7,900 research outputs found
Validity of clinical measures of the AC/A ratio
No full textThe gradient stimulus and heterophoria stimulus measures of the accommodative convergence/accommodation (AC/A) ratio are simple and widely-used techniques. We compared these two clinical methods with the more accurate and objective gradient response AC/A ratio method. Twenty-three subjects, with ages ranging from 17 to 42 years and with normal binocular vision, were assessed. The response AC/A ratio was determined using a Canon AutoRef R-1 to measure accommodation and an IRIS eye movement monitor to measure eye movements. Stimulus AC/A ratios were determined by taking phoria measures with the presentation of lenses (gradient method) and by varying the working distance (heterophoria method), with the accommodation taken as dioptric value of each stimulus. The mean AC/A ratios were 4.37 ± 1.31 Δ/D (mean ± standard deviation), 3.49 ± 2.17 Δ/D, and 5.81 ± 1.03 Δ/D, for the gradient response, gradient stimulus and heterophoria stimulus AC/As, respectively. The gradient stimulus AC/A showed only moderate validity, as measures corresponded with the ratios of the response method mainly when identifying very high and very low AC/A ratios (r2 = 0.388). Inaccuracies in the gradient-stimulus AC/A arose because the accommodation often varied markedly from the value of the stimulus lenses. The heterophoria stimulus AC/A appeared to be invalid for the subjects tested, as no correlation was evident between it and the response AC/A (r2 = 0.003). Since the heterophoria technique is susceptible to both errors of focus and the influence of proximal vergence, it may reliably detect only very high AC/A ratios
AC-DC Power System Analysis
No full textThe following topics are dealt with: AC-DC converter; power flow solution; harmonic solution; three-phase power and harmonic flow; electromagnetic transient simulation; electromechanical stability and transient converter simulatio
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
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
Experimental High-Frequency Parameter Identification of AC Electrical Motors
No full textIn 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
Bruce Pickett, Student
No full textCharles Bruce Pickett was a student at Jacksonville State University in the 1960s. In 1967-1968 he was a member of SGA, SEA, and ROTC.https://digitalcommons.jsu.edu/lib-ac-histimg/18446/thumbnail.jp
Bruce Pack, Football Player
No full textBruce Pack was a student at Jacksonville State College (now Jacksonville State University) in the 1960s. This photo was taken in 1965 for the Mimosa.https://digitalcommons.jsu.edu/lib-ac-histimg/13236/thumbnail.jp
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
Bruce Johnson, 1968-1969 Delta Chi Member
No full textBruce Johnson was a student at Jacksonville State University in the 1960s. In 1968-1969 he was a member of Delta Chi.https://digitalcommons.jsu.edu/lib-ac-histimg/21684/thumbnail.jp
Judy Bruce, ROTC Sponsor 1
No full textJudy Bruce was a student at Jacksonville State College (now Jacksonville State University). In 1962-1963 she was a Sponsor for the Charlie Company, Honorary Captain, as part of the ROTC.https://digitalcommons.jsu.edu/lib-ac-histimg/28113/thumbnail.jp
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