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The Cause of Undervoltage Fault of Diesel Generator Set

Sep. 19, 2019

After using diesel generator set for a period of time, there will always be some faults, such as the generator cannot start normally, low oil pressure alarm, low frequency, overspeed, abnormal noise, etc. In this article, we mainly discuss the cause of diesel generator undervoltage faults.

1. Diesel engine mechanical speed regulation

The speed regulation of diesel engine is divided into electronic speed regulation and mechanical speed regulation. If it is mechanical speed regulation, then there is a fuel  pump mechanism on the diesel engine that controls the oil quantity and oil paths. There is a pull rod on the top to control the oil quantity. It is called speed control pull rod. A speed limit (high speed) push rod and a speed control push rod are distributed on both sides of the speed control pull rod. When the power is turned on for 20 seconds, the low voltage is reported. Don’t know whether the voltage and frequency is always not to go up. If it is not going up, it may be the speed, you can adjust the speed control push rod. However, if the diesel engine fails, there is usually a major fault. The main fault is solved, and a series of minor faults are solved.


diesel generator set

2. The voltage sampling line is loose – The voltage cannot be measured

3. Remanence

If the generator has no residual magnetism, then the voltage system of the generator cannot be built at the beginning. For this problem, we need to know how much volt the excitation output of the generator AVR voltage regulator is, and then connect the corresponding voltage source to the excitation output line for magnetization (the voltage type should be corresponding and the polarity should not be reversed).

4. Ground fault

If three-phase grounding occurs, the voltage and current are very low. At this time, it is mainly necessary to check whether the grounding discharge device is closed or grounded.

5. Voltage regulating plate failure

Due to changes in environmental factors, the parameters of the AVR pressure regulating plate are no longer applicable and need to be re-adjusted. Generally speaking, non-parallel diesel units basically do not have such problems, because the parameters of the pressure regulating plate are fixed values ( 400V), which is generally not adjustable. This problem may occur only for gensets that are used in parallel because the AVR voltage regulator is regulated in accordance with the main bus voltage, it is not static. At this time, the parallel device usually has a voltage regulating signal sent to the AVR voltage regulating plate. In this case, either check whether the voltage regulating signal is connected incorrectly, or try to use the electronic control quickly when the power is turned on (parallel device, voltage regulating plate, etc.) to reset the voltage.

6. The varistor or rectifier bridge diode on the generator winding is damaged

The function of the varistor is: when the overvoltage fault occurs, the varistor is turned on to reduce the voltage. If the varistor is broken down or turned on for other reasons, it is conceivable that the voltage must be very low. The rectifier bridge has six diode, the set DC power supply is used to supply the voltage regulating plate and the excitation device. If the rectifier bridge diode is damaged, the functions of the voltage regulating board and the excitation device will be greatly reduced.


In order to avoid generator failure, regular maintenance is very necessary. This can not only give a full play to the excellent performance of the diesel generator set, but also extend the service life of the genset. Due to different brands of diesel generators, the maintenance items are different. In order to maintain your diesel generator accurately, the units should be maintained strictly in accordance with the user's maintenance manual provided by the supplier.


Jiangsu Starlight Power is as a professional diesel generator set OEM with more than 40 years in the field of generator manufacturing. We can provide multiple brands of emergency diesel generators with good quality, such as Cummins, Perkins, Volvo, Deutz, Yuchai, Weichai, Shangchai, etc. If you are interested, please contact us to get the price now!

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