为在挤压压延及锻造前加热铝锭、钢锭和其他材料用,分使用工频(50或60赫)及中频两种加热炉,中频由闸流管变流器供电。在感应加热炉中,金属坯外表面感应的涡流最大,而向中心迅速减弱。由外表面向中心电流减至约37％处的距离为电流穿透深度δ,是选择加热频率的主要因素。 In order to heat ingots, ingots and other materials prior to extrusion calendering and forging, the furnaces are operated with a power frequency (50 or 60 Hz) and two intermediate frequency furnaces. The intermediate frequency is supplied by a thyristor converter. In an induction furnace, the eddy current induced on the outer surface of the metal billet is maximized and rapidly decreases towards the center. The distance from the outer surface to the center current reduced to about 37% is the current penetration depth δ, which is a major factor in selecting the heating frequency.