A fundamental problem of fault-tolerant distributed computing is for the reliable processes to reach a consensus. For a synchronous distributed system of n processes with up to t crash failures and f failures actually occur, we prove using a straightforward bivalency argument that the lower bound for reaching uniform consensus is (f + 2)-rounds in the case of 0 < f ≤ t −2, and a new lower bound for early-stopping consensus is min (t + 1, f + 2)-rounds where 0 ≤ f ≤ t. Both proofs are simpler and more intuitive than the traditional methods such as backward induction. Our main contribution is that we solve the open problem of proving that bivalency can be applied to show the (f + 2)-rounds lower bound for synchronous uniform consensus. / Singapore-MIT Alliance (SMA)

This thesis has been performed in order to earn a master's degree in electrical engineering. The task was to implement an automatic voltage regulator, AVR, to control the terminal voltage of the synchronous generator Svante at the Division of Electricity at Uppsala University. The AVR uses Bluetooth technology to transfer a control signal produced by a programmable logical controller, PLC, to phase- mounted SSRs that decides what proportion of the AC from a six-phase brushless exciter that is to be used for the magnetization of the rotor. Test runs of the AVR were preformed with a regulator optimized according to the Ziegler-Nichols method and a static exciter that uses brushes and slip rings to apply voltage to the rotor winding. The results obtained complies with the assigned requirements set for this thesis and the AVR. The primary focus of this thesis mainly lies in the construction of the control system, which include the programming of both PLC and Bluetooth communicating devices.

This thesis adresses the problem of estimating the parameters in a permanent magnet synchronous motor (PMSM). There is an uncertainty about the parameters, due to age and tolerances in the manufacturing process. Parameters such as the resistance and the current to torque factor Kt, changes with respect to temperature as well. The temperature in the motor varies in normal motor operation, due to variations in angular velocity and torques. Online estimation methods with the model reference adaptive systems technique (MRAS) and offline methods are presented. The estimation algorithms are validated in simulations with Matlab/Simulink and also evaluated with experimental data. Experiments were performed on a range of different motors, in realistic scenarios. Relevant factors such as the angular velocity of the rotor and the impact of the gravity force are investigated. The results show that it is possible to estimate the motor factor $K_t$, with an accuracy of two percentage from its reference value in normal industry conditions. The estimated value of the motor inductance is within 25 percentage of the calculated reference value. The resistance however is affected by the resistance in the cables from the motor to the measurement device. With the cable resistance included in the calculations, the estimate still often exceeds double the value of the reference value.

Synchronous generators play a vital role in power systems. One of the major mechanical faults in synchronous generators is related to bearings. The popular vibration analysis method has been utilized to detect bearing faults for years. However, bearing health monitoring based on vibration analysis is expensive. One of the reasons is because vibration analysis requires costly vibration sensors and the extra costs associated with its proper installation and maintenance. This limitation prevents continuous bearing condition monitoring, which gives better performance for rolling element bearing fault detection, compared to the periodic monitoring method that is a typical practice for bearing maintenance in industry. Therefore, a cost effective alternative is necessary. In this study, a sensorless bearing fault detection method for synchronous generators is proposed based on the analysis of electrical signatures, and its bearing fault detection capability is demonstrated. Experiments with staged bearing faults are conducted to validate the effectiveness of the proposed fault detection method. First, a generator test bed with an in- situ bearing damage device is designed and built. Next, multiple bearing damage experiments are carried out in two vastly different operating conditions in order to obtain statistically significant results. During each experiment, artificially induced bearing current causes accelerated damage to the front bearing of the generator. This in-situ bearing damage process entirely eliminates the necessity of disassembly and reassembly of the experimental setup that causes armature spectral distortions. The electrical fault indicator is computed based on stator voltage signatures without the knowledge of machine and bearing specific parameters. Experimental results are compared using the electrical indicator and a vibration indicator that is calculated based on measured vibration data. The results indicate that the electrical indicator can be used to analyze health degradation of rolling element bearings in synchronous generators in most instances. Though the vibration indicator enables early bearing fault detection, it is found that the electrical fault indicator is also capable of detecting bearing faults well before catastrophic bearing failure.

The design, analysis, and control of Disc-type Linear Permanent Magnet Synchronous Machines are presented in this dissertation. The machine is designed in order to have the features of high power density, high efficiency, direct drive without extra transmission device, simple mechanical structure, and wide operating speed range for the electric vehicle applications. The Feasible Triangle Technique for permanent magnet synchronous machine design is employed to increase torque density and reduce torque ripple. Magnetic theory and the finite element method are involved to establish machine mathematical model to simulate and analyze the proposed machine. The coupling field between rotor and stator can be analyzed from flux distribution by using three-dimensional finite element method. The numerical simulation and experimental results are used to verify the machine model and the operational characteristics of machine. The dissertation provides the procedures for design, analysis, and control of the proposed machine.

As a result of personal computers and broadband Internet connections become very popular, online learning converts asynchronous learning into synchronous learning or blended learning. This implies that synchronous communication becomes an essential part of online learning and learners now are able to see others face-to-face online just like in traditional classrooms. In the past, the major difference between cyber classroom and traditional classroom is that cyber classroom cannot provide face to face interaction. However, a synchronous cyber classroom features real-time and multiple video communication channels can overcome this shortage and even promoted better attention and engagement for learners. Social presence is an indicator to measure the degree of social interaction, determined by the perception of an individual to environments and other individuals. The prior researches have indicated that increasing social presence is helpful to enhancing learning participation and satisfaction in a synchronous cyber classroom. Hence, we argue that promoting social presence in a synchronous cyber classroom would have a positive effect on learners¡¦ learning participation and satisfaction. The aim of this study is to explore the factors affecting social presence in a synchronous cyber classroom. We will apply social learning theory as a basis to verify if social presence does have a positive effect on learning outcome and what factors affect social presence in a synchronous cyber classroom learning environment. Because the perception about social presence is accumulated as time goes by, a questionnaire survey is used in this study. The target population is drawn upon the learners who have had real experiences of participations in learning with synchronous cyber classrooms. The questionnaires were carried out on the Web. The subjects were learners from the National Sun Yan-Sen Cyber University, the K12 digital school and some others from the PTT BBS. The total number of valid questionnaires is 252. The research result indicates that the factors affecting social presence in a synchronous cyber classroom are intimacy, user friendliness, responsiveness, extraversion, and cue richness. Moreover, social presence does have a positive effect on learning participation and satisfaction. The contribution of this study is finding out the factors affecting social presence from the social learning theory perspective. If teachers could pay attention to these factors and try to apply them for enhancing learners¡¦ social presence, learners¡¦ learning outcome is greatly improved in a synchronous cyber classroom environment. This study is concluded by pointing out some suggestions for practice and future research.

Automated Material Handling Systems are practically ubiquitous across industry and many rely on the operation of timing belt drives, which require that the belt be tensioned properly in order to function correctly. A series of experiments was carried out in which the frequency spectrum of acoustic radiation caused by transverse vibrations of a timing belt was used to calculate the belt tension. A lumped-parameter model was developed that considers the geometric complexity of the timing belt's construction by calculating the stiffness of a single belt tooth using commercially-available finite element analysis software. This was used to modify the belt's effective stiffness suggested by the manufacturer. Based on the experimental data, a set of material properties was determined that, when used in the lumped-parameter model, resulted in tension predictions that agreed with the experiment within the 95% confidence intervals of the means. / text

published_or_final_version / Electrical Engineering / Master / Master of Philosophy

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