Characterization of Bimetal Strips Using an Accurate Measuring Methodology

摘要

Bimetals strips are a kind of simple, highly reliable, precise and low cost actuators. Made by the combination of two metallic strips with different thermal expansion coefficients, they show the property to bend when the temperature varies compared to the reference conditions; this makes bimetals particularly suitable to be used in electromechanical breakers to prevent overloads in electrical circuits. Bimetals strips are part of the kinematic chain of magneto thermal circuit breakers. Due to the heat produced by current flow, their temperature increases and a bending occurs; if the current exceeds a certain limit, the bimetal is bent in such a way to actuate the device's mechanism, causing the opening of the electrical contacts. The simplicity of bimetals makes them, at moment, an essential component of safety devices. The increasing competition in the world market, however, makes necessary to optimize the design and production of circuit breakers, and bimetal itself is part of this optimization process. Therefore, a fine characterization of the bimetal is required. In this paper we present a methodology for a high accurate measurement of both deflection and blocking force of bimetals. The measurement of the bimetal deflection has been realized with a non-contact technique, based on the use of an optical triangulation distance laser sensor and precise positioners. The blocking force of the bimetal, defined as the external force needed to completely restrain to zero the displacement at some driving current, has been measured putting in contact the accessory of a load cell with the bimetal at no current condition, and supplying current till the thermal equilibrium is reached. During both displacement and force measurements, the temperature of the bimetal and the current have been continuously monitored and measured; an acquisition system has been used for data recording. The measurement set-up has been realized to guarantee a high thermal and electrical insulation of the bimetal, preserving the set-up from electrical leakage and over heating and preventing influence of the measuring equipments on the bimetal behavior. The bimetal has been clamped in an appropriate vice made by a thermal resistant and electrical insulating material. Furthermore, during the force measurements, the load cell accessory bore in its contact point a synthetic ruby ball, which allowed small contact friction, together with high thermal and electrical insulation. Results of measurements of common bimetals are also presented and discussed. Data analysis and uncertainty estimation show the validity of the methodology.