The influence of switching arcs on contact resistance of Ag/SnO2 materials
Silver tin oxide (Ag/SnO2) contact materials are widely used for relays and contactor applications. Device approbation in accordance to standards requires several types of temperature rise test in new and switched conditions. Significant differences in temperature rise after the various switching sequences are experienced. In addition, differences between individual phases of one device - especially in switched condition - are known from test. In this work, the contact resistance evaluation under different load conditions has been studied. Therefore the arcing events at make and break operation during different endurance tests (AC-3 and AC-4) have been analyzed and correlated to the contact resistances at rated current level during temperature rise test. Surface layer changes of contact materials after load are characterized by metallurgical methods. The paper shows the formation of different surface layers as a function of arcing energies during make and break operation. The results demonstrate that make and break arc together are creating the surface layers defining the final contact resistance.
Effect of Ambient Temperature and Contact Force on Contact Resistance and Overtemperature Behaviour for Power Engineering Contacts
With switching devices for power engineering applications the question of overtemperature at steady state current becomes more important as the size of said devices decreases and/or rated currents increase. In order to evaluate the influence of parameters as contact material, contact force, ambient temperature, current and current flow time a PC driven model switch is build up with the ability to set these parameters easily. This paper describes the setup of the model switch and gives first results of tests related to typical conditions of MCCB with a rated current of 160 A. Parameter variations include contact force (5N to 30 N), ambient temperature (25°C to 125°C) and contact material (Ag/C, Ag/Ni, Ag/W, AgNi0,15). Results obtained are discussed on the background of contact surface roughness as well as contact material properties as electrical conductivity and hardness as a function of temperature.
Numerical and experimental investigation on air distributor design of fluidized bed reactor of sawdust pyrolysis
Abstract An improved biomass conversion in a lab-scale fluidized bed reactor (FBR) is achieved by optimizing the air distributor orifice size and number. Two approaches for introducing air were numerically and experimentally examined. These included perforated plate and downward orifice sparger. 3 mm and 5 mm orifice diameters, and 7, 9 and 13 holes were simulated at flow velocities of 2, 4, 6, and 8 times the minimum fluidization velocity, Umf. The simulation results revealed that increasing the orifice number has a positive effect on gas holdup, particularly for the perforated plate, which showed a uniform distribution of bubbles. The sparger configuration produced larger bubbles sizes, which could lead to better recirculation of solid flux, while stagnant particle zones were observed at the lower section of both distributors. A pyrolysis unit is also inserted at the top of freeboard of the FBR. For perforated plate with 6Umf, the pyrolysis process achieved thermal and cold gas efficiencies of 82.91% and 52.03%, respectively, while combustible gases with 41.6%mol CO, 8.7%mol CH 4 and 2.17%mol HC, were obtained. For sparger pipe and 6U mf _sp, 35.33 ppm NOx and 111 ppm SO 2 were measured in flue gas from combustion process at bed region.