The Fault Analysis and Maintenance Process of Medium Frequency Heating Furnace
When the medium frequency heating furnace is started, the given voltage is suddenly applied. Because there is no voltage feedback, the output of voltage regulator increases rapidly. Due to the drift of control parameters, the current feedback coefficient is reduced. The same if current is given, the instantaneous current of if device is relatively large, resulting in impulse current. Only when the voltage loop is overshoot, the output current begins to decrease, which is finally determined by the load. Because of the large reactance in the medium frequency furnace, the system response is slow. With the increase of the given voltage, the impact current is larger, and the impact on the thyristor is also large, which accelerates the degradation of the thyristor, resulting in the fatigue breakdown of the thyristor.
With the aging of components and the parameter drift of analog control system, the lead angle triggered by thyristor in the inverter control circuit becomes larger (42.8 °), and the power factor is low (0.733). Even if the same intermediate frequency voltage and current are output, the active power output of intermediate frequency heating furnace is not high and the heating effect is poor. Under normal conditions, in the process of medium frequency heating, according to Ohm's law, the load parameters change with the increase of temperature, such as the increase of intermediate frequency equivalent impedance; but when the temperature changes slowly, the intermediate frequency equivalent impedance changes little. When the medium frequency voltage is constant, although with the operation of the heating device, due to the poor heating effect, the intermediate frequency equivalent impedance changes little, and the voltage feedback increases slowly, resulting in the slow change of medium frequency current, that is, the system works in the state of high current for a long time, which also easily leads to the deterioration of thyristor.
In general, the failure of intermediate frequency furnace can be divided into two categories according to the fault phenomenon: completely unable to start and unable to work normally after starting. As a general principle, when there is a fault, the whole system should be comprehensively inspected under the condition of power failure, which includes the following aspects:
Rectifier: the rectifier adopts three-phase full control bridge rectifier circuit, which includes six fast fuses, six thyristors, six pulse transformers and one freewheeling diode. There is a red indicator on the quick fuse. Normally, the indicator is retracted inside the shell. When the fast fuse is burnt out, it will pop up. Some of the fast fuse indicators are tight. When the fast fuse is burnt out, it will be stuck in it. Therefore, for the sake of reliability, the on-off gear of a multimeter can be used to test the fast fuse to determine whether it is burnt out.
Inverter: the inverter includes four fast thyristors and four pulse transformers, which can be checked according to the above methods.
Transformer: each winding of each transformer should be connected. Generally, the primary side resistance value is about dozens of ohm, and the secondary pole is a few ohm. It should be noted that the primary side of the intermediate frequency voltage transformer is in parallel with the load, so its resistance value is zero.
Capacitor: the electric capacitor in parallel with the load may be broken down. The capacitors are generally installed on the capacitor rack in groups, and the group where the capacitor is broken down should be determined before inspection. Disconnect the connection point between the bus bar of each capacitor and the main bus bar, and measure the resistance between the two bus bars of each capacitor, which should be infinite under normal conditions. After confirming the bad group, disconnect the soft copper sheet of each electric thermal capacitor leading to the bus bar, and check one by one to find the broken capacitor.
Water cooled cable：The oscilloscope can be used to check the fault. Clamp the oscilloscope probe at both ends of the load to observe whether there is attenuation waveform when pressing the start button. When the cable core is broken, disconnect the water-cooled cable from the output copper bar of the electric capacitor, and measure the resistance value of the cable with the resistance block (200 Ω block) of the multimeter. The resistance value is zero under normal conditions and infinite when it is disconnected. When measuring with a multimeter, the furnace body should be turned over to the dumping position to make the water-cooled cable fall off. In this way, the broken part can be completely separated, and the core can be judged correctly.