Trouble Analysis of Thyristor Breakdown in Induction Melting Furnace

There are many reasons for the breakdown of the thyristor in the induction melting furnace, including the circuit and the quality of the thyristor itself. The main faults of the thyristor breakdown are analyzed below.

(L) The winding resistance of the resistance-capacitance absorption circuit of the thyristor of the induction melting furnace is blown or the wire is broken, which causes the thyristor to breakdown or deteriorate the characteristics. Due to the existence of line inductance in the circuit (transformer leakage inductance LB, reactor), the thyristor causes a turn-off overvoltage during the turn-off process, and its value can reach 5-6 times the peak working voltage, so it is easy to cause the thyristor to breakdown Or the characteristics become worse.

(2) Induction melting furnace inverter bridge conversion contactor due to contact sintering, mechanical failure, or the setting value of the conversion potentiometer is too large, after the inverter is switched, the contactor cannot be opened or switched, resulting in a current-limiting magnetic ring Does not work, causing thyristor breakdown. In the commutation process of the thyristor, due to the commutation current, capacitor discharge, etc., it will cause a larger current rise rate du/df, and a larger current rise rate will make the internal current of the thyristor too late to spread to all PN junctions. As a result, the PN junction near the gate of the thyristor is burned due to excessive current density, causing the thyristor to break down. The magnetic ring set on the inverter bridge can effectively limit the current rise rate d//df and protect the thyristor.

(3) After the overcurrent protection action of the induction melting furnace occurs, the rectification trigger pulse disappears, causing the rectifier thyristor to turn off, causing the thyristor to breakdown.

We know that when an over-current protection action occurs, the rectifier trigger pulse is shifted to 150 degrees, so that the rectifier bridge is in an active inverter state, and the energy stored in the filter reactor is sent back to the grid to prevent the thyristor from being over-current. , The impact of overpressure. When an over-current action occurs, the rectifier trigger pulse disappears. When the rectifier thyristor is turned off, a high turn-off over-voltage will be generated, so that the thyristor will withstand the impact of over-current and over-voltage, which can easily cause the breakdown of the thyristor. This kind of failure is generally caused by the deterioration of the characteristics of the output low-power thyristor on the blocking protection board or the increase of the power supply. It can be solved by connecting a 4.7k potentiometer in series in the circuit, and the actual resistance value is determined by debugging on the computer.

(4) The cooling water pipe of the thyristor of the induction melting furnace is blocked, causing the thyristor to breakdown due to excessive humidity.

(5) The quality of the thyristor itself is not good enough or it has been subjected to the impact of overcurrent and overvoltage for many times, which causes the characteristics of the thyristor to deteriorate and breakdown.