Patent GB-328270-A (GB)
Title: Improvements in a method and means for electrically heating metals
IPC: H05B3/00
Filed: 1928-10-18
Abstract: 328,270. Thompson, F. W. Oct. 18, 1928. Heating by electricity.-In heating metal bars or the like by passage of current therethrough the change of resistance of the bar causes automatic operation of a switch to connect the bar to a lower voltage 'source when a. predetermined temperature is attained. The lower voltage may merely maintain the bar at this temperature or may cause a further rise to a predetermined final temperature. The current is supplied by a transformer having two primary windings P<1>, P<2>, and a secondary winding comprising a single turn S completed by the bar A to be heated. The water-cooled electrode clamps 47, 48 for gripping the bar are mounted on copper slabs 53, 49. The slab 49 is rigidly bolted to one end of the secondary. The slab 53 is slidably mounted on bearer bars 54 secured to the other end 43 of the secondary. Ball bearings 56 are arranged between the ends of the slab and brackets 55 on the secondary. When the bar expands the slab 53 moves against the action of springs 59 secured to the bar 43 and which normally hold the slab 53 against set screws 58 in brackets 57. Alternatively expansion may be allowed for by building up the limb 43 from sheets of copper to render it flexible. The movable electrode jaws are mounted on rods 63 carrying discs 77 pressed by springs 76 against cams 82, 83 fitted with operating levers 86, 87. A handle 89 on the lever 86 loosely engages the lever 87 so that movement of the handle 89 in one direction closes both jaws but movement in the other direction to open the movable jaw of clamp 47 does not affect the position of the lever 86 and the corresponding jaw. When the jaws are closed a lever 104 secured to the shaft of cam 82 operates the switch 10 against the return of a spring 19 through links 107, 111, 103 to close the circuit of the primary P<2> to apply a high voltage to the bar. The links are retained in position by a trip catch 116 on a lever 118. The primary current is sufficient to open the solenoid switch 33 with consequent de-energization of electromagnet 22 with the result that the pawl 21 engages a detent 35 to lock the switch 35 in position. When the predetermined temperature is reached the primary current is insufficient to hold the switch 33 open. Consequently the magnet 22 is energized, the pawl 21 is pulled out of engagement with the detent 35, and the spring 19 causes rotation of the switch to trip the lever 118 and connect the primary P<1> to apply the low voltage. When the lever 118 is tripped the links 111, 107 are actuated by the spring 122 to open the movable jaw of clamp 47. The solenoid switch may be fitted with a dashpot 39 if the predetermined temperature is above the temperature at which the resistance does not increase proportionally to the temperature. Auxiliary electrodes or short circuiting connections may be employed where the bar is of non- uniform section. The laminations forming the transformer core 123 are clamped between U- shaped iron heads 124 by tie bolts 125. The primary windings are of pancake form. There are two primary windings P<2> and two subdivided primary windings P'. Pins 129 and wedging blocks 130 hold the windings in position. Tappings connected to switches 6 and 7 are arranged at one end of the windings. The secondary comprises bars 41, 43 interconnected at one end by a bar and supported on columns 44. The bar may be replaced by electrodes.