The method of intermediate frequency furnace electrical work principle and common fault processing
Operating principle of medium frequency power supply: First, alternating current is rectified into direct current by three-phrase full-controlled bridge rectifier. Second, direct current is turned into direct-current power supply after smoothing by reactor. Last, direct-current power supply is converted into one-phrase medium frequency current with certain frequency(general 1000-8000HZ). Load is composed of induction coil and compensation capacitor, which the two are linked into parallel resonant circuit (Generally IGBT power supply also adopts series resonance, as well as parallel resonance).
In general, according to malfunctions phenomenon, the medium frequency power supply malfunctions have two main types: completely cannot start and cannot operate after starting. Operators should inspect the whole system completely when malfunctions happen. It included the following aspects:
(一) Power supply: Using multimeter to check whether the main circuit switch(contactor) and the controlled fuse have electricity to exclude the possibility of interruption.
(二）Rectifier: It employs three-phrase full-controlled bridge rectifier, which comprises of six fast-acting fuses, six thyristors, six pulse transformers and one fly-wheel diode. There is a red indicator on the fast-acting fuse. It is inside the shell in normal situation. But it pops up when fuse is burned out. Sometimes it is stuck inside because some indicator is tight. So for safety, operators can use multimeter to test the fast-acting fuse to make sure whether it is burned out.
The simple method to test the thyristor is using electrical blocking multimeter to test negative pole--positive pole resistance and gate pole--negative pole resistance. It is not necessary to take down when testing. In general, the resistance between positive pole and negative pole is infinite while the resistance between gate pole and negative pole is between 10 and 50Ω. Oversize or undersize shows the lose efficacy of the gate pole, which can not be triggered.
The secondary side of pulse transformer links to the thyristor and the primary side with the 50Ω resistance links to the main control panel. The fly-wheel diode is not easy to break down. Using the multimeter diode to test the both ends of the fly-wheel diode when checking. The multimeter displays pressure drops about 500mv in forward direction, not in reverse direction.
(三）Inverter: It includes four fast-acting thyristors and four pulse transformers. It can be checked according to the method mentioned above.
(四）Transformer: Each winding of transformer should be interlinked. Generally the resistance of the primary side is about dozens of ohm and the second side is a few ohm. There is one point should be noticed: the primary side of the medium frequency mutual inductor is linked to load, so the resistance is zero.
(五）Capacitor: Capacitor paralleled with load is generally installed on the capacitor bracket and may be broken down. So the broken-down capacitor group should be checked first. Interrupt the point of junction between bus-bar and main bus-bar and test the resistance between two bus-bar of each capacitor. The resistance should be infinite in normal situation. When the broken group is confirmed, then interrupt annealed copper sheet from each capacitor to bus-bar and check one by one to find the broken down capacitor. Each capacitor is composed of four core. The shell is one pole. The other pole is led to the end cover through four insulators. Generally only one core is broken down. The capacitor can still be used if keeping away from the leading wire. And the capacity is 3/4 of the original capacity. The other malfunction of capacitor is oil leaking. It does not affect the normal use but should pay attention to fire prevention.
The angle steel used to install capacitor is isolated from capacitor bracket. The major loop will connect ground if the insulation is broken down. To test the resistance between the leading wire of capacitor shell and capacitor bracket can make out the insulation condition.
（六）Water-cooled cable: The water-cooled cable with each 0.6-0.8 dimension red copper wire twisted together links the medium frequency power supply with induction coil. The sectional area is 480mm2 for 500 kg furnace and 300-400mm2 for 250kg furnace. The material of the water-cooled cable shell adopts the pressure rubber tube with 5kg compression resistance. There is cooling water inside and the part of load loop. Because of tension and twisting force, The flexible joint of rubber tube is easy to crack after a long time. The cracking process of water-cooled cable is most part is broken first, then the small part is burned out quickly when the high power operates. Then the medium frequency power supply will generate high overvoltage. The thyristor will be burned out if the overvoltage is fail to protect. The medium frequency power supply can not start to work after interrupting the water-cooled cable. If starting repeatedly without find out the reason, the medium frequency voltage mutual inductor may be burned out. Oscilloscope is applied to check malfunction. Put the oscilloscope in the both ends of load, and observe whether the waveform is attenuation when starting the button. The water-cooled cable is isolated from capacitor output copper bar first when making sure the lead-breaking of pencil. Then multimeter resistant block is applied to test the resistance of the cable. The resistance is zero in normal situation and infinite in off-state. When using multimeter, the furnace body should be in inclinable state and lifted by water-cooled cable to break away from the cracking part completely. Only in this way can make sure whether the pencil is leading-breaking correctly.
Through checking the several aspects mentioned above, the cause of trouble can be found out. Then we can connect the controlled power supply and check further.. The medium frequency power supply main circuit switch has two types: manual and automatic. For automatic switch system, The power line should be interrupted temporarily to make sure main circuit is not closed. We can check the following aspects after connecting controlled power supply.
1. The detector of oscilloscope is connected with gate pole and negative pole of rectification thyristor. The oscilloscope is synchronized with power supply. We can see pulse waveform after pressing button and the waveform should be double pulse and the range is more than 2V. The pulse will disappear immediately after pressing stop button. Repeat six times and check each thyristor. If there is no pulse in gate pole, then move the detector to the pulse transformer. If there is pulse in the primary side and no pulse in the secondary side, it shows the pulse transformer is damaged or the problem may happens on the transmission line or the main control panel.
2. The detector of oscilloscope is connected with gate pole and negative pole of inverted thyristor. The oscilloscope is internal synchronization. We can see a series of sharp pulse and the range is more than 2V. We can read the pulse period though the clock of oscilloscope and count the frequency, which is lower than nominal frequency of power cabinet about 40% in normal situation. Then the frequency is called starting frequency. The pulse frequency will rebound to starting frequency after pressing stop button.
The trouble of unable to start completely will be excluded through checking the aspects mentioned above. The operation is abnormal after starting, generally show the following aspects:
1. Default phrase of rectifier: It shows the sound is abnormal while operating, the maximum output voltage is less than rated voltage and the strange sound of power cabinet is larger. Then the output voltage should be reduced to about 200V. The oscilloscope is applied to observe the output voltage waveform( the oscilloscope is synchronized with power supply). There are six input voltage waveforms each period in normal situation and two waveforms when default phrase happens. This is caused by some thyristor of rectifier does not trigger the pulse or conduct. Then the oscilloscope is applied to check the gate pole pulse of six rectification thyristors. Using multimeter resistant block to test each gate pole resistance after power off if there is pulse. It will work after replacing the non-conduction or high resistance of gate pole.
2. Three- arm inverter: The trouble shows huge output current even no-load furnace, and the working sound of power cabinet is heavy. the medium frequency output voltage is higher than normal after transferring power to the minimum. The oscilloscope is applied to observe the voltage waveform of positive pole and negative pole of four inverting thyristors. If operating in three-arm manner, there are two normal waveforms of adjacent thyristor and there is no waveform and sinusoidal wave of other adjacent thyristor. The wave between positive pole and negative pole is sinusoidal wave if KK2 does not conduct. At the same time the non-conduction of KK2 will cause KK1 unable to turn off. So there is no wave in both ends of KK1.
3. The trouble of induction coil: It is the load of medium frequency power supply. And it is made of square red copper pipe with 3-5mm wall thickness. The trouble is as following aspects:
Induction coil is water leaking. It may cause the coil turn-to-turn flint, must be timely welded to run.
Molten steel sticks on the induction coil, steel slag fever, redness, cause copper pipe burn through, must keep clean timely.
There happens short circuit between induction coil turn-to turn. This kind of trouble happens easily in small size medium frequency induction furnace. Because of small furnace and deformation caused by thermal stress, the current is larger and working frequency is higher than normal situation.
To sum up, in order to maintain the medium frequency power supply with correct methods, we must be familiar with the characteristics of medium frequency power supply troubles and reasons. Only in this way can we exclude troubles as soon as possible and recover the normal operation of medium frequency power supply to make sure the smooth going of production.
The main characteristics of medium frequency power supply with the following:
With wide frequency
range from 1KHZ to 20KHZ and wide applications, it can match according to specific
size of heating work piece or the requirements of diathermanous depth.
It employs traditional medium
frequency parallel resonance with simple load matching, high efficiency and
Inverting process: it
makes the device greatly improve the operational reliability with high power
and develop in the field of high-power. The device duty cycle has been achieved
High efficiency and
energy conservation: Compared to SCR medium frequency power supply, it is
characteristic of high power factor and power efficiency within the full power
range and can save energy at least 20%.
It is not only
appropriate for factories but also for schools and research institutions with
the advantage of small size, light weight, complete models and wide optional range.