MF Power Failure Check (II)
The above checks will generally identify most of the causes of the malfunction. Next, you can turn on the control power for further inspection. There are two types of IF power supply main circuit closures: manual and automatic. Automatic closure of the system, you should first temporarily disconnect the power cord to ensure that the main circuit will not close. After connecting the control power, you can make the following aspects of the check. 1.
1.oscilloscope probe connected to the rectifier thyristor gate and cathode, the oscilloscope is placed in the power supply synchronization, press the start button can be after When you see the trigger pulse waveform, it should be a double pulse with an amplitude of more than 2 V. Press the stop button, the pulse will disappear immediately. . Take a look at each thyristor and if there are no pulses on the gate, you can move the oscilloscope probe to the primary side of the pulse transformer and look at it. If there are pulses on the primary side and not on the secondary side, the pulse transformer is damaged, otherwise the problem may be in the transmission line or the main control board. 2.
2.the oscilloscope probe connected to the inverter thyristor gate and cathode, the oscilloscope is placed in internal synchronization, turn on the control power supply can be seen Inverter trigger pulse, which is a series of sharp pulses, the amplitude should be greater than 2V, through the oscilloscope’s time mark to read the pulse period, calculated trigger The frequency of the pulses, which should normally be about 20% higher than the nominal frequency of the power supply cabinet, is called the start frequency. When the start button is pressed, the spacing of the pulses increases and the frequency becomes lower, which should normally be about 40% lower than the nominal frequency of the power supply cabinet, and this frequency is called the start frequency. Stop button, the pulse frequency immediately jumps back to the start frequency.
The above list of checks will basically eliminate the failure to start at all. It does not work properly after starting, which is generally manifested in the following areas.
1.rectifier phase loss: the fault manifests itself as abnormal sound during operation, the maximum output voltage does not rise to the rated value, and the power cabinet noise becomes louder. The output voltage can then be lowered to about 200V, and the output voltage waveform of the rectifier can be observed with an oscilloscope (the oscilloscope should be placed in the middle of the screen). (power synchronization), the input voltage waveform normally has six waveforms per cycle, and two are missing during phase loss. If one of the rectifier thyristors does not have a trigger pulse or the trigger does not conduct, then you should first use an oscilloscope to look at the six rectifier thyristors. Gate pulses, if any, after shutdown with a multimeter 200Ω to measure the resistance of each gate, will not pass or gate resistance particularly large that thyristor can be replaced.
2.Inverter three bridge arm work: the failure of the output current is particularly large, the same when the furnace is empty, and the power supply cabinet at work very loudly. Heavy, start up and turn the power knob to the lowest position, you will find that the IF output voltage is higher than normal. Use an oscilloscope to observe the voltage waveform between the anode – cathode of the four inverter thyristors in turn. If the three bridge arms are working, you can see that the waveform of two adjacent thyristors in the inverter is normal, and one of the other two adjacent There is no waveform, the other one is a sine wave, as shown in Figure
3.KK2 trigger is not available, the waveform between its anode – cathode is a sinusoidal waveform. At the same time, KK2 does not conduction will lead to KK1 can not be turned off, so there is no waveform KK1 two ends.
4.Induction coil fault: the induction coil is a medium frequency power supply load, it uses a wall thickness of 3 to 5 mm square copper tube.
The induction coil fault: the induction coil is the load of the IF power supply, which is made of square copper tube with wall thickness of 3 to 5 mm. It has the following common faults.
The induction coil is leaking, which may cause the coil to fire between turns and must be re-welded in time to run.
Steel water sticks to the induction coil, steel slag is hot and red, which can cause the copper tube to burn through, and must be removed in a timely manner.
Induction coil turn-to-turn short circuit, this kind of failure in small medium frequency induction furnace is particularly prone to occur, because the furnace is small, in the work of heat stress The role of deformation, resulting in turn-to-turn short circuit, the fault manifests itself as a larger current, the operating frequency is higher than usual.
In summary, in order to be able to use the correct method of IF power supply failure maintenance, you must be familiar with the characteristics of common failures of IF power supply and reasons, in order to reduce the detours, save time, as soon as possible to troubleshoot, restore the normal operation of the IF power supply, thereby ensuring the production of The smooth running.