considered to increase the capacity of the frequency converter; If the frequency converter has vector control function, vector control method should be adopted.
2). Check if the motor is heating up. If the temperature rise of the motor is not high, first check whether the electronic thermal protection function of the frequency converter is pre-set reasonably. If there is still room for the frequency converter, the pre-set value of the electronic thermal protection function should be relaxed.
If the temperature rise of the motor is too high and the overload occurs is considered normal overload, it indicates that the motor is overloaded. At this point, the first thing to consider is whether it is possible to increase the transmission ratio appropriately to reduce the load on the motor shaft. If possible, increase the transmission ratio. If the transmission ratio cannot be increased, the capacity of the motor should be increased.
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