1、 The intermediate frequency device can work at low power, but when the rectifier voltage UD is adjusted high, the overcurrent protection acts; Or when the rectifier voltage UD and intermediate frequency voltage are both high, but the rectifier current ID is very small, when UD rises to the maximum value, the intermediate frequency power is still very low.
The reasons for the failure are as follows:
(a) The trigger time TF before leading is too small. When the current rises, the commutation fails, and TF should be increased.
(b) With the rise of rectifier voltage, the intermediate frequency power increases, and the interference often increases. The overcurrent and overvoltage protection misoperate due to interference.
(c) If the AC equivalent resistance of the load is too large, the UD will be increased and the ID will be small after starting, so the if device cannot meet the power output.
(d) The inter turn or inter layer insulation of the induction coil is poor, and it can operate when UD is low. When the intermediate frequency voltage is high, the insulation breakdown causes inter turn short circuit, and the AC equivalent resistance decreases rapidly, causing the inverter to overturn.
2、 The inverter bridge arm is closed or short circuited. If a fault occurs at low power output, the inverter may continue as usual, but the operation has been very abnormal. At this time, the medium frequency sound frequency emitted by the device suddenly decreases, the rectified current ID increases significantly, and the rectified voltage UD decreases. Use an oscilloscope to observe that the voltage at both ends of the thyristor that is not turned off in the inverter bridge is a straight line.
The reason for the failure is that the performance of the thyristor itself is unstable, and it gradually deteriorates after being used for a period of time. Check whether the group of thyristors that cannot be turned off is short circuited, or the turning off time is too long.
3、 One arm of inverter bridge is not connected. If this fault occurs at full power output, the inverter will immediately fail and the overcurrent protection will act. If a fault occurs at low power output, the inverter may continue. At this time, the intermediate frequency frequency suddenly decreases, the intermediate frequency voltage decreases, the DC current increases, and the voltage of the non conducting bridge arm is equal to the load voltage. In case of the above fault, first determine whether the thyristor is not conductive or the bridge arm is open elsewhere. If the thyristor is not conductive, you can further use the oscilloscope to check the trigger pulse, and then determine whether it is the trigger system fault, the thyristor gate fault, or the line fault.
4、 A thyristor in the inverter bridge arm turns forward. When the current of the main circuit is small, the sound of the intermediate frequency device is normal. However, when the power is increased, the current of the main circuit also increases, and the system suddenly loses control and the protection acts.
The cause of the fault is that the temperature of the thyristor rises, causing the turning voltage to drop and losing the blocking ability. The reason for the rise of thyristor temperature may be that the external heat dissipation of thyristor is poor. Check the radiator area, installation, cooling water system, etc. The temperature rise may also be the result of poor thyristor characteristics. For example, the forward voltage drop is too large, causing the average loss to increase; Or due to the high working frequency, the switching loss increases and the temperature rises.