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建立人际资源圈澳洲essay代写：Factors that Affect the Durability of Brushed Electrical Machine
2017-05-08 来源: 51due教员组 类别: Essay范文
本篇澳洲essay代写讲了拉丝电机是基于碳刷和滑环之间的电流换向原理，能够将机械能转换为电能或电能到机械能的装置。当这种设备将机械能转换为电能时，例如风力或水力发电机以及汽油或柴油发动机，习惯于将设备称为发电机。本篇澳洲essay代写由51due论文代写机构整理，供大家参考阅读。
The term of brushed electrical machine denotes a device which is able to transform either mechanical energy to electrical energy or electrical energy to mechanical energy based on principle of current commutation between carbon brush and sliding ring (Chapman, 2005). It is accustomed to calling a device as a generator when such this device converts mechanical energy to electrical energy, for instance, wind or water turbines, and gasoline or diesel engines. On the other hand, a device is referred to as a motor when it gets adapted to convert electrical energy to mechanical energy, such as electro motors serviced in the Tesla vehicle, and food waste disposers used in our daily life.
Electrical machines play an important role in the society surging for higher efficiency. However, once an electrical machine reaches the end of life, or tolerable durability, it will be condemned. The issue of operational durability of electrical machines has to be concerned due to their ubiquity. Motor in an automobile engine cooling fan system is a concise example, its function is to disperse the excess heat and maintain a beneficial thermal environment for the vehicle engine. Designed life of thecooling motor might simulated by a vehicle engineer, but when the time carbon brush in the motor worn out or any unexpected death before achieving designed target of this motor is harmful to the vehicle and costly to the driver.
The mission of this essay is to discover already known factors that affect the durability life of an electrical machine, with design that carbon brush build in its internal electric circuit, within the field of tribological science.
REVIEW OF LITERATURE
An electrical machine is able to rotate based on the principle of sliding electric contact. The term of sliding electric contact denotes an electrical connection between two energized conductors in intertransition region during relative rotating (Holm & Holm, 1967).
Performance of sliding contacts involves frictional wear phenomenon. A particular number of factors which identify the amount of friction wear, or we can say the overall duration life of a brushed electrical machine, have been discovered by researches such as oxidation film between brush and commutator, commutator surface temperature, environmental humidity (Fakih & Dienwiebel, 2015), and brush spring pressure (Yasar, Canakci, & Arsan, 2007). In addition, friction wear relates to mechanical installation defects as well, such as the smoothness of commutator surface.
Oxidation Film between Brush and Commutator:
The friction between carbon brush and pure copper is large; however the existence of an excellent oxidation film assists to reduce the friction coefficient acting as interval lubrication. Friction coefficient is decreased to 10% while having oxidation film comparing with original carbon brush versus pure copper wearing.
Oxidation film is produced based on three conditions which exist at the same time: water vapor, current, and carbon-copper connection. An excellent film presents a well-proportioned color of sepia, aeneous, or puce. The production of oxidation film is a continuous process, that is, both forming and peeling off procedures exists all at once.
The thickness of oxidation film was studied by Fakih and Dienwiebel using the Focused Ion Beam (FIB) and Energy-Dispersive X-Ray Spectroscopy (EDX) methods which came to the conclusion of 400 nanometers.
Commutator Surface Temperature:
Frictions wear changes along temperature variation. Hamilton (2000) states that fiction coefficient is reduced during temperature increased at first, and later then has a harmonious and coordinated growth with temperature. For instance, when friction coefficient is 0.15, the commutator surface temperature reaches 140ºF; but whenfriction coefficient is scaled down to 0.08, the temperature is at 220ºF.
It is generally accepted that temperature at commutator surface should not be less than 140ºF. If a sliding carbon-copper electric contact operating at low temperature, less than 140ºF, excellent oxidation film is not generated. In other words, temperature in the surface of an operating commutator should be at a level between 140ºF and 239ºF. The optimum condition is the most of the running time for a motor stay around 212ºF. Either over-heated or low situation leads to an excess of friction wear, which will indirect cause durability life decreasing.
Environmental Humidity:
Based on the principle summarized by Savage (Savage & Schaefer, 1956) and Lancaster (Lancaster & Pritchard, 1981) that a high proportion of water molecules in high humidity environment results in higher proportion of exposed carbon brush been covered a transient water vapor monolayers. Study also shows that in certain elevated density, the rate of brush wear, resistance and other coefficients divide, resulting in a change of in the wear system (Arbigay, Bares, & Sawyer, 2010). Environment with high humidity gathers volume of electrical resistance that brings an interval insulation situation. In this case, electrical machine is stop to run due to open circuit issue.
Brush Spring Pressure:
Yasar, Canakci, & Arsan (2007) found that higher brush spring pressure applying to carbon brush enlarges frictional wear in sliding carbon-copper connection. Oppositely, low brush spring pressure results in electrical spark and greater voltage drop. What is more, the friction layer matters as a key factor in brushes’ behavior control. Extreme situations like the arc erosion and abrasion will be obvious when pressure is below 30 kPa or above 120 kPa.
In conclusion, the literature mentioned all raise questions to be solved on how to postpone the life of brushed electrical motors. With the development of science, there might be better solutions.
DISCUSSION
In this paper factors that affect durability life of brushed electrical machine are discussed. Low friction wear rate and good commutator film are the key elements to predicate whether an electrical machine is able to reach a long durability life. Study by Zhao, Barber and Liu (2001) also shows that the electrical current itself might affect the friction wear of brushed machine. It can also be implied from their study that besides looking for various factors, the relations between the electrical current and the brushed electrical machine should be noted for the highest efficiency combination of friction wear and amount of electricity consumed. Study by Bhushan, Israelachivili, and Landman (1995) offers a new angle to consider the effects between materials in contact, which might also be informative for brushed electrical machines. They resort to atomic/nano scale simulations and find that films containing different types of molecules (branched chain, straight chain, etc) have various effects in lubrication. Having been focusing on the macro scale, it is interesting to see what will happen in the future with the help of nano technology, which will eventually provide possible solutions for longer life of brushed electrical machines. There is also discussion that previous researches put much effort on the materials and environment factors, but less on the basic performance of friction wear and the process during which metallurgical factors might appear (Yasar, Canakci, & Arslan, 2007). REFERENCES
Arbigay, N., Bares, J. A., & Sawyer, W. G. (2010). Asymmetric wear behavior of self-mated copper fiber brush and slip-ring sliding electrical contacts in a humid carbon dioxide environment. Wear , pp. 455-463.
Bhushan, B., Israelachvili, J., & Landman, U. (1995). Nanotribology: Friction, Wear and Lubrication at the Atomic Scale, Nature, 374, pp. 607-616.
Casstevens, J., Rylander, H., & Eliezer, Z. (1978, 5). Influence of high velocities and high current densities on the friction and wear behavior of copper-graphite brushes. Wear , 48 (1), pp. 121-130.
Chapman, S. J. (2005). Electric Machinery Fundamentals (4 ed.). New York, NY, United States: McGraw-Hill Companies, Inc.
Fakih, B., & Dienwiebel, M. (2015). The structure of tribolayers at the commutator and brush interface: A case study of failed and non-failed DC motors. Tribology International , 12, pp. 21-28.
Hamilton, J. R. (2000). DC motor brush life. IEEE Trans Ind Appl , 36 (6), pp. 1682-7.
Holm, R., & Holm, E. (1967). Electric Contacts: Theory and Application. Berlin, Heidelberg: Springer-Verlag.
Hu, Z., Chen, J., & Xia, J. (2008). Study on surface film in the wear of electrographite brushes against copper commutators for variable current and humidity. Wear , pp. 11-7.
Lancaster, J., & Pritchard, J. (1981). The influence of environment and pressure on the transition to dusting wear of graphite. Journal of Physics , pp. 747-762.
Savage, R. (1945). Carbon Brush contact films. General Electrical , pp. 13-20.
Savage, R., & Schaefer, D. (1956). Vapor lubrication of graphite silding contacts. Journal of Applied Phyics , pp. 36-38.
Yasar, I., Canakci, A., & Arsan, F. (2007). The effect of brush spring pressure on the wear behaviour of copper-graphite brushes with electrical current. Tribology International , pp. 1381-1386.
Zhao, H., Barber, G., & Liu, J. (2001). Friction and wear in high speed sliding with and without electrical current. Wear, 249, pp 409-414.
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