Induction melting furnaces are generally in the range of 100 kW to 1000 kW. Among them, the KP thyristor of the rectifier circuit and the KK thyristor of the inverter circuit have high power and high withstand voltage requirements. In practice, it is found that the thyristor damage is mostly overvoltage breakdown, and it can be seen that the reverse voltage breakdown voltage index is very important. In this amateur condition, the control pole of the thyristor can be inspected first by using the R×1 block of the multimeter. KP thyristor for rectification, whose resistance to the cathode is between tens of ohms and several hundred ohms; the resistance of the KK thyristor for inverter to cathode is between ten ohms and tens of ohms . If the resistance is zero or infinite, it can be judged as damage. The difference between the positive and negative resistances between the control electrode and the cathode is not large and is normal. Then use R × 10k to measure the resistance between the cathode and the anode, and the forward and reverse resistance should be infinite. If it is small or zero, it can be judged as thyristor damage.

After roughly judging by the multimeter, the reverse breakdown voltage of the thyristor can be measured as follows. Insert the positive meter's positive pen into the DC 2500V jack, where the Megohm strap grounding ring is the negative output and should be connected to the multimeter's black test lead. After connecting, first shake the megger shaker slowly. If the multimeter's hands are reversed, just reverse the two test leads. Then quickly turn the shaker of the megohm meter, at which time the multimeter's voltage indication is rising, and then fixed to a certain voltage value, no longer rising. This voltage value is the reverse breakdown voltage value of the thyristor. If the meter is connected to the meter and the voltmeter is connected, the current measured voltage is the forward breakdown voltage of the thyristor. This data is generally slightly smaller than the reverse breakdown voltage, but the difference is not allowed too. Big. Especially for the KK thyristor used in the inverter circuit, if the difference between the forward and reverse breakdown voltage is too large, the switching characteristics are not good. The breakdown voltage of the KP thyristor for rectification is generally selected to be greater than or equal to 1200V, and the breakdown voltage of the thyristor for inverter should be greater than or equal to 1400V. Since the megohmmeter is high internal resistance, its maximum output current does not exceed lmA. During the test, only a reversible breakdown of the thyristor is formed, and the forward and reverse breakdown voltage is measured without Cause any damage. Practice has proved that this method is very practical and safe.