| 摘要 |
PCT No. PCT/CH79/00059 Sec. 371 Date Dec. 28, 1979 Sec. 102(e) Date Dec. 11, 1979 PCT Filed Apr. 26, 1979 PCT Pub. No. WO79/01000 PCT Pub. Date Nov. 29, 1979. Electric welding sleeves used more particularly with larger pipe diameters have a heating wire winding which is under compressive stress due to the shrinkage of the sleeve member. This compressive stress and the elongation of the resistance heating wire occurring during the welding process can lead to a lateral displacement and contact with adjacent turns. The welding apparatus (4) used for welding such sleeves obviates these difficulties in that the overall welding time of the electric welding sleeve (15) is subdivided into a preheating time and a welding time. During the preheating time a partial power is used and during the welding time the full welding power. If the welding power is taken from an alternating current system the welding process can start with half the power by arranging a diode (8) in the welding circuit (2) and at the end of the preheating time the diode is bridged by a switching stage (10). |
