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Title: An investigation of tribological characteristics of an electromechanical system
Other Titles: Ji dian xi tong de mo ca te xing yan jiu
Authors: Louie, Yuen Tung (雷遠東)
Department: Dept. of Manufacturing Engineering and Engineering Management
Degree: Master of Philosophy
Issue Date: 2007
Publisher: City University of Hong Kong
Subjects: Electromechanical devices
Notes: CityU Call Number: TJ1075.L68 2007
Includes bibliographical references (leaves 119-126)
Thesis (M.Phil.)--City University of Hong Kong, 2007
xvi, 149 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: The life of a conventional electric motor depends much on the wear rate of carbon brushes, which slide against a rotational commutator in operation. The damage of carbon brushes is attributed to both mechanical wear and electrical erosion. This study aims at finding the tribological characteristics of the sliding contact between the motor brush and commutator under both excited and unexcited conditions with different PV (load x velocity) factors. The mechanical wear of carbon brushes under the effects of different surface temperatures was also studied. In order to conduct a parametric study, a friction and wear test rig which is able to simulate the real running contact of motor brushes and a commutator was developed. The range of running conditions includes brush load from 50 to 800 gf, rotational speed from 1,000 to 35,000 rpm, brush current from 0 to 6 A (220 V AC) or 0 to 20 A (18 V DC), and measuring frictional force from 0 to 550 gf during operation. Most studies on friction and wear of sliding contacts focus either on the individual or combined effect of sliding speed and normal load in terms of PV factors or energies. Temperature change is treated as a by-product of the sliding action. As the temperature of the sliding surfaces changes, their physical properties such as hardness may also vary. As a result, the wear rate of the sliding contact, in turn, may be affected. In this investigation, the relationship between the wear of carbon brushes and the PV factors under different temperatures was experimentally deduced. The experimental results show that the brush wear is dominated by the electrical wear when under low brush load. It is mainly caused by the unstable contact between the brush and commutator surface under dynamic condition. When the brush and commutator come into contact, they are not fully in touch with each other throughout the surfaces. A surface is supported by the other only through the asperity contacts. The true area of contact is equal to the sum of the individual asperity contact area. The size of the real contact area is closely proportional to the brush load. As the brush load is too small, the relatively small asperity contact area gives rise to the contact resistance. Therefore, more electrical energy is consumed for a fixed current load at the contact which is susceptible to wear in operation. Furthermore, as the rotational speed increases under low brush load, the vibration of the brush becomes more serious than under high brush load. The sliding interfaces can be instantaneously separated and an air gap appears when vibration occurs. The magnitude of the air gap depends on the vibration amplitude. As the air gap can be taken as a dielectric material with a relatively large resistance, large amount of energy can be dissipated at the interface. If electricity flows through the dielectric material (e.g. air, wear debris) in the forms of ionized gases or electrical discharge, electrical sparking or arcing occurs. It can then further damage the contact surfaces. On the other hand, the brush wear is dominated by the mechanical wear when under high brush load. When the brush load is sufficiently high, the relatively large asperity contact area assists in keeping the contact resistance at a low value. In addition, the high brush load can help to suppress the vibration at high speed. Therefore, less electrical energy is consumed at the interface and the wear is highly dependent on the mechanical properties of the sliding pairs. The heat-up pin-on-disk (POD) experimental results show that two wear behaviors can be observed. They are either a linear relation between the wear rate and temperature or a constant wear rate at different temperatures, as predominantly happened under low or high PV factors respectively. It is most likely caused by the change in the physical properties of the carbon brush or interfacial conditions at high temperatures. The tribological characteristics of an electromechanical system can be complicated. It is because the effects of various parameters such as brush load, sliding speed, current magnitude, surface conditions, and environmental factors (e.g. temperature and humidity) are all interrelated. The tester that was developed in this project can be used to study the effects of those parameters under realistic motor running conditions. Furthermore, the tester can be used by industry for a quick evaluation of carbon brushes and as a base for accelerated life test of a motor.
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