In the process of generator manufacturing and installation, due to process and other factors, the air gap is always not so uniform, and the impedance in the coil installation is not absolutely the same, resulting in the generator will induce a voltage on the large shaft of its rotor during operation, which is called the shaft voltage. Due to the presence of shaft voltage, the voltage can break down the oil film through the pads at both ends of the rotor to form a shaft current, which can burn out the bearings. Therefore, we need to ground the large shaft through the grounding carbon brush.
The specific method is to add an insulating layer to the bearing seat at the excitation end of the generator, and to ground the large shaft at the steam end with a grounding carbon brush. The large shaft grounding carbon brush has multiple functions: first, it can eliminate the electrostatic voltage of the large shaft to the ground. In the operation of the generator, the large shaft is subjected to magnetic flux leakage to generate a levitation potential. This levitation potential can cause electric shock erosion to the pad and deteriorate the lubricant, further deteriorating the operating environment of the pad. Secondly, the large shaft grounding carbon brush also provides a role for the rotor grounding protection device. If the rotor is insulated and damaged, the large shaft grounding carbon brush can short it to the rotor. Without the action of grounding carbon brushes, it is difficult to detect one point of the rotor grounding, and it is even more difficult to avoid the occurrence of multi-point grounding caused by interlayer or interphase short circuits. Finally, the large-shaft grounding carbon brush also provides a function for measuring the voltage of the positive and negative poles of the rotor coil to the ground. Regarding the question of why the carbon brush of the large shaft grounding of the generator does not have a one-point rotor grounding alarm, we need to distinguish between the two concepts of "large shaft grounding" and "rotor grounding". The so-called large shaft grounding means that there can be no electrostatic charge accumulation on the "metal surface" of the entire rotor. Excessive electrostatic charge accumulation can cause an increase in the potential to ground, which will break down the lubricating oil film of the rotor bearing and cause direct friction between the rotor journal and the bearing pad. Therefore, we need to connect the metal surface of the rotor to the earth to release the accumulated electrostatic charge in time. This is also one of the functions of the large shaft grounding carbon brush, that is, to eliminate the shaft voltage and prevent the generation of shaft current. Rotor grounding refers to the grounding of the electrical winding rods inside the rotor, which is not allowed. Because the internal winding circuit of the rotor is direct current, the voltage to the ground of the positive and negative poles of the winding is evenly distributed under the condition of good insulation. When a little grounding occurs, the voltage to ground of the other pole of the ungrounded pole will rise to the full voltage, and it is easy to break down the insulation of the ungrounded pole. Therefore, after the occurrence of one ground fault, it will quickly develop into two points of grounding. Therefore, we need to set up a little grounding protection, when a little grounding occurs, an alarm will be issued in time or the generator will be thrown out of operation by tripping. Why install grounding brushes on the shaft, the core part of the generator? It's like installing a safety barrier for the powerful heart of the machine. Due to the possible unevenness of the air gap, the impedance of the coils varies, causing the generator to induce shaft voltages on the rotor while it is running. If left unchecked, this shaft voltage will break down the oil film through the bearing, creating a devastating shaft current that will burn out the bearing. To combat this, we employ two strategies. First of all, insulating material is added to the bearing seat at the excitation end to insulate the whole bearing from the ground. Secondly, install a grounding carbon brush on the large shaft of the steam end to directly connect the large shaft to the ground. In this way, the resulting shaft current can be smoothly channeled into the ground by the grounding carbon brushes, avoiding damage to the bearing. In addition to this, grounded carbon brushes have an important task: to measure the voltage of the positive and negative poles of the rotor to ground. How is this seemingly impossible task accomplished? Originally, the grounding carbon brush was connected to the large shaft through a "rotor grounding relay", which was connected to the positive and negative poles of the rotor at the same time. As soon as any point of the positive or negative pole of the rotor leaks onto the rotor, the potential of the large shaft will change, and the "rotor grounding relay" can be detected immediately, providing a basis for the operator to deal with it. Under normal circumstances, the excitation circuit of the generator is not grounded, and there is no voltage on the large shaft of either the positive or negative pole. It is only when a certain point in the circuit is grounded or the insulation drops to a non-conformity that the positive and negative electrodes produce the corresponding voltage. And this voltage is measured by grounding carbon brushes. In practical applications, large generators usually use insulated bearing housings at the excitation end and grounded carbon brushes at the steam end. This is because the various protections of the generator excitation and the ground insulation of the rotor measurement are achieved by connecting the large shaft of the steam end of the carbon brush. Once the rotor coil leaks electricity to the rotor core, the excitation positive or negative electrode will generate a grounding voltage, indicating that the rotor may fail, and it should be shut down immediately for maintenance. In addition, the grounding brushes are usually connected to the rotor insulation monitoring loop, which will send an alarm signal whenever a current flows through. Publish a collection of dragon cards to share millions of cash
In short, the existence of grounding carbon brushes is like installing a sophisticated safety monitoring system for the generator, guarding the normal operation of the machine at all times and ensuring the stability and safety of the electric power.