1. Factors in the use of thyristors in intermediate frequency melting furnaces
Considering the thyristor component parameters themselves, some factors may cause damage to the components, which can be seen from the analysis of a large number of damaged thyristor component chips.
(1) The standard of the thyristor component stipulates that the internal P-N junction temperature of the chip shall not exceed 115 degrees Celsius. When the junction temperature exceeds the critical allowable junction temperature, the blocking voltage that the component can withstand will drop sharply.
(2) Effect of voltage rise rate Du/dt and current increase rate DI/dt on the use of thyristors. The thyristor has a limited voltage rise rate. When the thyristor is turned from on to off, the voltage is suddenly applied to both ends of the device. Because the P-N junction has a certain capacitance, if the voltage rise rate is too large, a certain amount will occur. The leakage current causes the component to be damaged without being turned off. The rate of current rise of the thyristor is also limited. When the component is used in a relatively large current and a relatively high frequency, the current rise rate is large, the current on the chip is too late to diffuse, and the local junction temperature is too high and damaged.
(3) The effect of turn-off time on the thyristor. The turn-off time of the thyristor has a great influence on the operation of the inverter circuit. The higher the operating frequency, the smaller the turn-off time required. The thyristor takes a certain time from turn-on to turn-off, and the charge stored in the P-N junction The flow is to be gradually released. If the turn-off time is too long, the thyristor cannot be reliably turned off, causing the inverter circuit to fail to commutate and causing overcurrent phenomenon to cause damage to the components.
2. Factors in the control circuit of the intermediate frequency melting furnace
The imperfection of control circuit design is the key to unstable and unsafe power supply operation, and it is also one of the main reasons for thyristor damage. It is manifested in the following aspects:
(1) The control line of the power supply cannot track and control the change of the power load in real time. That is to say, when the current voltage fluctuates, such a wave signal cannot be fed back in time, thereby causing instability of the current circuit voltage and the occurrence of overcurrent and overvoltage. This is also a common problem in domestic IF power control lines.
(2) The protection measures for overcurrent and overvoltage in the control circuit are not perfect enough, and the cause of overcurrent and voltage is not fundamentally solved. The first is the problem of protection mode. At present, most of the power supply protection adopts the method of pulling the inverter, that is, the rectification pulse at the time of failure is pulled back to the control angle position of 150°, so that the rectification is in the inverter state, and the huge amount generated in the tank path is generated. The energy is sent back to the grid. However, due to the loss of the trigger pulse of the inverter bridge during the fault, the channel for energy bleed of the energy storage element is turned off. Therefore, this method can not solve the threat of the main circuit energy storage component to the inverter thyristor in the fault; nor can it solve the adverse effect of the overcurrent caused by the short circuit or overload of the tank on the turn-off of the thyristor. This is a disadvantage of the thyristor protection. Second, it is the improper measures taken to protect. Most power supply protection adopts the method of pulling the inverter and blocking the pulse, but adopts the relay control. The action of the relay lags behind the overcurrent and overvoltage, which will lose the meaning of protection. This is also a factor that cannot be ignored in thyristor damage in IF power supplies.
3. Thermal instability of the intermediate frequency melting furnace line
The thermal instability of the control line is also a potentially unreliable factor in the operation of the power supply. After the power supply is running for a period of time, the parameters of each part drift. This drift occurs in the rectifier, which will cause instability of the power supply voltage; the occurrence of the reverse-transformed flow in the inverter triggering part will cause the loss of the power self-regulation ability in the feedback link.
4. Influence of the use condition of intermediate frequency melting furnace on the safe operation of thyristor
From the point of view of use, the main causes of thyristor damage are as follows:
(1) The intermediate frequency power supply is a water-cooled intermediate frequency melting furnace. The cooling water is like the blood of the power supply. It is very important. The size of the displacement and the quality of the water quality. If the requirements of the intermediate frequency melting furnace cannot be met, the scale is easy to scale. It is easy to be blocked, which is not conducive to the diffusion of the junction temperature of the thyristor, resulting in a decrease in the carrying capacity and the commutation ability of the thyristor, which eventually leads to the damage of the thyristor.
(2) The high ambient temperature, large dust and humidity will cause poor contact of the conductive parts of the power supply and local short circuit.
(3) Sudden power failure on the power supply side and poor contact of the high voltage part cause component damage. Due to frequent power outages in areas with tight power supply, the protection link in the power supply cannot play a normal protective role during a sudden power failure. The high voltage generated by the inductance of the circuit in the event of a sudden power failure often causes the thyristor voltage to breakdown.
The contact of one phase of the switch on the power supply side of the workshop is poor, and the ignition or the high-voltage incoming line drop protection switch has insufficient capacity, which will cause the power supply to “lack of phase”, causing the intermediate frequency power supply to suddenly damage the thyristor.
