GB530967A - Improvements in and relating to thermal relays - Google Patents

Abstract

530,967. Thermal switches. BRITISH THOMSON-HOUSTON CO., Ltd. July 7, 1939, No. 19871. Convention date, July 9, 1938. [Class 38(v)] (Also in Group XIII] A bi-metallic thermal relay rendered highly insensitive to mechanical shock and vibration, comprises a pair of oppositely-acting strips 10 each secured to a yieldable member 11 on a bracket provided with an adjusting screw 14. Each strip carries a magnetizable member 19, a hollow iron sleeve 21, a silver contact 22 within the sleeve 21 and a copper sheet 23. Members 19 co-operate with poles 18 of an adjustable permanent magnet 16 of high retentivity and coercivity and consisting, for example, of an alloy of aluminium, nickel, cobalt and iron. The two strips, together with the parts carried by them, are substantially similar, so that the system is inherently insensitive to any vibration to which the relay is subjected, but any movement of the strips relative to the magnet 16 produces eddy currents in the sheets 23 and members 19 which tend to damp the movement producing them. This damping is aided by hysteresis losses in members 19 and, in the closed position, by the friction produced between the contacts due to collective movement of the strips. The magnetic force also tends to accelerate the closing and opening movements of the contacts, in the latter case, because the magnetic force diminishes more rapidly than the bending-force of the strips, increases due to the temperature-change.