Medium frequency power supply maintain
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Medium frequency power supply maintain

ⅠThe system defense of the medium power supply supply
There are three parts in the system defense, which respectively are water system, hydraulic system and electric system with emphasis on defense for electric system
The practice proves that most of the faults happened in medium frequency system has direct relationship with water, so water requires the water quality, water pressure, water temperature and flow rate must reach the demands prescribed by equipments
The defense for the electrical system: the electrical system should be checked and repaired regularly, because the linking parts in the main loop is easy to be fevered, which arouse firing and many faults happened
Ⅱ The detection method for the common faults in medium frequency power supply system (only introduce the detection method for electrical system)
(1) Detect the usually used equipments and instruments
Digital multimeter, insulative megohm meter, inductive capacitor meter, oscilloscope, which are used by professional personnel.
(2)The square frame principle drawing of the system’s main loop
Three phases AC input
→ breaker → three-phase full-wave rectifier and filtering → inverter and MF load
Breaker, three-phase full-wave rectifier & filtering, three-phase AC input of the inverter and MF load
(3)The system detection
The system detection has four parts
1. The detection for the control system (breaker and its control parts)
This part’s detection is simple: the common electrician will detect the control system according to the breaker’s brochure and the control principle of the system main loop.
The detection result should be the operation of the breaker is normal with the normal push-button & indicating light.
2. The detection for rectifier part
Firstly, the system must link water.
The main loop is broke in the input line of the filtering reactor and connects a resistor load (≤500 Ω, ≥ 500 W) in both input terminals of the three-phase full-wave rectifier input. After switch on the machine, the DC voltage meter should be indicated in the position of about 1.35×UL (UL is AC input line voltage).
3. The detection for the inverter and medium frequency load
The inverter and medium frequency load will be connected after the control system and rectifier part become normal, if the machine can not be switched on normally, you can check the line connection in the main circuit board, restart the machine after changing the location of 114 & 115, the main circuit should be changed if the machine can not be switched on, and the inverter& medium frequency load may have problems if the machine can not be switched on normally after doing the above methods, in that case, every component should be examined one by one.
(4)the detection for the main components
1. The examining methods of thyristor

The right/opposite resistance should be between 10KΩ ~ 100KΩ after examined by digital multimeter of 200KΩ (the resistance value is influenced by water).
The gate-level resistance should be between 10 KΩ ~ 20KΩ after examined by digital mutimeter.
2. The examining method of capacitor
The connecting copper plate of the capacitor is taken part, and every column of the capacitor is examined by the insulative megohm meter to see whether they are charge & discharge (normally they should be charge& discharge.
Note: The voltage of the chosen insulative megohm meter should not exceed the rated voltage of the capacitor, and the insulative megohm meter is used to examine whether the capacity value of every column in the capacitor is normal.
3. The examining method of furnace
Observe whether the turn is short circuit, which means whether the coil insulation for the protection location is well.
4. The circuit board should be examined by professional personnel with oscilloscope, and it can be replaced if it is suspected to have problem.
(5)the common faults and maintenance of the medium frequency induction heating power supply
The medium frequency power supply can be used in areas of melting, heating, quenching and welding, and different area have different demand for the power supply, so the medium frequency power supply’s control circuit & main circuit have different structural form, and the faults can be analyzed& judged quickly& correctly and got rid of through effective measures based on mastering proficiently the basic working principle of the circuit & the basic characteristics of the power components. Here we just make discuss for the typical circuit and common faults.
1. The equipment used to switch on cannot be started normally
1.1 The faults phenomenon: the DC current is great, DC voltage & medium frequency voltage is low, the equipments’ voice is oppressive and has over current protection when the machine is started.
Analysis and treatment: the thyristor in one bridge leg of the inverter bridge maybe short circuit or closed circuit, which results in the operation of the three-leg bridge of the inverter bridge. Oscilloscope is used to observe the voltage drop waveform of the thyristor in the four bridge legs of the inverter bridge. If the voltage drop waveform of the thyristor in any bridge leg becomes a straight line, which means this thyristor is conducted successfully, if the voltage drop waveform of the thyristor is sine wave, which means the thyristor is not conducted successfully, in that case the conducted thyristor should be replaced and searching for the reason of the thyristor that is not conducted.
1.2 The faults phenomenon
The DC current is great, DC voltage and medium low frequency voltage can not be set up successfully.
Analysis and treatment: compensating the capacitor’s short circuit, breaking the capacitor, searching for and replacing the capacitor that is short circuit.
1.3 Faults phenomenon
Every electrical parameter and voice is normal when the heavy duty cold furnace is started, but the power can not rise and the over current protection turns up.
Analysis and treatment: ⑴ using the oscilloscope to observe the commutation angle of the inverter thyristor and adjust it to suitable value, if the inverter commutation angle is too small
⑵ Using megohm meter to monitor the resistance value of the furnace body, and removing the short circuit point;
⑶ The relative induction circle of the furnace material is low, and using megohm meter to monitor the resistance value of the furnace material’s relative induction circle, relining if the resistance is low.
1.4 The faults phenomenon: it is hard to start the sweeping start circuit of zero voltage
Analysis and treatment: ⑴ checking the dotted terminal of the current transformer, if the negative feedback of the current is not adjusted suitably.
⑵ The signal line may too long or too thin
⑶ Paying special attention to the short circuit in transformer’s turn to turn, readjusting the current’s negative feedback and replacing the broken components.
1.5 The faults phenomenon: it is hard to start the provoked sweeping start circuit of zero voltage
Analysis and treatment:
⑴ choosing the beginning sweeping frequency again if it is not suitable.
⑵ using the oscilloscope to observe the waveform and frequency of the sweeping circuit and remove its faults.
1.6 The faults phenomenon: every electrical parameter and voice is normal, but the current disappeared when the power rises, and there is over pressure & current protection when the voltage reach rated value.
Analysis and treatment: the loaded open circuit is in charge of the loaded copper plate’s interface and water cooled cable.
2. The working state of the equipment has problem although it can be started
2.1 The faults phenomenon: the equipment in empty load can be started, but the DC voltage can not reach the rated value and the DC flat-wave reactor has impact sound with shaking.
Analysis and treatment: closing the inverter control power supply, connecting false load in the output terminal of the rectifier bridge, using oscilloscope to observe the output waveform of the rectifier bridge, this can make you see the output loss phase waveform of the rectifier bridge. The reasons for the loss phase respectively are: ⑴ the rectifier triggered pulse is lost; ⑵ the amplitude of the triggered pulse is not wide enough, which result in the loss of the triggered power and make the unstable state of the thyristor; ⑶ the double-pulse triggered circuit’s pulse sequence is wrong or lose its pulse; ⑷ the control gate of the thyristor has open circuit, short circuit or bad contact.
2.2 Faults phenomenon: the equipment can be started successfully, and it will have over pressure or over current protection when the power rises in a specific value.
Analysis and treatment: there are two steps to search for the reason of the faults.
⑴ Operating the equipment in empty load firstly, observing whether the voltage can reach the rated value; if the voltage can not reach the rated value and have over flow protection near the specific value of the voltage many times, the reason for this phenomenon may be there is not enough voltage endurance to compensate the capacitor or thyristor (not exclude the phenomenon is aroused by the firing in some parts of the circuit);
⑵ The equipment can be shifted to the heavy duty operation and observed whether the current value can reach the rated value, if the voltage can reach the rated value; if the current can not reach the rated value and have over flow protection near the specific value of the current many times, this phenomenon many be aroused by the interference of the big current, especially the interference of the medium frequency big current’s electromagnetic field for the control part and signal lines.
3. The faults easily emerged in the normal operation of the equipment
3.1 Faults phenomenon: many KP thyristors and quick fuses are burnt down during the normal over current protection, although the equipment operates successfully.
Analysis and treatment: during the over flow protection, in order to release the energy of the flat-wave reactor to grid, the rectifier will shift from the rectifier state to inverter state, at this time, if αis high than 120°, which may result in the subversion of the active inverter, many thyristors & quick fuses be burnt down and the switch tripping with short circuit’s enormous blasted sound of the current, which will bring in rather large current and electromagnetic impact to the transformer, and even destroy it in serious situation.
3.2 The faults phenomenon: the equipment operates successfully, but its working state is unstable near some point of the high voltage area with the DC voltage meter shaking and equipment has noise, which is easy to make the inverter bridge subverted and destroy the thyristor.
Analysis and treatment: this faults is hard to delete, and it is often happened in some components’ high voltage firing of the equipment: ⑴ The loose screw connecting the main joints of the copper plate results in the firing; ⑵ The oxidized main joints of the breaker results in the firing; ⑶ The loose connection socket’s screw of the compensated capacitor results in firing, and the discharging resistance in the compensated capacitor absorbs the capacitor’s firing; ⑷ The insulative parts of the water-cooled radiator is too dirty or carbonizing the striking in earth; ⑸ The inductive coil of the furnace strike fires to furnace shell or furnace board, the inductive coil’s turn distance is too close, which result in the turn firing or beating, and the high temperature carbonized insulative column of the fixed inductive coil of the furnace will fire & discharging. ⑹ The inner part of thyristor fires.
3.3 The faults phenomenon: the equipment operate successfully with sharp noise, meanwhile the DC voltage meter shakes slightly.
Analysis and treatment: using oscilloscope to observe the voltage waveform in both DC terminals of the inverter bridge, and the reason for the failure of one cycle wave, brief failure of the indefinite cycle, the brief failure of the parallel resonant inverter circuit and the self-recovery cycling failure generally is the control part of the inverter interfered by rectifier pulse, and the non-cycling brief failure usually is aroused by bad insulation in medium frequency transformer’s turn.
3.4 The faults phenomenon: the equipment has abnormal voice after operating a period of time, and the electric meter reading shakes that resulting in the unstable working state of the equipment.
Analysis and treatment: the equipment will have abnormal voice after working a period of time that leads to its unstable working state, the main reason for which is the equipment’s electrical parts have bad hot property. In that case, the electrical parts can be examined respectively after dividing it to two parts – weak & strong electricity. Examining the control part firstly can prevent the damage of the power components of the main circuit; connecting the power supply of the control part under the condition of off power supply’s switch, and using the oscilloscope to examine whether the triggered pulse in the control board is normal after the control part works in a period of time.
After insuring the normal of the control part, starting the equipment, using oscilloscope to observe the voltage drop waveform of the thyristor and finding the thyristor that has bad hot property after the abnormal phenomenon is emerged; the other electrical parts should be noticed, especially the breaker, capacitor, reactor, the joints of the copper plate and main transformer, if the voltage drop waveform of the thyristor is normal.
3.5 The faults phenomenon: the equipment works successfully but the power can not rise.
Analysis and treatment: the normal working state of the equipment indicates that every components of the equipment is well, while the power can not rise indicates that the parameter adjustment of the equipment is not suitable. The main reasons that influence the equipment’s power respectively are: ⑴ The rectifier part is not adjusted suitably, the rectifier pipe is not conduct completely, and the DC voltage does not reach the rated value that influences that power output; ⑵ The unsuitable adjusted medium frequency voltage can influence the power output; ⑶ The unsuitable limited flow & voltage value makes low power output; ⑷ The unmatched furnace and power supply can seriously influence the power output; ⑸ The unsuitable deployed compensated capacitor can not get the power output with best electric & heat efficiency, which means it can not get the reasonable economic power output; ⑹ The distributed inductance of the output loop and the added inductance of the resonant loop re too big, which influence the maximum power output.
3.6 The faults phenomenon: the equipment has abnormal shaking voices and the indication of the electrical instrument shakes when the power rises & down in specific power section during the time of the equipment’s normal operation.
Analysis and treatment: this faults is often happened in the potentiometer with given power, some section of which jumps not smoothly, which results in the unstable working state of the equipment, even leads to the subversion of the inverter and damage of the thyristor.
3.7 The faults phenomenon: the equipment operates successfully while the reactor in other line is burnt down.
Analysis and treatment: The main reason that result in the damage of the reactor in other line respectively are: ⑴ The reactor in other line has bad quality; ⑵ The inverter circuit operates asymmetric, the reason for which mainly is signal loop.
3.8 The faults phenomenon: the equipment operates normally, but it often breakdown the compensated capacitance.
Analyze and treat the reason for the faults: ⑴ The medium frequency voltage and its working frequency is too high; ⑵ The deployment of the capacitance is not enough; ⑶ In the voltage rise circuit of the capacitance, the capacity difference between series and parallel capacitor is too big, which results in the capacitance breakdown by voltage unevenly; ⑷ It is hard to breakdown the capacitance.
3.9 The faults phenomenon: the equipment operates normally with frequent over flow.
Analysis and treatment: When the equipment operates with normal electrical parameter, waveform and voice, but the over flow is too frequent. When this fault emerges, please pay attention to see whether the electromagnetic interference and coupled interference is resulted from the improper wiring and distribution between lines, such as the strong & weak electric lines wired together, the power frequency line & medium frequency line wired together, signal line & strong electric line and the bus bar of the medium frequency line interweaves together etc..
4. DC reactor
Fault situation: equipment works in unsteady condition, electricity parameter fluctuates, abnormal sound in the equipment, frequent over-current protection and burn fast-speed SCR.
Analysis treatment: In terms of maintenance of MF power supply, the fault of DC reactor is very difficult to judge and deal with. The common seen faults are as follows:
(1) That customers randomly adjust the air gap of the reactor and change of coils turns alters the inductance and influences the filter function, which makes output DC current being interrupted frequently to cause inverter bridge works unsteadily and inverter fails, so inverter SCR is burnt badly. As adjusting the air gap of the reactor and coils turns, the ability that reactor prevents the current rising will decline and burn SCR when Inverter Bridge is in short way. The random inductance adjustment of reactor will influence the start function.
(2) Coil of reactor is loose. If coil of reactor is loose, electromagnetic force makes coils shake when equipment is in working condition, inductance change suddenly and cause inverter failure when start with light load and small current runs.
(3) Coil insulation is bad. Earth short-way or coil turn short-way will cause the equipment works unsteadily. There are several reasons for bad insulation and short-way: A. bad water cooling, higher temperature will make insulation layer worse. B. Coil of reactor is loose. The friction between coil insulation layer and coil insulation layer, between coil insulation layer and iron core causes the insulation layer to be badly damaged. C. When dealing with water dirt of reactor coil, acidic liquid sinks into the coil and acidic liquid corrupts the copper pipe and acidic liquid becomes copper salt to break the insulation layer.
5. SCR
5.1 Fault situation: SCR will be broken when start if SCR change.
Analysis disposal: If there is fault about the equipment and burn SCR, you need to replace SCR, at this situation, do not start the machine at once after you replace SCR. The first thing you should do is to check the whole system and rule out the faults. Confirm that there is no fault, and then start the machine, otherwise, SCR will be burnt the moment the machine starts. More attention should be paid to the balanced pressure when installing new SCR, otherwise, the chip machine in the SCR will be broken and cause the descent of enduring pressure value of SCR greatly and SCR will be burnt the moment the machine starts.
5.2 Fault situation: The machine is in normal condition after replacing SCR, but SCR is burnt after working for a while.
Analysis disposal: There are several reasons for causing these kinds of faults:
(1) Heating features of electric units for control parts is not so good.
(2) The wrong installation between SCR and radiator.
(3) Radiator is overused to cause the platform center of it concaved and bad
connection between platform of radiator and SCR to burn SCR. 
(4) Units is burnt because of overheat produced by water dirt in the heater.
(5) Fast-speed SCR temperature increases because of bad giving out the heating.
The “stop” time will increase as temperature rises, and finally units can not be closed to overthrow the inverter and burn SCR.

(6) SCR working temperature is over high and door limit parameter decreases and
anti-interruption declines, easy to cause wrong firing to damage SCR and equipments. 
(7) Check RC snubber circuit. 
The above description is the usual examining way of MF power supply system and common faults, which is just for customers’ reference only. The faults are various and like kaleidoscope, therefore, specific analysis should be taken toward specific fault, for MF power supply system is more complex. It is very necessary that the professional check and maintain MF power supply and is equipped with rich practical experiences.
Finally, most important of all, the equipments must be carefully examined after replacing SCR. The equipments should be examined systematically even if the faults are eliminated.

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