SIMPLE STRUCTURE MULTILEVEL INVERTER USEFUL FOR AC MOTOR DRIVES

ABSTRACT

In this paper, a new simplified configuration for a multilevel PWM inverter and its control strategy is proposed. The proposed configuration consists of an auxiliary circuit with four switching devices, and a single full-bridge inverter. For medium power applications, the new circuit design, in which the number of switching devices is reduced, offers characteristics similar to conventional NPC inverters. The control method is based on closed loop regulation of the instantaneous line-to-line output voltages. The proposed scheme is simple, has fast response, and is effective for reducing the size of the DC-link capacitors. The performance of the system is verified by computer simulation and by practical experience     

ON THE PROPAGATOR OF THE STOKES EQUATION AND A DYNAMICAL DEFINITION OF VISCOSITY

The propagator of the unsteady Stokes equation is shown to be dominated by the solution of a purely diffusive equation, whose dispersion coefficient is the viscosity. Pressure plays an indirect role only, by creating instantaneously a steady velocity field which decays slowly in space. Viscosity appears to measure the temporal growth of the second moment of the unsteady Stokes propagator.     

THERMAL CONDUCTIVITY OF LIQUID FOODS—A REVIEW

Theoretical and semi-empirical models for estimating the thermal conductivity (k) of liquid foods are reviewed. Seven models were considered; five, all containing temperature as an independent variable, were selected for their simplicity and apparent accuracy for further study and statistical evaluation. Experimental k values of 158 food samples, such as fruit and vegetable juices, sugar solutions, egg white and dairy products, were obtained from the literature. The model by Riedel (1949) described the data the best, producing the lowest standard error in the estimation of experimental values. Using thermal conductivity of water would give errors almost four times as much as if the Riedel model was used. These semi-empirical models could also be used to estimate average k values in processes where temperature and/or solids content were varying. Theories of thermal conductivity applicable to liquid food systems were reviewed.     

ISOLATION AND GENERAL PROPERTIES OF LECTINS FROM THE BEAN (PHASEOLUS VALGARIS VAR. ROSINHA G2)

A purified lectin was prepared from 0.15 M NaCl extracts of bean (Phaseolus vulgaris var. Rosinha G2) by affinity chromatography on ConA-Sepharose followed by gel chromatography on Sephadex G-200. The haemagglutining material isolated by affinity chromatography (CII-Af) contained at least three active protein bands as shown by polyacrylamide gel electrophoresis. By chromatography on a Sephadex G-200 column, the (CII-Af) fraction gave only a major peak (CII-β) with haemagglutininating activity, which showed a single broad band on gel electrophoresis. Gel isoelectric focusing of the CII-β component gave a single active band in the pH range of 5.5–5.7. The purified lectin was a glycoprotein, containing 8.30% of neutral sugars (as mannose) and 2.12% of hexosamine (as glucosamine) and only trace amounts of sulphur-containing amino acids. The protein had a MW of 136,000 (6.9 S) with a Stokes radius of 43 Å, a calculated diffusion constant (D_20,w) of 5 times 10^?7 cm² s^?1, and a partial specific volume (v) of 0.75 ml.g^?1. From its frictional ratio f/fo = 1.3 and assuming a hydration capacity of 0.3 to 0.4 g of water/g of protein, the axial ratio for the lectin molecule would be in the range of 3–4 if a prolate ellipsoid of revolution is chosen as a model.     

Computer-based Automation and the Governance of Vertical Transactions

The purpose of this paper is to analyse the impact of computer-basedproduction technologies on firms' choice of governance modefor vertical transactions. We argue that in the long run diffusionof the new technologies should lead to a wider use of arm'slength relationships with suppliers of increasingly standardizedintermediate goods. In the early stage of diffusion, however,vertical transactions should tend to be internalized under bierarcbicalcontrol. In any case, network configurations based on subcontractingare negatively affected. Evidence supporting these argumentsis provided through a ‘laboratory’ study of make-or-buydecisions made by 100 adopter firms in the Italian metalworkingindustry.     

Wettability and Work of Adhesion of Nonreactive Liquid Metals on Silica

Wettability of silica by gold, silicon, and lead has been determined by the sessile-drop method. The contact angle, θ, and the work of adhesion, W , have been found to be 143° and 227 mJ·m⁻² for gold at 1353 K, 87° and 869 mJ·m⁻² for silicon at 1723 K, and 120° and 203 mJ·m⁻² for lead at 1000 K. Thermodynamic adhesion between silica and nonreactive pure metals has been analyzed using models for metal-oxide bonds. Models based on the assumption that only van der Waals interactions and/or metal-oxygen chemical bonds exist at the metal-oxide interface are unsuited for explaining the relative variations in the W values. A valid model can be accomplished by using an empirical equation which takes into account both metal-oxygen and metal-oxide chemical bonds. It appears that chemical bonds exist at the interface even for nonreactive metal-ionocovalent oxide systems.