Resistance annealing furnace to anneal a metal wire, strand, string, wire rod or strap

A resistance annealing furnace for annealing a metal wire, strand, string, wire rod or strap, the annealing furnace (1) comprising at least two electric axes (5-7), which are provided with respective pulleys (8-10) to convey said metal wire (2), strand, string, wire rod or strap, and a DC voltage generator (14) suppliable with an AC voltage (Uac) in order to generate an annealing voltage (Uann) applied between the at least two electric axes (5-7), so as to produce an electric current in a section of the metal wire (2), strand, string, wire rod or strap comprised between the at least two electric axes (5-7) that provokes the annealing due to Joule effect;

• said DC voltage generator (14) comprising an active power supply (19), which is supplied with said AC voltage (Uac) so as to generate an intermediate DC voltage (Udc), a pulse width modulator (PWM) (20) to transform the intermediate voltage (Udc) into a first PWM voltage (Um1) with the same amplitude, a voltage transformer (21) to transform the first PWM voltage (Um1) into a corresponding second PWM voltage (Um2) with a smaller amplitude, and a voltage rectifier (22) to transform the second PWM voltage (Um2) into the annealing voltage (Uann);
• and
• wherein said active power supply (19) comprises a low-pass filtering stage (23), which receives said AC voltage (Uac), a voltage wave form distortion detector (24), which is connected to a node (25) of the low-pass filtering stage (23) so as to provide a signal indicating a distortion of the AC voltage (Uac), an AC/DC converter stage (26), which is connected to an output of the low-pass filtering stage (23) so as to convert a filtered voltage into said intermediate voltage (Udc), a first voltage measuring unit (30) that measures the intermediate voltage (Udc), and a first controller (27) configured to control the AC/DC converter stage (26) as a function of the signal provided by said voltage wave form distortion detector (24) and of measured values of the intermediate voltage (Udc), so as to reduce reactive power at an input (19a) of the active power supply (19).