Why Does the Generator Set Generate Shaft Current

May. 06, 2022

Under normal conditions, the insulation resistance of the rotating part of the generator set to the ground is required to be greater than 0.5m Ω. If both ends of the main shaft are grounded at the same time, shaft current may be generated. The shaft current causes serious damage to the generator set with long operation cycle. If it is not found for a long time, it will cause serious damage to the mechanical parts of the generator set. Why does the generator set generate shaft current?


The reasons are as follows:

When the motor set is a motor rotor, the reason is mainly caused by the asymmetric distribution of magnetic lines of force and the magnetization effect of the rotating shaft. The asymmetric distribution of magnetic lines of force is usually caused by the asymmetry of lamination gap.


There is also a phenomenon that shaft current is generated due to the application of external potential. The reason is that the rotating magnetic field of the mobile motor unit cuts the conductor or unbalanced rotor, which leads to the magnetization of the rotating shaft. Friction, fusion, collision and eddy current devices may cause the magnetism of the equipment and establish a magnetic field.


Why Does the Generator Set Generate Shaft Current


When the rotating magnetic field cuts the conductor, these parts will induce a certain potential. When the potential rises enough to break through the oil film, a current circuit will be formed. This current loop may pass through the whole rotor, or it may only form a local short-circuit current in the bearing or floating ring seal. These currents in turn generate new magnetic fields that magnetize shafts or other parts. Therefore, this mutual conversion of magnetoelectricity will form a strong magnetic field and high current in the generator set. The voltage causing the shaft current of the generator set is more than 20V, and the damaged voltage 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 lower than 1V, the shaft current will not be generated. 


When using the generator set, we should always pay attention to whether the shaft current is generated. If it is generated, we should timely analyze and find out the causes, so as to avoid the generation of shaft current.


What if the generator shaft current rises?


The following precautions are usually taken:

(1) During design and installation, insulating pads are usually installed between the bearing support and the base at the excitation end of the generator, and insulation measures are taken for all oil pipes, screws and screws.


(2) In order to prevent shaft voltage caused by one point grounding short circuit of rotor winding, put the two-point grounding protection device of excitation circuit into operation.


(3) In order to cut off the shaft current, the exciter side includes the generator bearing, the oil seal of the generator, the water inlet and outlet support and water inlet and outlet pipe flange of the water-cooled generator rotor, and the insulating pad is installed between the exciter and auxiliary exciter bearing and the base plate of the base. The fasteners of the bearing pedestal and the oil pipe connected to the bearing pedestal shall also be insulated from the bearing, and double-layer insulation measures can be adopted.


(4) Avoid magnetic circuit asymmetry in motor design.


(5) The axial magnetic flux shall be avoided during the design, manufacture and operation of the motor.


(6) Insulate the bearing pedestal from the ground.


(7) Install grounding brush on the shaft.


(8) Adopt non-magnetic bearing pedestal or additional coil.


(9) Add a bypass capacitor to the ground at the armature outlet end of the DC motor.


Guangxi Dingbo Power Equipment Manufacturing Co., Ltd has focused on high quality diesel generators for more than 15 years, we have solved many questions for clients and provided many generator sets to clients. Therefore, if you are interested in diesel generators, we welcome you to contact us, our email address is dingbo@dieselgeneratortech.com.

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