Patent GB-710152-A (GB)
Title: Improvements in electric power regulators, particularly for electric heating apparatus
IPC: H01H61/06
Filed: 1952-01-29
Abstract: 710,152. Thermal switches. STENSHOLMS FABRIKS AKTIEBOLAG. Jan. 29, 1952 [Jan. 31, 1951; Jan. 18, 1952], No. 2401/52. Class 38(5) In a thermal interrupter for controlling the temperature of a hot plate &c. wherein the circuit is periodically broken by means of a bimetallic strip 30 or other thermal member responsive to a heating winding 31 carrying a current proportional to the current in the hot plate, a rapid action being given by the co-operation of a magnet 25 and armature 24 carried by the contacts 1, 2, the magnetic attraction tending to hold the contacts together is caused to diminish with rise of temperature so that a first connecting-in period of extended duration is obtained and the hot plate is rapidly heated to its desired temperature. In the form shown in Fig. 1, the 'fixed' contact 1 is mounted on an arm 3 pivoted at 4 and biased by a spring 6 into engagement with an adjustable stop 13 with a cam surface engaged by a fixed pin 20. The stop 13 may thus be adjusted by rotation of a knob 19 or temporarily raised by axial movement of the knob, a spring 19 returning the knob in the latter case when hand pressure is removed. The stop 13 may engage a screw 9 mounted on the arm 3 by means of a bimetal strip 11 to compensate for variations of ambient temperature. The movable contact 2 is carried by a bimetal strip 21 mounted on a heat-accumulating body 22 mounted on a heat-conducting rod 27 clamped together with the heater 31 to the free end of the bimetal strip 30 which is fixed to the frame at 35. In use, the knob 18 is rotated to a desired setting, raising the stop 13 and bringing the contact 1 into engagement with the contact 2. Current flows through the heater 31 and the bimetal strip 30 bends down raising the contact 2. The contact 1 and arm 3 follow it owing to the magnetic attraction between armature 24 and magnet 25 till heat flowing along the rod 27 causes bending of the bimetal strip 21 carrying the contact 2 and forces armature and magnet apart. The arm 3 then rapidly moves back into engagement with the stop 13 and the circuit is broken. The bimetal 30 now cools and eventually the contact 2 re-engages the contact 1. The bimetal strip 21 is, however, still warm so that the armature 24 is not this time so near to the magnet 25 with the result that when the rise of temperature due to the heater 31 again causes the contact 2 to rise, the contact 1 does not follow it and during subsequent interruptions the arm 3 remains permanently in engagement with its stop. In a modification, Fig. 6 (not shown), the desired variation in magnetic attraction is obtained by placing between the armature and magnet a plate which loses its magnetic properties with rise of temperature, the contact 2 being screwed into the body 22 and not mounted on a bimetallic strip. Additional contacts are also opened by the arm 3 when its original " off " position. In another modification, Fig. 7, the magnet 71 is mounted on the heataccumulatory body 64 and attracts not only a combined armature and ' fixed ' contact 80 but also an armature 72 which is mounted with the movable contact 68 on a bimetal strip 69. To start with, when the contacts 68, 80 engage, the armature and contact 80 is close to the magnet 71 and thus follows the contact 68 when this rises due to the bending of the bimetal strip 58 under the influence of the heating coil 61. Later, as the body 64 warms up, the bimetal strip 69 bends and forces the magnet 71 and armature 80 apart releasing the contact-carrying arm 73 as in the first construction. Simultaneously, the armature 72 approaches and is held by the magnet 71 even when the body 64 cools so that for subsequent interruptions, the contact 80 remains ' fixed' and does not follow the contact 68. Should it be desired, however, to provide a connecting-in period of extended duration after ordinary regulation has set in, this may be effected by pushing the knob 82 axially. This rotates the arm 73 about its blade pivot 74 and presses the contact 80 hard against the contact 68 to forcibly separate the armature 72 from the magnet 71. In this modification a bimetal strip 75 provides ambient temperature compensation.