What Is the Cause of Overspeed in A Diesel Generator?

Generally, diesel generator sets are installed with a governor to keep a specified speed range. The governor can quickly and automatically adjust the fuel supply based on the change of load. It keeps balance between the generator output power and the load, ensuring the stable operation of the diesel generator set. For some reason, sometimes generator overspeeding problem may occur. Here we’ll explore the possible causes of overspeed in a diesel generator and what you need to do next.

Causes of overspeed in a diesel generator

Typically, the generator overspeed problem may be caused by the following reasons:

Faulty governor

The governor is designed to control the fuel to the engine cylinders so as to control the speed of the unit. If the governor malfunctions and cannot perform its function for some reason, the engine will over speed. Failure of the governor’s internal parts may cause failure of the governor. Governor drive failure is the common reason for the over speeding of engine. You can replace the governor and see if that works. Sometimes, overspeed problems can also occur when mechanical engine governing systems are not installed, commissioned and serviced by qualified personnel.

Wrong settings in governor

Check your over speed setting on the governor and make sure it is correct. If it's too low, the generator will trip unnecessarily. If the governor response time is slow or if it is not properly calibrated, it may not be able to adjust quickly enough to the sudden decrease in load, causing the generator to overspeed. In addition to the wrong settings of the governor, excess wear in the linkages between the governor and the fuel rack/pump/throttle can also lead to the over speeding issue. You should check governor linkages and make sure they are in good conditions.

False sensor/ electric/solenoid signal

The speed signal could be corrupted by an incorrectly mounted or adjusted sensor or system configuration errors, which causes an incorrect input in the system logic. The false signal could affect the control system and lead to an overspeed situation.

Sudden load loss

If the load connected to the generator suddenly decreases or goes offline completely, the generator will continue to produce the same amount of power as before. With less load to power, the generator output will increase, causing it to overspeed. When the load is reduced but fuel racks/ fuel pumps/ governors staying stuck at high output, the over speeding problem may occur.

MPU fault

The Magnetic Speed Pickup (MPU) is an instrument for detecting the speed of an Engine. Faulty MPU can also lead to engine over speed. Occasionally it has been metal filings from the ring gear transferring to the MPU. You need to pull the MPU and give it a clean.

In addition to the causes mentioned above, increased throttle due to mechanical or electrical device, increased starting fuel limit, faulty injectors and disturbed setting in control panel can also cause the problem. In most cases, the diesel generator overspeeding appears due to the malfunctioning of the engine governor and fuel injection pumps.

When the generator overspeed occurs, you need to force stop the machine to prevent serious accidents. Measures should be taken to shut down the generator immediately:

Shut off the oil supply system.

For the fuel injection pump with oil supply rod or toothed bar exposed, the rod should be quickly pulled back to the oil stop position.

For the diesel engine with a decompression device, the decompression handle should be quickly pulled to the decompression position, so that the diesel engine will stop due to no compressed gas.

If the closing device is installed in the intake pipe, the fire extinguishing device needs to be stopped immediately.

At present, the control modules of most diesel generator sets feature the function of overspeed shutdown protection. It means that if the control module detects that the engine speed is higher than its set point, the control program will automatically power off to shut down the engine for protection. If your diesel generator doesn’t have this function, it’s recommended that you shut off the engine and troubleshoot problems timely.

Dingbo Power is a professional diesel generator manufacturer with more than 17 years’ experience. We provide high-quality diesel gensets including the brands such as Cummins, Perkins, Volvo, Ricardo, Yuchai, Shangchai, etc. Feel free to contact us with any questions, concerns, or inquiries to learn more about the products & services we provide.

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