The Reasons of Shaft Current Damage in Generator Sets

Let's analyze the causes of shaft current damage in the motor set together. In the case of a generator set with an electric motor rotor, the main reason is the asymmetric effect of the magnetic field line distribution and the magnetization effect of the shaft, which generates shaft current. The asymmetric distribution of the magnetic field line is usually caused by the asymmetric gap of the laminated layer. Of course, there is also a phenomenon where shaft current is generated due to the application of external potential. The reason is that the rotating magnetic field of the generator set cuts the conductor or the rotor is unbalanced, resulting in shaft magnetization. Friction, welding, collision, and eddy current devices may all cause the equipment to become magnetic and establish a magnetic field.

When a rotating magnetic field cuts a conductor, it induces a certain potential within these parts. When the potential rises enough to break through the oil film, a current circuit is formed. This current circuit may pass through the entire rotor, or it may only form local short-circuit currents in the bearings or floating ring seals, which in turn generate new magnetic fields to magnetize the shaft or other parts. Therefore, this type of magnetic and electrical conversion will form a strong magnetic field and high current in the generator unit. The voltage that causes the generator set to generate shaft current is above 20V, and the voltage that causes damage is between 30-100V. Therefore, ensuring the normal operation of the generator set can effectively limit the voltage of the generator set. Once the voltage is below 1V, shaft current will definitely not be generated. The main method to reduce or eliminate damage caused by shaft current is to limit the increase in shaft voltage. It is generally believed that the voltage sufficient to cause shaft current damage is above 20V, and the typical bearing damage voltage is between 30-100V.

If the shaft voltage is reduced to below 1V, the faults caused by shaft current can basically be eliminated. There are various methods to limit and reduce shaft voltage, and various types of grounding devices are commonly used, such as using carbon brushes, metal sliding shoes, mercury grooves, water seals, etc. to connect the rotor to the casing, making the rotor conductive to ground and eliminating static potential of the shaft. In addition, there are some methods for conducting charges on the rotating shaft, such as using ionized air channels, using highly conductive lubricating oil, or adding certain additives to the oil to make the lubricating oil a good conductor.

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