Improvements in and relating to electric overload protective systems and apparatus therefor

摘要

557,190. Protective cut-out arrangements; electromagnetic switches; resistances BRITISH THOMSON-HOUSTON CO., Ltd. May 27, 1942, No. 7193. Convention date, May 27, 1941. [Class 38 (v)] An overload protective system having instantaneous and inverse time-limit characteristics, and suitable for the protection of a motor 45, comprises a relay winding 26 energized in series with a mass 35 of negative temperaturecoefficient resistance material adapted to be heated conductively by short-circuited secondary windings of a primary winding 16 carrying the load. The mass 35 is preferably of the composition set out in Specification 540,344, [Group XXXVI], and is positioned between the rectangular-sectioned and channel-sectioned copper annuli 17, 18, respectively, to which the circuitleads are also connected, the winding 26 being energized either from the supply 37, 38 at constant voltage or from a variable-voltage extra secondary 19 to primary 16. The circuit may also include a remote milliammeter 39 calibrated directly in temperature units, and a further winding 26a of few turns in series with the load may constitute the instantaneous relayelement. With sustained moderate overloads, the higher induced currents in annuli 17, 18 heat the mass 35 and lower its resistance at a rate depending on the current and also, when energized by winding 19, upon the voltage of that winding; but if the supply is from a constant source the calibration of meter 39 is more uniform and the inverse-time characteristic more pronounced. The effect of 26a is negligible except at sudden high overloads, when it acts at once, but if this winding is omitted the winding 26 alone, when sufficiently energized by winding 19, provides instantaneous operation. The two windings 26. 26a, together with an anti-chatter shading ring 27, are assembled on a core 20 which preferably comprises a lateral extension 12 of the threelimbed transformer core. The armature 25 pivots in the adjacent side limb of the latter, and when attracted, de-energizes at contacts 28, 29 the circuit of holding coil 48 of the main contactor.