Analysis of the causes of thyristor (SCR) burnout
During the induction furnace working period, thyristor often burnout, this problem often boring the maintainer. According to our many years' induction furnace repair records, Dmelting Technologies has compiled some information for repairman reference.
Inverter thyristor water cold shell no water or heat dissipation effect low. The user should change the water cold shell and check the water output pressure and volume of water cold shell, sometimes, because of the water quality problem, there is lots of water scale on the inner surface of water cold shell, and water scale has not any thermal conductivity, so even lots of water through the shell, but no any thermal conductivity. Judging method: keep the furnace running under its over-current value about 10 mins, then stop furnace and use hand to test thyristor core area immediately, if it overheats, probably the fault caused by this reason.
Tank circuit has imperfect contact or breakage problem. The user should abide by the actual condition to diagnosis fault. If it is imperfect or breakage problem, the sparking will happen when power reaches a high value, impact on furnace running and make the furnace to start its protection function. Sometimes, sparking can make transient overvoltage between thyristor both side, if overvoltage protection action is slow, probably burn thyristor, SCR. This phenomenon often has overvoltage and overcurrent action together.
The thyristor during the opposite phase cut off, it suffers overvoltage. The user should check the resistance-capacitance snubbers circuit.
Load insulation against ground fault, make sparking and interrupt the pulse trigger time, make high voltage between thyristor both side, burn out the SCR.
Pulse trigger circuit fault, if pulse trigger cannot work in furnace running period, that will cause inverter open circuit, medium frequency power output terminal make high voltage, burn the thyristor. This fault most cause by inverter pulse, output circuit fault, the user can use an oscilloscope to check fault.
Load in open circuit during furnace running - when the furnace running in high power condition if the load turn to open circuit suddenly, that cause high voltage in output joint and burn out the thyristor.
Load short circuit during furnace running - when the furnace running in high power condition, if the load turn to short circuit suddenly, that cause a big short circuit power blow on thyristor, if the overcurrent protection does not work, the thyristor would burn out.
Protection system fault, the thyristor is safe or not, mostly depend on protection system, if protection system has the problem, a little fault might impact on thyristor, so, check protection system after thyristor burns out is a necessary process.
Thyristor cooling system fault, the thyristor can radiate huge heat during work, itself need cooling protection, there are two normal methods, one is water cooling, another is wind cooling, more user like water cooling method, the wind cooling method is good at less than 100KW power equipment. Water cold medium frequency equipment has water pressure protection circuit, but that is common general water input protection, if the water pipe has block fault, the protection function will useless.
Reactor fault, reactor inner sparking make the inverting current intermittent fault.