Sharif Digital Repository

Nanomachines are molecules inspired by natural molecular machines and are able to move and transport at the nanoscale. Due to the advancement of nanotechnology, the need to move materials and energy on a small scale is necessary more than ever. Nanocars are artificial molecular machines with chassis, axles, and wheels designed for nanoscale transport at materials’ surfaces. Understanding the dependence of surface dynamics of nanocars on the substrate’s physico-chemical properties is critical to the design of the transport properties of such man-made nanoscale devices. Among the multitude of potential substrates for the nanotransporters, graphene exhibits intrinsic ripples on its surface,... 

Mostly, the high voltage DC power supplies have output voltage ripple. In such an applications like power supply of microwave lamps in radar systems, low output voltage ripple is an important point of operational characteristics. On the other hand, measuring the low voltage ripple will be a challenge that can lead to recognize of its reasons and reduce its amplitude. In this thesis, different ways of ripple measurement constraints is described, then a method based on high pass filters for output voltage ripple measurement is implemented. The impact of components tolerance and their parasitic components like shunt parasitic resistance of a capacitor in filter operational characteristics is... 

Permanent magnets play an essential role in the structure of the electric motor, but due to limited resources, many efforts have been made to meet the need for them. One of the most important types of non-permanent electric motors is reluctance motors. Despite their antiquity in design, torque ripples have led to the limited use of reluctance motors, but advances in science have made it possible to control these ripples.As a result of the major advantages such as low manufacturing costs, high efficiency, and uncomplicated structure, these motors have been the subject of many researches. One way to eliminate the problem of torque ripples is to increase the number of teeth, but in the common... 

Torque variations caused by intermittent combustion and inertia effects of reciprocating components, exert significant torsional excitation on crankshaft of internal combustion engines. These excitation induces considerable torsional vibration on crankshafts. Therefore, modeling and analysis of crankshaft torsional vibration and absorber systems, especially in automobiles engine that crankshaft tolerate high amplitude excitations coupled with heavy inertia effects has high importance. In this study, ability of centrifugal pendulum vibration absorber and dual mass flywheel which used in rotating machinery for vibration reduction, will be examined. At first, cranktrain will be torsionaly... 

With the increasing expansion of modern DC loads and utilizing energy storage systems along with distributed renewable energy resources, grid-following bidirectional voltage-sourced converters (GFBVSCs) with fast dynamic performance are required. Due to the presence of single-phase loads and asymmetrical short circuit faults, unbalanced grid voltage conditions are available in the distribution system. Under unbalanced conditions, an oscillating component with a frequency of twice the grid angular frequency appears on the DC side. Removing double-frequency ripple in the DC link voltage without third harmonic current injection and reactive power injection to the grid based on the grid codes to... 

Some microwave tubes can function as a narrowband power amplifier and their output power spectrum is an indicator of their amplification and efficiency. One of the factors affecting its power spectrum is the noise and ripple of its HVDC power supply. To reduce the negative effects of power supply, first there should be an exact measurement of ripple and noise in the output of the HVDC power supply. Destructive factors which affect voltage ripple measurement in the case of high DC voltage are follows as conductive interference, radiative interference, intrinsic noise of electrical elements, and nonideal frequency response of voltage divider. After identifying and suppressing the mentioned... 

In this thesis, a new wind turbine scheme based on Electrically-Excited Synchronous Generator (EESG) and diode-bridge rectifier is proposed and developed. This new scheme is mainly based on the idea of generator excitation current employment as a means of wind turbine control. That is why the new scheme is called Excitation Controlled Synchronous Generator (ECSG) wind turbine. The most important aspects of the proposed ECSG wind turbine scheme are cost-effectiveness, simplicity and higher reliability, as well as higher efficiency. Considerable cost reduction for the converter unit of ECSG wind turbine is so important since costly converter has been one of the main challenges of the... 

The TWT is highly sensitive to the ripple amplitude of its power supply output voltage. If the amplitude of the output voltage's ripple exceeds a certain limit, the amplitude of the undesired harmonics in the power spectrum of the TWT is greatly increased. Increasing the amplitude of undesired harmonics in the TWT output power spectrum reduces the accuracy and performance of the TWT. Therefore, for optimum TWT performance, a low-ripple power supply is essential. Typically, a low-frequency rectifier or high-frequency switching structure is used to build a high-voltage DC power supply. In this thesis, the different structures of high voltage DC power supplies are studied. Next, by focusing on... 

In some high voltage applications it is necessary for high voltage power supply (HVPS) to operate over a wide load and gain range. For instance a HVPS may be used to supply various microwave tubes. Our goal in this thesis is to design a HVPS which can provide a DC voltage between 0-15 KV and 0-5 KW. In convetional approaches, operating over wide load and gain range necessitates the switching frequency or duty cycle to swing over a wide range. This approach may disturb soft switching, consequently switching loss will increase. The control process will be more complex too. the wide swing range of switching frequency means that the HVPS needs to operate with low switching frequency too.... 

The purpose of this Thesis is designing and constructing of high voltage power supply with very low ripple. Proposed method of this thesis is formed of two sections. In this proposed structure, in first section, structure is on basis of diodes rectifier that has been used for producing main portion of output load energy. Second section uses with aim of regulating of first stage voltage. In attention to existence of noises and available offsets in different sections of proposed circuits, model is basis of two stages structure. Sampling of output voltage by using of resistance divider for main loop of circuit has been done. So noises and offsets available in circuit act as prevented factor... 

EEG Signal Processing has really important role in study of nerves system and nervous diseases. In most applications, signals occur in the frequency band ranging below 100 Hz are considered. In this study, the higher frequency components of the signal, especially in patients with epilepsy, reported which they are called high frequency oscillations (HFOs). Some sources considered that HFOs are up to 1000 Hz and some other considered up to 2500 Hz. In study of HFOs, the high frequencies between 100 and 200 Hz that are called ripple and seen in normal mode are separated from the faster frequencies from 200 to 500 Hz and these components can accurately help to detect epileptic foci generator (to... 

High voltage dc power supplies are commonly used in medical equipment, microwave lamps, X-ray equipment and radio transmission systems. In radio Transmission power supplies the output voltage quality is of great importance and the output voltage ripple must be very low. In addition usage of low frequency transformer and rectifier needs large bulk capacitors to reduce output voltage ripple. Moreover, to reduce energy discharged into load in load short circuit faults the output capacitance should be as low as possible. An approach to solve this problem is to use a diode rectifier and a switching power supply in series configuration. The Advantage is that the switching power supply provides a... 

In this thesis we would like to investigate structural disorder of Graphene and its effect on electronic properties. Graphene is a one-layer crystal of Carbon atoms which was the first two dimensional crystal observed experimentally. Low energy current carriers on Graphene behave like mass-less Dirac fermions and this interesting electronic property attracts much attention on studying Graphene. Structural disorder of Graphene highly affects its electronic properties. We study thermal fluctuations of Graphene sheet, disorder super lattice, and static ripples, as structural disorders of Graphene. Disorders due to thermal fluctuations of graphene induce a disordered gauge field on the graphene...