Shaft Voltage in Diesel Generator Sets

For many users, understanding the working principles and professional knowledge of diesel generator sets can be challenging. However, lacking this knowledge may lead to difficulties in operation and even shorten the unit's service life. As the saying goes, "knowing the enemy and yourself ensures victory in every battle," so mastering essential unit knowledge is crucial. Today, the editor from Dingbo Power Generator Factory will explain key information about shaft voltage.

Shaft voltage refers to the voltage generated between the two bearing ends of a motor or between the motor shaft and bearings during operation. Under normal circumstances, when the shaft voltage is low, the lubricating oil film between the generator shaft and bearings provides good insulation. However, if the shaft voltage rises to a certain level due to various factors, it will break down the oil film and cause discharge, forming a circuit for shaft current. Shaft current not only destabilizes the oil film and degrades the lubricating and cooling oil quality over time but also generates high temperatures at the small metal contact points between bearings and the shaft due to high current density. This can cause local melting of bearings and, in severe cases, damage the bearing pads, leading to forced shutdowns.

Causes of Generator Shaft Voltage

Shaft voltage is primarily caused by: magnetic asymmetry, electrostatic charges, static excitation systems, residual magnetism, and other related phenomena.

Prevention and Elimination Measures

To mitigate the severe hazards of shaft voltage, the following preventive measures are typically adopted:

1.During design and installation, an insulating pad is usually added between the bearing bracket at the generator's excitation end and the base. Additionally, all oil pipes, screws, and bolts are insulated.

2.A grounding brush for the turbine-side main shaft is designed to discharge static charges from the low-pressure section of the turbine, ensuring the shaft and ground maintain the same potential.

3.To reduce shaft voltage caused by magnetic circuit asymmetry in turbogenerator sets, generators are designed with measures to eliminate or reduce third or fifth harmonic components in the shaft voltage. This includes adopting an optimized generator structure and strictly following the manufacturer’s technical and design requirements during installation to prevent rotor eccentricity.

4.To prevent shaft voltage from rotor winding single-point ground faults, an excitation circuit two-point ground protection device is activated during operation.

5.To interrupt shaft current, insulating pads are installed at the excitation end, including between the generator bearings, oil seals of hydrogen-cooled generators, inlet/outlet water supports and pipe flanges of water-cooled generator rotors, and between the tail bearing and the base plate. Fasteners of the bearing housing and oil pipes connected to it are also insulated from the bearing, often using double-layer insulation.

6.Avoid magnetic circuit asymmetry during motor design.

7.Prevent axial magnetic flux during motor design, manufacturing, and operation.

8.Insulate the bearing housing from the ground.

9.Install a grounding brush on the shaft.

10.Use non-magnetic bearing housings or additional coils.

11.Add a bypass capacitor to the armature outlet terminal of DC motors for grounding.

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