Today, starting with this diagram, it looks like this:
Ordinary thyristor, yesterday I talked about its measurement method in general, and today I will look at its main types of failures.
First of all, the open circuit fault usually occurs between the G–K poles or between the A-K poles, resulting in the inability to conduct forward current between the two poles, and the resistance becomes infinite.
Under normal circumstances, the forward resistance between the G–K poles is small and the reverse resistance is large, but if the forward resistance between the G–K poles is large and tends to be infinite, the thyristor will not turn on even if there is a forward pulse signal. If there is a break between the A-K poles, the thyristor will be ineffective.
In addition, if the control signal (trigger pulse) of the thyristor is lost or abnormal, it will also cause the thyristor to break the circuit, so it is also important to determine whether the trigger signal is normal.
Here pay attention to the trigger form of the thyristor, it can be a square wave signal, it can also be a sinusoidal signal, it can also be other waveforms, only when the forward signal is high to a certain extent, the thyristor can be triggered to turn on, negative and zero values can not trigger the conduction, and the forward voltage is too low to trigger the conduction.
In the half-wave rectifier or full-wave rectifier circuit of the inverter, once the thyristor of one bridge arm is broken, the average DC bus voltage will be reduced, and the direct manifestation on the oscilloscope is the lack of two wave heads. In this case, there will be a poor load capacity, and in the case of a load situation, the three-phase input current will be severely unbalanced.
Let's talk about the leakage of thyristors, the so-called leakage, is not good insulation. Under normal circumstances, the forward and reverse resistance between the A-K poles is very large and tends to be infinite, but if it is used for a long time, under the influence of various factors, the resistance value between the A-K poles decreases, at this time, there is no gate trigger, and there will be current in the loop. Thyristor leakage is also manifested between the G–K poles.
In the half-wave rectifier or full-wave rectifier circuit of the inverter, once the thyristor of a bridge arm leaks, the DC bus voltage will fluctuate, and the three-phase current will be unbalanced under the load situation, which will trigger the protection. If the protection is not timely, it will cause the pipe fitting to heat up, which will lead to thermal breakdown, and once the breakdown, it means "blowing up".
Once there is a short circuit, it will cause a short circuit in the power supply, because, in the semi-control or full-control rectifier circuit, the upper and lower half bridges are conducted in pairs, once there is a bridge arm straight through, it will cause a short circuit between phases, and its destructive force is very strong!
Thyristor short circuits can be caused by the following reasons, the first is voltage breakdown, when the thyristor exceeds the maximum allowable voltage, the PN junction will break down, resulting in a short circuit. This situation is mostly caused by electrostatic high voltage, so it is mandatory to wear an anti-static bracelet when overhauling the inverter. Of course, there are also sudden overvoltages, but this is not common, because there are generally measures to prevent overvoltages at the incoming end.
Then there is the high temperature. High temperatures are caused by electric currents, and high ambient temperatures are also determining factors, as well as heat dissipation conditions. When the thyristor operating temperature exceeds its maximum allowable temperature, it causes damage to the PN junction and forms a short circuit. Therefore, it is necessary to avoid overcurrent of equipment and control the ambient temperature to provide favorable conditions for heat dissipation.
Then there is the overcurrent, when the current through the thyristor exceeds its rating, it will generate a large amount of heat, to put it bluntly, it is still the thermal breakdown of the PN junction caused by the high temperature.
There is also the aging of pipe fittings, and the thyristor will produce loss with the increase of service time, especially in complex environments, the more prone to breakdown damage. Therefore, it is necessary to replace the pipe fittings regularly.
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