S.M. Abu Sharkh

The evaluation of arc erosion on electrical contacts using three-dimensional surface profiles

Three-dimensional surface profiling is a useful technique for the evaluation of electrical contact erosion. A review is given of the various methods by which a 3-D surface profile can be measured, and numerical techniques are described which can be used to analyze electrical contacts to define erosion in terms of volume and height changes. Experimental data on the rate of change of mass of Ag-CdO contacts are used to compare with volume and height changes over a number of switching cycles. Emphasis is given to the evaluation of 3-D surface profile in the particular condition where the change of mass of the contacts is nearly zero after a large number of break only switching cycles, and it is shown that in this condition the contacts exhibit volume changes both above and below the datum surface of a new contact.

Design and performance of an electric tip-driven thruster

A 250 mm diameter demonstrator electromagnetic tip-driven propeller (ETDP) unit has been designed, built and tested in a towing tank at zero, advance speed and for a range of advance speeds. The aims have been to assess the performance, efficiency and general practicalities of such a propeller for use on underwater vehicles. An overview is given of the general design philosophy and practical design issues of the demonstrator. The testing procedure is described and performance results are presented.

Voltage steps in atmospheric low current arcs between opening silver metal oxide contacts

When breaking a low current dc circuit (typically <10 A), the arc voltage exhibits a stepped rise as the contacts move apart. As the current increases, these steps gradually disappear, and the voltage rises smoothly. The paper presents experimental results for atmospheric arcs between silver metal oxide contacts. Up to 5 steps can be observed for currents less than 0.5 A. The probability of occurrence of the steps decreases as their voltage level increases. The number of steps also decreases with increasing current. At currents above 7 A the steps disappear and the voltage rises smoothly. The contact gaps at which the steps occur were found to increase as contact opening velocity increases. The paper discusses the different theories proposed in the literature to explain the phenomena, in the light of the presented experimental results.

Assessment of industrial electrostatic hazards using finite-element electric field analysis

Charged clouds, generated in industrial processes, can lead to large values of space potential and electric field inside a vessel. Isolated masses of airborne liquid within can carry induced charge to the wall resulting in electrostatic discharges. Critical values of space potential can be found using a relationship between the jet safety factor and the material hazard coefficient. A computer package is described which allows complex space potential profiles and electric fields to be modelled for a prescribed space charge density. This allows the central space potential, a key indicator of electrostatic risk, to be evaluated. Examples are presented of two recent hazard investigations made by Wolfson Electrostatics.