The slip ring is a key component of a wound rotor motor . Today, we will use the slip ring of a three-phase wound rotor motor as an example to illustrate this point.
The slip ring of a motor consists of three insulated copper or steel rings, separated by an insulator. The slip ring of a small motor has a complete cylindrical surface, while the slip ring of a large motor has spiral grooves machined on its surface. The purpose of this is to collect the carbon powder generated by the friction between the carbon brush and the slip ring during motor operation. Because the grooves on its surface are spiral, it can ensure that the carbon brush surface is subjected to uniform force and contact during motor operation, thus ensuring the reliability of motor operation.
In actual production and processing, in order to ensure the coaxiality of the slip ring and the rotor, many motor manufacturers machine the slip ring surface during the rotor surface turning process and ensure that the surface roughness of the slip ring meets the requirements. However, for slip rings with spiral grooves, most motor manufacturers directly install and use them. Therefore, the machining accuracy of the slip ring itself, as well as the dimensional conformity of the slip ring hole and the coaxiality conformity between the inner hole and the ring surface, are particularly important. These are issues that both motor manufacturers and slip ring manufacturers must pay attention to.
Brief description of slip ring applications
Slip ring series synchronous motors, as well as asynchronous motors, which use slip rings are widely used in industrial sectors, and most of them operate under various harsh conditions. Although these motors do not have the commutation function of DC motors, they, like commutators, also suffer from problems such as abnormal wear of slip rings or brushes, brush vibration, and sparking.
Especially considering the brush material, slip ring brushes not only use graphite brushes, but also sometimes metal-graphite brushes to increase the current density. Therefore, factors such as abnormal residual expansion must also be considered. Even for high-speed motors like steam turbine generators or motors operating in fully enclosed gaseous or hydrogen media, there are still many problems.
The material of slip rings requires high mechanical strength, good electrical conductivity, and corrosion resistance. When in sliding contact with the brushes, they must possess wear resistance and stable sliding contact characteristics. Generally, steel slip rings have good wear resistance and high mechanical strength, and are therefore mostly used in synchronous motors where polarity causes significant differences in slip ring wear.
Generally, steel slip rings have good wear resistance and high mechanical strength, so they are mostly used in synchronous motors where the wear difference of slip rings is large due to polarity. Steel can be processed into complex structures, and it is an easily obtained and inexpensive material, so it is widely used in synchronous motors, including hydro-generators with low circumferential speeds.
For slip rings used in applications like steam turbine generators, where mechanical strength and wear resistance at high circumferential speeds are paramount, forged steel is sometimes employed. Additionally, stainless steel can be used when corrosion resistance is required; however, the sliding characteristics of stainless steel are unstable, and improper combination with brushes can often cause brush skipping, leading to excessive brush temperature rise or abnormal wear. Therefore, extra care must be taken when using stainless steel slip rings. Compared to steel slip rings, bronze castings and other copper slip rings offer better sliding characteristics and are therefore widely used. However, when they contain a large amount of low-melting-point lead, lead desorption can occur due to heat generation at the energizing point, easily causing slip ring wear or abnormal brush wear.
In the mating of slip rings and brushes, when the brush's abrasiveness is too strong and the slip ring material is too soft, stepped wear equivalent to the width of the brush often occurs on the slip ring. This is especially true in fully enclosed motors operating at high temperatures and low humidity, which are more prone to excessive wear on the brushes or slip rings. This type of wear occurs as follows: initially, there are only very small scratches, and the brush fails to collect current at these points, generating sparks. Once sparks occur, the scratches gradually worsen and expand, eventually forming scratches the same size as the sliding contact surface of the brush. Therefore, even very small sparks generated by slip ring brushes must be taken seriously.
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