Cause inspection of thyristor failure in medium frequency power supply
In industrial production, medium frequency power supply is widely used in smelting, heat penetration and other fields due to its high efficiency and stable performance. However, its core component, thyristor (silicon control device), often fails for various reasons, affecting the normal operation of the equipment. This article will discuss in detail the main causes of thyristor failure in medium frequency power supply and the corresponding inspection methods to help technicians quickly locate and solve the problem.
Damage caused by overvoltage
1. Failure of RC absorption circuit: Overvoltage is one of the main factors causing thyristor breakdown. When the resistor or capacitor in the RC absorption circuit breaks down or opens, the instantaneous high voltage cannot be effectively suppressed, resulting in damage to the thyristor. At this time, a multimeter should be used to measure the value of the resistor and capacitor, and an infrared thermometer should be used to detect the temperature abnormality of the absorption resistor. If a resistor is found to be not heated, it may be caused by an open circuit of the capacitor or a burnt resistor.
2. Pulse trigger circuit failure: The loss of the pulse trigger signal will also cause the inverter to open, causing high voltage at the output end and burning the thyristor. An oscilloscope can be used to monitor the pulse waveform, or the wire connector can be gently shaken to find the location of poor contact.
Damage caused by overcurrent
1. Heat dissipation system obstruction: The thyristor will generate a lot of heat when working. If the heat dissipation water jacket is blocked or the water channel is not smooth, it will cause local overheating or even burning. It is necessary to check whether the cooling water circulation is smooth and whether the contact surface between the water cooling jacket and the thyristor is flat and tight. In addition, scale formed by poor water quality will reduce the thermal conductivity, and even if there is sufficient water, it may cause overheating problems.
2. Load short circuit shock: Sudden load short circuit will generate huge instantaneous current, which exceeds the maximum load capacity of the thyristor. If the overcurrent protection fails to act in time, the component will be permanently damaged. Therefore, it is necessary to regularly check the sensitivity and response speed of the protection device.
Degradation of insulation performance of the main circuit
1. Discharge and sparking to the ground: Poor insulation of the load circuit may cause sparking to the ground, interfere with the timing of pulse triggering and form high voltage at both ends of the thyristor. The insulation condition between the main circuit and the ground should be checked in particular, especially the integrity of the insulation layer between the induction coil and the yoke.
2. Loose connection of the tank circuit: Poor contact or broken wires in the tank circuit will cause arcing when the power is increased, inducing overvoltage risks. Such hidden dangers can be initially eliminated through visual inspection and manual tightening.
Impact of insufficient off-time
In high-frequency circuits, the off-time (tq) of thyristors must be short enough to meet the needs of fast switching. If the model is not suitable (such as tq is too large), the on-time will be prolonged, and it will be damaged by large current impact. Therefore, when selecting, ensure that the device parameters meet the circuit frequency requirements.
Other potential factors
1. Line virtual connection: Loosening of the power line or pulse transformer pressure bolts may cause unstable power supply. It is recommended to check and re-tighten all joints one by one.
2. The intermediate frequency output current is too high: The current beyond the design range will make the thyristor speed unable to keep up with the change and accelerate aging. The maximum output current can be limited by adjusting the gate line voltage through the control board.
3. Failure of the protection system: Failure of the overvoltage/overcurrent protection device will directly threaten the safety of the thyristor. It is necessary to regularly test whether the protection function is started normally.
4. Reactor abnormality: internal sparking or insufficient inductance may cause the inverter side current to be intermittent, resulting in high voltage and damage to components. When replacing, new parts that meet the specifications should be selected.
In general, the failure of medium-frequency power supply thyristors is mostly caused by factors such as overvoltage, overcurrent, poor heat dissipation and insulation failure. By systematically checking key links such as the RC absorption circuit, pulse trigger system, cooling water circuit, main circuit insulation and protection devices, faults can be effectively prevented and diagnosed. In daily maintenance, it is recommended to use professional tools such as multimeters, oscilloscopes and infrared thermometers for regular inspections to ensure that the equipment is in the best operating condition.