Regarding the issues of sun low and undervoltage in solar pump system.

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The solar pump system is mainly composed of solar panel, VFD and water pump. The main fault of the system is most likely to come from these three devices. However, the failure of the water pump will not generally lead to the sun low or undervoltage alarm of the VFD.
Therefore, we will mainly discuss the problems of sun low and undervoltage caused by solar panel and VFD.

In the following text, alarm refers to sun low or undervoltage alarm.

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The following problems of the solar panel may cause alarm of the VFD:

The nameplate of the solar panel is falsely labeled;
Used second-hand solar panels;
Poor installation angle of solar panels;
The solar panel is obstructed by obstacles;
The power of the solar array is insufficient;
Poor weather.

The following alternative solutions:
A. Increase the number of solar panels in series, the motor can only operate at rated voltage when the Vmp of solar strings is greater than √2 ≈ 1.414 times of the motor voltage. Pay attention to detecting the Voc of the series, and do not exceed the input range of the VFD.
B. Increase the number of parallel connections of the solar strings. As long as the voltage of the series is within the input range of the VFD, the number of parallel connections can be increased at will. Of course, this will increase the cost, but will not damage the VFD and motor.

For the 1st and 2nd problems, it is recommended to choose high-quality solar panels, or solution A.
For the third and fourth questions, it is recommended to adjust the installation plan. If it is not possible to adjust, consider using solution B.
For the fifth question, both option A and option B can be considered, and they can be considered simultaneously
For the sixth question, you can wait for the weather change. If it is an area with insufficient sunlight for a long time, you can refer to the solution for the fifth question.

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Inappropriate parameter setting of VFD may cause alarm of VFD.
Here are several possible inappropriate settings and solutions:

Inappropriate setting of undervoltage protection points
For 380V series models, the default undervoltage point is 350V, and for 220V series models, the default undervoltage point is 190V.
The undervoltage point can be modified by percentage using H5-06. For example, for a 380V VFD, when H5-06=90, the undervoltage point is 350V * 90%=315V.
Inappropriate minimum operating frequency setting
P0-14 is the minimum operating frequency of the motor, and P8-14 should be set to 1 so that the motor will stop when it is below this frequency.
Please note that do not set the minimum operating frequency too low, it may damage your motor. We recommend setting it to the frequency at which the water pump can just discharge water.
The motor accelerates too quickly
The faster the motor accelerates, the greater the power required. When the solar array is insufficient to provide such a large power, the VFD may alarm.
The acceleration time of the motor can be modified through P0-17, depending on the power of the solar array and the motor.
The motor decelerates too slowly
When the weather suddenly changes, such as clouds blocking the solar array, the power of the solar array will suddenly decrease and not be enough to support the motor to run at its current speed. If the motor cannot be slowed down quickly, the VFD may alarm.
The deceleration time of the motor can be modified through P0-18, and it is recommended to set it to 2-3 seconds.

Tip: All the energy required for the operation of the water pump comes from the solar array. If the voltage or power of the solar array is insufficient, the relevant fault cannot be eliminated by modifying the parameters of the VFD or replacing a larger VFD.