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.
Traceability strategy for gear-pitch-measuring instruments: development and calibration of a multiball artifact
There is a strong demand to ensure the traceability of gear-measuring instruments (GMIs). We propose a multiball artifact (MBA) for the evaluation of pitch-measuring accuracy. The aim of the MBA is to transfer the minimum uncertainty from a calibrated value at the National Metrology Institute of Japan (NMIJ) to a measured value of GMIs at a shop floor. The MBA is composed of equally spaced high-accuracy balls around an axis. The pitch-measuring accuracy of GMIs is evaluated by measuring the angular pitch deviation of the balls instead of the angular pitch deviation of a gear. We calibrated the angular pitch deviation using a coordinate measuring instrument (CMM) at the NMIJ and adapting a multiple-orientation technique. We proposed a calibration strategy of the angular pitch deviation. The calibration value of the pitch deviation for the MBA at the NMIJ was obtained with a measurement uncertainty (U95) of 0.2 µm. We evaluated the pitch-measuring accuracy of a GMI using the calibrated MBA. Each value of U95 for the cumulative and single pitch deviations for the left and right flanks was less than or equal to 0.5 µm. The small uncertainty was transferred from the NMIJ to the GMI.