ON THE SAMPLE STABILITY OF NONLINEAR CHATTER MACHINING

This paper is concerned with the determination of sufficient conditions guaranteeing stability of nonlinear regenerative chatter machining subject to a stationary stochastic process of small intensity at a Hopf bifurcation point. The first step in the investigation is the identification of boundaries of linear stability and instability in the parametric plane of the machining model. Next, the conditions for nonlinear sample stability are derived using the mathematical framework of the classical Hopf bifurcation theorem, the center manifold theorem, the integral averaging method and the Lyapunov exponent. Sub- and supercritical bifurcations are established qualitatively. The investigations are analytical and they have been conducted without the assumption of small time delay between successive tool passes.     

More efficient balancing of rotors

The balancing of equipment may be improved by reducing the number of cycles required. That calls for a dynamic model of the part being balanced. It is established that this model must be based on vector coefficients characterizing the influence of imbalances in the correction planes of the part on the vibrations of its bearing in the balancing machine. Formulas for these coefficients are obtained, as well as an algorithm for their computer calculation. By simulation, the changes in the vector influence coefficients are determined as a function of the balancing variables.     

Study of the Development of Positive Streamers Along an Ice Surface

This paper presents the results of fundamental investigations on the inception and propagation of corona discharge on an ice surface stressed with a standard lightning impulse voltage. High-speed photography and photomultiplier techniques were used to observe and record the propagation of the streamers. The effects of several experimental parameters such as freezing water conductivity and HV rod electrode radius on the streamer inception parameters were investigated. Moreover, time to first streamer, inception voltage and corresponding field, as well as streamer propagation velocity and charge deposited by a streamer on ice surface were measured. The results are discussed and emphases are laid on the main factors influencing the development of positive streamers on ice surface.     

Challenge of mixed insulating liquids for use in high-voltage transformers. II. Investigations of mixed liquid impregnated paper insulation

As society depends more and more on electrical power, the demands for highly reliable power supplies are increasing. This paper examines the compatibility of oil mixtures with cellulose and aramid papers that are used in transformer insulation.     

A well balanced omega-6/omega-3 ratio in developing flax bolls after heating and its implications for use as a fresh vegetable by humans

Flax seed (Linum usitatissimum L.) can be used for both industrial and food products including a wide variety of nutraceuticals and health foods and for which a restricted and specific fatty acid (FA) profile is required. As functional foods, flax meal is provided to humans in the form of ground vacuum-packed grains or flaxseed-enriched baked products. However, no reports are yet available on the potential food use for fresh flax products. A comparative study was conducted using two flax cultivars, AC McDuff and Linola, to evaluate FA accumulation in developing bolls at 12 different developmental stages and to determine stages having well balanced omega-6/omega-3 FA ratio. The lipid hydroperoxide level in fresh bolls and seeds, and the cyanogenic glucoside content in developing bolls were also measured in AC McDuff following different heating treatments. The findings indicated and suggested that the use of flax bolls as health promoting vegetable can be considered if adequately processed and palatable food attributes are obtained.     

Asymptotic stability of a stochastic delay equation

The investigation of a nonlinear stochastic delay equation with structural tool and regenerative time delays is presented. The conditions of Hopf bifurcation are computed in order to describe the regions of stability and instability. Explicit expressions characterizing the influence of nonlinear and stochastic perturbations, valid in the first order centre manifold approximation, are derived. In addition to this, we describe the underlying mathematical ideas of the centre manifold reduction of delay differential equations to ordinary differential equations for fixed time delays.     

A rotor speed estimation algorithm in variable speed permanent magnet synchronous generator wind energy conversion system

A rotor speed estimation algorithm in a direct vector controlled permanent magnet synchronous generator wind energy conversion system is proposed. The proposed method is based on a simple equation obtained from the flux model of the machine and contains only stator flux and current. Constant gain recursive least squares estimator is used for implementing the speed estimation algorithm. Rotor position information used for coordinate transformation is computed using the estimated speed. Stator flux information required by the speed estimator is obtained using the stator voltage equation by implementing a programmable low pass filter. The estimated speed is used as the feedback signal for the speed control loop of the vector controlled machine side converter control system whose command speed is obtained from a wind speed sensorless maximum power point tracking controller, thus, we obtain a complete rotor speed and wind speed sensorless permanent magnet synchronous generator wind energy conversion system. Simulation is carried out to validate the performance of the proposed method. Copyright © 2012 John Wiley & Sons, Ltd.     

Modelling and experimental validation of high performance low platinum multilayer cathode for polymer electrolyte membrane fuel cells (PEMFCs)

The aim of this paper is to demonstrate numerically the feasibility of using a sputtering technique for the design of Polymer Electrolyte Membrane Fuel Cell cathodes having low platinum loading and offering high performance. A two-dimensional steady state model accounting for electrochemical reactions, momentum, saturation, species and charge conservation equations have been developed. It has been applied to three different cathode configurations where the platinum is assumed to be sputtered one, two and three times on the same number of carbon–Nafion layers (CNL). The predicted polarization curves were validated with the experimental data obtained in our laboratory. Our results showed that the best performance is obtained with a three Pt sputtered layer cathode. A parametric study proved the dependence of the PEM fuel cell cathode performance on the CNL porosity, the (Pt/C) agglomerate radius and the thickness of this agglomerate structure.     

近年来国内重大食品安全案例的回顾及分析

通过回顾近几年来国内发生的重大的食品安全案例,并对其原因及危害进行了分析,指出违反自然规律的生产和发展都是不切实际的。     

Drying of transformer insulation using zeolite

Power and distribution transformers are some of the most important components in the power network. The presence of moisture in a transformer deteriorates the transformer insulation by decreasing both the electrical and mechanical strength of the solid insulation. It is of great importance to pay attention to the moisture content of composite insulation systems in order to maintain low dielectric loss, high dielectric breakdown strength and low rates of degradation of the insulating system in HV equipment. In this paper, an approach to the said problem is presented. With the application of a synthetic hygroscopic mineral (molecular sieve zeolites) - a revolutionary, environment friendly, multipurpose mineral - a continuous drying process of the transformer oil during operation without the need for service interruption is achieved. A study of the effects of zeolite on the electrical and dielectric behaviour of the insulating liquid of the transformer is included in this paper.