A Special Issue: Electric Machinery and Transformers

As the demand for electrical energy increases worldwide, engineers and scientists have been investigating new electrical systems and materials to meet this demand economically, having large-scale planning and employing environmentally friendly energy production and energy-efficient systems for consumption to minimize adverse environmental effects. New applications such as renewable energy production, e-mobility, and aerospace technology can be considered within the scope of this perspective. Thus, it is a necessity to establish new paradigms in the design, construction, and selection of new materials and drive systems for electric machines and transformers, where such stringent requirements, as high power density, low weight, compact size, and low cost, should be complied with. Therefore, the objective of this Special Issue is to facilitate a platform for disseminating new findings on any aspect of electric machines and transformers with certain topics of interest, including new materials used in electric machines and transformers, investigations of the performance of electric machines and transformers at dynamic state as well as at steady state, and acoustic analyses of electric machines as well as transformers due to vibrations. In this Special Issue, 11 articles address these subjects of interest

Electrical Characteristics of Polypropylene Mixed with Natural Nanoclay

Polypropylene has been used in radio-frequency capacitors and has also started to be employed in cables as insulation. The objective of this study was to evaluate the electrical properties of polypropylene filled with natural clay as a nano-material. Polypropylene samples having 0%, 2% and 6% natural clay by weight were exposed to 60-Hz sinusoidal voltages at two different rates of rise. The breakdown voltage of each sample was recorded at these different ramp rates. Also, the Root-mean-squared (rms) current was measured as the voltage was increased across the test samples. The important findings of this study were (a) the breakdown strength of the natural nanoclay-filled polypropylene was higher than the unfilled polypropylene, and the optimum concentration of nanoclay appeared to be 2% by weight; (b) the current density as a function of the electric-field intensity indicated a non-linear behavior with saturation, and the saturation onset took place at a higher electric-field intensity in nanoclay-filled polypropylene, wherein 2% nanoclay seemed to be the optimum concentration as well for the onset electric field of saturation

Influence of AC voltage on the positive DC corona current pulses in a wire-cylinder gap View Document

In the development of hybrid HVDC and HVAC transmission lines, the study of radio interference is an important issue. Positive corona current pulses from high voltage transmission lines are the main source of radio interference. In this paper, the design of a wire-cylinder gap electrode system is presented to study the influence of AC voltage on the characteristics of positive corona current pulses. The study shows that the mode of the current pulses is different from that of either DC or AC corona discharge. Waveform parameters of the pulses, such as rise time, half wave time, duration time, repetition rates, average amplitude, and time intervals of secondary pulses are all statistically analyzed in this study. The empirical formulas for the repetition rates with different AC voltages are presented. A theoretical explanation based on an ion cloud model is given to reveal the mechanism behind the influence of AC voltage on positive corona discharge. The experimental results could provide some references for the prediction of radio interference from hybrid AC/DC transmission lines

Electrical Property of Polypropylene Films Subjected to Different Temperatures and DC Electric Fields

A polypropylene (PP) film is usually used as a dielectric material in capacitors as well as cables. However, PP films may degrade because of the combined effect of temperature and electric field. In an earlier study, plain PP films and PP films loaded with nano-metric natural clay were studied under sinusoidal (AC) electric fields at power frequency and temperatures above the ambient. To better understand the electrical characteristics of PP film under various conditions, the objective of this study is to determine the time-to-breakdown of the plain PP and PP filled with 2% (wt) natural nano-clay when subjected to time-invariant (DC) electric fields at elevated temperatures. In order to achieve this objective, the effects of uniform as well as non-uniform electric fields were compared at the same temperature for the PP film. In this study, experimental results indicated that the time-to-breakdown of all PP films, plain or filled with nano-clay, decreases with the increase in electric field intensity, non-uniformity of the electric field, and temperature. It was also found that the time-to-breakdown of PP film filled with 2% (wt) natural nano-clay under DC electric field is longer and less sensitive to temperature. Furthermore, when compared with the results under the uniform electric field, PP film filled with 2% (wt) nano-metric natural clay indicates shorter time-to-failure under non-uniform DC electric fields. Finally, the morphology of the samples was observed by digital camera, optical micrography, and SEM, to better understand the mechanism of the breakdown

Analysis of Shielding Effectiveness against Electromagnetic Interference (EMI) for Metal-Coated Polymeric Materials

Lightweight materials, such as polymers and composites, are increasingly used in the automotive and aerospace industries. Recently, there has been an increase in the use of these materials, especially in electric vehicles. However, these materials cannot shield sensitive electronics from electromagnetic interference (EMI). The current work investigates the EMI performance of these lightweight materials using an experimental setup based on the ASTM D4935-99 standard and EMI simulation using the ANSYS HFSS. This work studies how metal coating from zinc and aluminum bronze can improve the shielding performance of polymer-based materials, such as polyphenylene sulfide (PPS), polyetheretherketone (PEEK), and polyphthalamide (PPA). Based on the findings of this study, a thin coating (50 μm) of Zn on the surface of PPS and a thin coating of 5 μm and 10 μm of Al-Bronze, respectively, on the surface of PEEK and PPA have indicated an increase in the shielding effectiveness (SE) when subjected to EMI. The shielding effectiveness significantly increased from 7 dB for the uncoated polymer to approximately 40 dB at low frequencies and up to approximately 60 dB at high frequencies for coated polymers. Finally, various approaches are recommended for improving the SE of polymeric materials under the influence of EMI

Effect of Nano-clay Filler on the Thermal Breakdown Mechanism and Lifespan of Polypropylene Film under AC Fields

The wide application of nanocomposites in the insulation system has greatly contributed to the performance improvement of power equipment. However, nano fillers are not omnipotent for improving the properties of composite dielectrics. In some situations, nano-modified materials are in fact a compromise of improving some performance features while sacrificing others. In this work, the breakdown characteristics and time-to-failure of polypropylene film with nano-clay fillers have been evaluated under combined thermal stress and AC electric fields. Experiments on plain polypropylene (PP) samples have also been carried out under the same test conditions as control. Test results indicated that the time-to-failure of the samples with nano-clay filler was shorter than those without nano filler, which is different from the previous experience. SEM and EDS analyses were conducted to study how the failure mechanism had taken place in both plain polypropylene and the nano-clay filled polypropylene. The failure phenomenon in these materials can be explained by molecular thermodynamics. The main reason for the premature thermal breakdown of PP nanocomposite is essentially due to the weak coupling between nano-clay filler and polymer matrix. Finally, suggestions are proposed for nano modification methods and lifespan prediction models of composite dielectrics

Conference report: 2009 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP)

This year, out of 242 abstracts submitted, 233 were accepted, and finally 180 papers were included in the CEIDP Annual Report. These papers were submitted from 25 different countries, with the largest numbers from Japan, China, USA, Canada, France, and UK

Effect of Electric Field and Temperature on the Behavior of Polypropylene Filled with Nanometric Natural Clay

During the last two decades, when compared with pure polymers, nanocomposites have exhibited improved mechanical, thermal, optical and physico-chemical properties, owing to the nanometric particles. Among many other applications polypropylene has found place in electrical systems as a dielectric material. One of these applications is the usage of very thin sheets of polypropylene in high-performance pulse and low-loss RF capacitors as the dielectric medium. Another application is in the power cables as an insulating medium. In an earlier study, the present authors have observed that the breakdown strength improved in polypropylene loaded with nanoclay of various contents. The aim of this study, therefore, is to evaluate the time-to-failure of polypropylene (PP) and polypropylene-based nanocomposite films under the influence of electric field and temperature at the same time because to our best knowledge there is a lack of information in relation to such data

Electrical breakdown of polyurethane-based nanocomposites

In this study the effect of nanometer-size mica fillers in polyurethane is investigated by evaluating electrical breakdown voltages. The particle size of mica fillers used in this study is 1 nm and 10 nm. The TPU-mica composites were prepared with various concentrations of filler content. Each composite sample was subjected to a sinusoidal voltage waveform having a frequency of 60 Hz at a certain rate of rise up to a level where a breakdown occurred. The corresponding rms voltage at this breakdown level was recorded as the breakdown voltage of the sample. Except mica contents of 2% and 10% by weight, the breakdown voltage of the composite was found to be higher with 1 nm particle size than those with 10 nm particle size. The nanocomposite with 1 nm particle size and a concentration of 1% by weight indicated a breakdown voltage almost twice the breakdown voltage of pure TPU