The First Part :What Causes the Shaft Voltage of Diesel Generator

With the gradual increase of single unit capacity, shaft voltage has become a serious problem after large generators adopt static self-parallel excitation system. The waveform of shaft voltage has complex harmonic pulse components, which is particularly harmful to oil film insulation. When the shaft voltage does not exceed the damage value of the oil film, the shaft current is very small. If the shaft voltage exceeds the breakdown voltage of the bearing oil layer, a large shaft current will be formed on the bearing, namely the so-called electric discharge machining current, which will ablate the bearing components and cause great harm. Asymmetric magnetic circuit, monopole effect, capacitive current, electrostatic effect, permanent magnetization of static excitation system, shell, shaft, etc. may cause shaft voltage.

Shaft voltage refers to the voltage generated at the two bearing ends of the motor or between the motor shaft and the bearing when the motor is running. Under normal conditions, when the shaft voltage is low, the lubricating oil film between the generator shaft and the bearing can play a good insulation role. However, if the shaft voltage rises to a certain value due to some reasons, the oil film will be broken down and discharged, forming the circuit of shaft current generation. The shaft current will not only destroy the stability of the oil film, but also gradually degrade the quality of the lubricating and cooling oil. At the same time, because the shaft current passes through the metal contact point of the bearing and the shaft, the metal contact point is very small, and the current density is very high, which will produce high temperature in an instant, causing the bearing to burn locally. The molten bearing alloy splashes under the action of rolling pressure, which will burn small pits on the inner surface of the bearing. Finally, the bearing will be damaged due to accelerated mechanical wear, and in serious cases, the bearing bush will be burnt, resulting in an accident and forced shutdown.

The generator shaft voltage has always existed, but it is generally not high, usually a few volts to more than ten volts. However, when the insulation pad loses its function due to oil pollution, damage or aging, the shaft voltage is enough to break through the oil film between the shaft and the bearing and discharge. Over time, the lubricating and cooling oil quality will gradually deteriorate, and in serious cases, the shaft and bearing bush will be burnt, resulting in shutdown accidents.

Why is the generator shaft voltage generated? Next,let Dingbo Power introduce you one by one.

1. Shaft voltage caused by magnetic asymmetry.

It is an AC type voltage that exists at both ends of the turbine generator shaft. Due to the magnetic asymmetry caused by the manufacturing and operation of the diesel generator such as the fan-shaped stamping sheet used in the stator core, the eccentricity of the rotor, the different permeability of the fan-shaped sheet, and the axial guide slot used for cooling and clamping, the alternating magnetic linkage circuit including the shaft, bearing and foundation plate is generated. Thus, a voltage difference is generated at both ends of the main shaft of the generator.

Each kind of magnetic asymmetry will cause the shaft voltage component of corresponding amplitude and frequency, and the superposition of each shaft voltage component makes the frequency component of this shaft voltage very complex. The amplitude of the fundamental component is the largest, the amplitude of the third and fifth harmonics is slightly small, and the amplitude of the higher harmonic component is very small. This AC shaft voltage is generally 1~10V, which has large energy. If no effective measures are taken, the shaft voltage will form a circuit through the shaft-bearing-base platen and generate a large shaft current. The electric arc caused by the shaft current is added between the bearing and the shaft surface, and its main consequence is to cause the tungsten on the bearing and the wear of the shaft surface, and to rapidly degrade the lubricating oil. This will accelerate the mechanical wear of the bearing, and even cause the bearing bush to burn out.

Dingbo Power is a professional manufacturer of diesel generators. If you want to know more, please continue to watch our next article, or do not hesitate to consult us via email E-mail: dingbo@dieselgeneratortech.com.