14,433 research outputs found
Influence of ac ageing on space charge dynamics in LDPE
No full textPolymeric materials have been widely used as insulation in power industry due to their excellent electrical properties. However, these properties deteriorate in time irreversibly when the material is subjected to electric stress. Although space charge is believed to play an important role in ac ageing, exact mechanisms are poorly understood due to very limited experimental data. In the present work efforts have been made to investigate the influence of ac ageing on space charge dynamics in low-density polyethylene (LDPE). LDPE films with 200mm were aged at 50 kV/mm at 50 Hz for various times at ambient temperature. Space charge dynamics in the samples prior to and after ageing were monitored using the pulsed electroacoustic (PEA) technique under dc electric stress. The results indicate that there is a significant amount of homocharge accumulation in the unaged sample due to charge injection. These injected charges are the captured by the deep traps originated from the interface between crystalline and amorphous regions in LDPE. Ageing under ac condition does not necessarily lead to an increase in amount of charge in the bulk but leads to an increase in mobility of charge carriers. Chemical analysis by infrared spectroscope (FTIR) reveals there are chemical changes taken place in the bulk of the material after ac ageing. It is believed that the chemical changes introduce shallow traps which promote the movement of charge carriers in the bulk. Consequently, the injected charges spread across the sample
<i>egl-43</i> and <i>nhr-67</i> repress <i>lin-12 Notch</i> expression to prevent AC proliferation.
No full text(A) Expression of LIN-12::GFP after control, lin-12, egl-43 or nhr-67 single and egl-43; lin-12 or nhr-67; lin-12 double RNAi. The left panels shows Nomarski (DIC) images, middle panels LIN-12::GFP expression in green and the right panels the GFP signal merged with the AC labelled by the cdh-3>mCherry::PH reporter in magenta. Elevated LIN-12::GFP expression was observed in the ACs of 93/128 egl-43 and 119/130 nhr-67 RNAi treated animals, while none of 111 control RNAi animals showed LIN-12::GFP expression in the AC. (B) Quantification of the AC numbers in mid-L3 larvae treated with the different RNAi combinations shown in (A). (C) Quantification of the AC invasion defects caused by the different RNAi treatments shown in (A). (D) GFP::MCM-7 expression together with LAM-1::GFP after control, egl-43 or lin-12 single and egl-43; lin-12 double RNAi, and in larvae expressing NICDΔCT::mCherry. The left panels show Nomarski (DIC) images and the right panels the GFP::MCM-7 signal in green merged with the AC labelled by the cdh-3>mCherry::PH reporter in magenta. 32/35 egl-43 single RNAi and 13/40 egl-43; lin-12 double RNAi treated animals showed GFP::MCM-7 expression in the AC. None of the 32 control and of the 40 lin-12 single RNAi treated animals exhibited GFP::MCM-7 expression in the AC. Expression of NICDΔCT::mCherry induced GFP::MCM-7 expression in 32/33 cases. (E) AC-specific expression of nicdΔct from the cdh-3 enhancer/promoter leads to the formation of multiple ACs. Left panels shows Nomarski (DIC) images and middle panels the BM marker LAM-1::GFP. The right panels show the mCherry expression from the cdh-3 promoter as a control (row 1) or co-expressed with nicdΔct from a bi-cistronic transcript (row 2). (F) Quantification of the AC numbers in three independent control lines expressing mCherry alone and in three lines expressing NICDΔCT together with mCherry in the AC. (G) Quantification of the AC invasion defects in control lines and in lines expressing NICDΔCT together with mCherry in the AC. The pooled results of the three indepdnet lines are shown. The error bars indicate the standard deviation and the horizontal bars in (B) and (F) the mean values. Statistical significance was determined with a Student’s t-test and is indicated with ** for p<0.01 and **** for p<0.0001. The black arrowheads point at the AC nuclei. The numbers in brackets in the graphs refer to the numbers of animals analyzed. The scale bars are 5 μm.</p
A power sharing series power BJT array with isolated low voltage control for AC power control applications
No full textA technique for a continuously variable AC resistance using a series BJT array is presented. This array provides high power dissipation capability and uniform voltage and power distribution across the individual transistors. The array, controlled using a set of optoisolators to maintain the electrical isolation between the control circuits and the power stage, could be used as the basis to develop several useful techniques including a solid state AC regulator with comparable performance to the commonly used ferro-resonant systems; a linear AC electronic load suitable for testing UPS and other power conditioners; and, in other AC power control applications such as switching capacitors in AC resonant circuits
Research on DC-modulated AC/AC converters
No full textAC/AC converter is popular equipment widely applied in various industrial applications. However, it has high total harmonic distortion (THD), low power factor (PF) and poor power transfer efficiency (η). A new idea of the DC-modulated AC/AC converter is advanced by using DC/DC conversion technology to improve the power factor. The main objective of this project is to analyze the circuits and discuss the simulation and hardware testing results of this new idea applied to a Buck converter.
The analysis in this project will be divided into three steps. Firstly, the author calculates the mathematical model to get a transfer function of this new type of Buck converters. Secondly, the author will use the software named PISM to simulate the circuit operation. Thirdly, in order to prove the practicability of this new design, the author would like to set up a hardware model after getting some correct parameters from the PISM.
From the discussion in this report, the most significant parameters are duty cycle and frequency. For this new technology requirement, these significant parameters which improve power factors distinctly will take a lot of new influences on the converter. The consideration of these influences will be discussed by simulation results and hardware testing results which give some helps to other researchers to consummate this new technology in future.Master of Science (Power Engineering
DC/AC/DC chopping converter
No full textThe purpose of this Final Year report is to provide a clearer overview to the student’s FYP Professor, Assoc Prof Luo Fang Lin of the knowledge and experiences the student has accumulated and how he has applied the theory he has learnt from his undergraduate course to meet the objective of the project. In this project, the student is required to design and build a DC-AC-DC chopping converter with a feedback controller to ensure that a constant output voltage of 33VDC despite fluctuations of input voltage (9.3VDC to 11.5VDC).Bachelor of Engineerin
Experimental High-Frequency Parameter Identification of AC Electrical Motors
Full text linkIn order to predict conducted electromagnetic interference in inverter-motor drive systems, high-frequency (HF) motor models are requested and the involved parameters have to be available. In previous studies, the authors have presented an accurate HF model for induction motors and they have defined the procedures to identify the model parameters. In this paper, these results are extended to several types and sizes of industrial ac motors such as induction, synchronous reluctance (without interior permanent magnets), and brushless motors. The model parameter-identification procedure has been improved, and it is based on a least-squares data fitting applied to the measured magnitude and phase-frequency-response curves of the phase-to-ground and the phase-to-neutral impedances. The aim of this paper is to provide quick indications to select the suitable values of the HF model parameters, with reference to the size and type of the ac motor, to evaluate the HF voltage and current components in inverted-fed ac motor system
Using the AC Drive Motor as a Transducer for Detecting Electrical and Electromechanical Faults
No full textCondition monitoring of AC motors is a subject area that has received extensive research. Whether this monitoring is carried out on a scheduled basis by engineer intervention, or continuously using an on-line unit, the results of this testing enable preventative maintenance work to be a carried out earlier, before any major failure occurs. Monitoring using vibration analysis is the most common and depending on the plant, can be done once or twice a year. This is usually limited to the condition of motor bearings and is not commonly used to detect failures in the motor electromagnetic systems. Monitoring units that use motor current measurements are also available, but these are less widely-used and usually on major plant motors (>250kW for example) that have a large capital outlay to replace.
The industry drivers – as always – are maximum plant and machinery uptime, with the minimal amount of scheduled maintenance. If maintenance is carried out too regularly, costs rise significantly not only due to the maintenance activity itself, but disruption to production schedules. Maintenance schedules that are too infrequent can result in an unacceptable rise in total failures of plant that are unexpected and may cause a significant amount of production disruption and downtime, especially if this occurs during out-of-hours working time. However, industry now faces another big challenge and one that has had a good share of exposure over the last few years. It is of course, the drive to reduce carbon emissions and with it the amount of energy that a plant itself consumes. What has brought this more to the fore recently is the significant rise in energy costs. Whilst product margins have to remain the same, many companies energy costs have seen a two to three-fold increase in energy budgets in the last few years alone. For industry processes that have a significant amount of fan and pump applications, the manufacturers of low-cost AC inverters have saturated the lower-performance market of inverter drives such that any drive can control these type of fan and pump applications, where accurate speed control is not a major driver.
Unfortunately, this can be a step backwards for end-users of plant that use equipment to monitor motor condition via motor current signals. Additionally, vibration analysis that relies upon ‘base-lining’ motor data when the AC motor is running at base speed may not give accurate readings when the motor is under inverter control and running at a different speed.
For manufacturers of AC inverter drives in this low-end market, it can be difficult to sell a product from one manufacturer over another without the unit having a “USP”, or Unique Selling Proposition. Most decisions taken on inverter equipment purchase at this level are usually in favour of the equipment that costs the least to purchase. Credibility of manufacturers based on product history and perceived reliability cuts little ground with an ever cost-conscious industry.
This is where the research into diagnosis of faults on inverter driven motor systems can provide just this USP for manufacturers. If the incorporation of on-line diagnosis for simple inverter applications can be brought to a typical inverter unit at a reasonable cost, the manufacturer who can offer this gains a unique foothold in the marketplace – a drive that can monitor and signal that the motor it is driving is showing signs of early failure.
It will be sensible to limit this research to simple inverter applications as high-end inverter drives that operate equipment such as high-speed printing presses, rotary shears will be more difficult to model and simulate than a fan or pump application.
It is hoped that a typical inverter drive can relay enough detailed information about the load which it is driving to allow this to be used for abnormal motor load conditions as this will provide a platform on which to extend the research beyond this MSc and into the realms of incorporating such technology into a drive manufacturers equipment. If this can be done without major modification to an inverter, then it may be easier to implement in equipment offered by different manufacturers.
It is quite possible that this technology could be licensed under a name that guarantees the performance of the condition monitoring algorithms and reliability from one drive manufacturer to another
Double actuator cylinder (AC) model of a tandem vertical axis wind turbine (VAWT) counter-rotating rotor concept operating in different wind conditions
No full textThe aim of this thesis is to evaluate the influence between the two counter-rotating VAWT rotors with the Double Actuator Cylinder (DAC) model which will be validated against the panel code. The DAC model is developed from the original Actuator Cylinder (AC) model which includes the Modified-Linear (Mod-Lin) correction. The original AC model for modelling single rotor is investigated, modified and validated against the panel code to be sufficient which is the premise for a sufficient DAC model. The original AC model is modified to become a dynamic AC model including Cheng’s modification, flow curvature effect and unsteady aerodynamics. The unsteady aerodynamics is included by coupling the modified Risø dynamic stall model with the AC model. An investigation with Migliore’s conformal mapping method as well as panel code revealed that the modified Risø dynamic stall model implicitly included the flow curvature effects. The derived dynamic AC model is validated to show good agreements with the panel code. It is also revealed that further improvements could be made to the Mod-Lin correction method. The DAC model is finally derived, and the performance on predicting the influence between the two rotors is then investigated.European Wind Energy Masters (EWEM
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