Present application status and prospect for the induction melting technology
The application history of induction furnace in foundry may be traced back to the 1970s,when the mains frequency induction furnaces were started to be applied in foundry.In peak time there have been 10t/2.8MW and 20t/4MW mains frequency induction furnace put into running.In China,the application of IF induction furnace widely began in the 1990s.With the rapid development of electronic technology,the technical level of domestic IF induction melting equipment has been approaching to international levels.From the temperature control technology,constant power control,power grid pollution prevention and the energy saving points of view,the development direction of the induction melting technology has been discussed in present article.
Induction melting,energy saving,power control technology,IF power supply,series resonant
1.Development progress of the induction melting technology
The application history of the induction furnace in foundry may be traced back to the 1970's in China,since that time the mains frequency induction furnaces have come into popularization and also the mains frequency induction furnaces sized 10T/2.8MW and 20T/4MW were being applied at the period of peak time.There were some defects at these devices,which eventually were not promoted widely,however,that has created a precedent of melting iron by the way of electromagnetic induction technology using electrical power,moreover has indicated the development direction for the more advanced melting method in future.
At the same time along with the appearance of the electric power semiconductor devices,the Intermediate frequency(shorted as IF below)induction furnace is invented overseas and consummates gradually(excluding IF power generator set).Soon afterwards,since 1970's in China the experimental nature application and relevant research had also gradually been carried out,but its scale was not all large.Till to 1990's under the vigorously promotion by some companies from Europe and America,the IF induction furnace only then entered practical application stage in the large-scale at the foundry in China.
At that time the major melting equipment in foundry were imported overseas including the IF induction furnace with the parallel resonance technology and later the more advanced IF induction furnaces with the series resonance technology have been imported into Chinese foundry,which can realize that one power supply to feed power to two induction furnaces at same time.INDUCTOTHERM named it by trade mark as Dualtrack.
Since that time,Xian Mechanical&Electric Institute(shorted as XMEI below)also began to carry out both research and promotion about the IF induction technology and in the beginning of the 21st century had produced the IF induction furnace with serial resonance circuit and power sharing technology based on the IGBT devices(Insulated Gate Bipolar Transistor),in which one power supply can feed power to two induction furnaces in any power ratios.It created an application precedent for domestic product of high power IF induction furnace equipped with power sharing technology.
Now the profession for the IF induction furnace has developed rapidly in China,the level of the domestically produced equipment has already approached to the level of international.In terms of the IF power supply,the current conversion mode of“AC–DC–AC”has been widely used on the main electric circuit,namely it is that the rectifier is applied to convert the mains frequency city electricity into a rippled direct current firstly,then using the filter to make the rippled direct current smooth,finally this smooth direct current is inverted to a single-phase IF AC current by the inverter
According to the connection type of the compensation capacitor to the load,there are two kinds of power supply,which are the IF power supply with series resonance and the IF power supply with parallel resonance.
In IF power supply with parallel resonance the compensation capacitor with the induction coil of the furnace has become a self-oscillation circuit,usually the current flowing through the power electronic devices for converting circuit would be 1/10～1/6 of loading current.It means that the IF power supply with parallel resonance may obtain bigger capacity under the condition of using the same converting power electronics.
The IF power supply with series resonance came into use quite late,which was promoted and began to be used after 1990’s in China.Due to its particular power sharing function(one power supply can provide power to two or more furnaces),thus it can provide iron liquid continuously;therefore this feature receives the user’s favor.
Generally,the regulation of the output power in the IF power supply with parallel resonance is completed at the leading rectifier by means of controlling the conduction angle on the rectifying thyristors to manipulate the output voltage thus the output power is adjusted.
However the IF power supply with series resonance regulates the output power by means of changing the working frequency of the inverter,while the working frequency closes to the resonance frequency the output power will increase,whereas the output power will decrease when the working frequency leaves from the resonance frequency;but the output voltage of the rectifier still keeps of constant.Duo to this features,the furnace configured with the IF power supply with series resonance to be named as the variable frequency furnace(Shorted as VF furnace below).Owing to this characteristic one rectifier could be loaded with several inverters by using proper interlock of power control,so long as that the sum of power output from all invertors does not surpass the actual load ability in rectifier,it is guaranteed that each inverter may be manipulated own output power individually to meet the working requirement.
A typical application may be one power supply feeds the power to two furnaces,which is using one rectifier to load two same sized induction furnaces,thus the power of one IF power supply can be distributed flexibly to two furnaces.If a big part of the power is distributed to the melting operation and the rest small part of power to the holding operation,thus that the operation mode“one furnace for melting and one furnace for holding”could be realized and this kind of furnace system will have the ability of providing iron liquid to molding line in foundry continuously and improve the productivity.
There are merits and defects respectively for the two above mentioned types of the IF power supply.Due to the rectifier needn’t to be controlled or only slightly controlled the IF power supply with series resonance appears high power factor at the electric network side always in spite of the higher or lower output power,so this application will not increase extra reactive loss of the electric network.That one power supply feeding power to two furnaces is a typical selection at the continuous casting producing line.This method of“one tows two”may provide iron liquid continuously to the casting line and this holding
furnace working with the principle of the series resonance will not bring an extra reactive loss for electric network,however the series resonance makes these inverting thyristors to flow through full load current of induction coil,which means that more thyristors should be parallel connected for same power output comparing with that with parallel resonance.This results in disadvantage about equipment invest cost rising.
For the IF power supply with parallel resonance,due to the output power is controlled by the rectifier,the bigger reactive power loss will occur while the output power is reduced,thus it is suitable for a pure melting operation(no holding operation or very short time of holding operation).such as be propitious to the producing shift system,in which one shift for molding operation,one shift for melting and casting as well one shift for cleaning up due to that the casting operation will be carried out soon when melting is completed,the IF power supply may not be operated under a low power output status,therefore no extra reactive power loss would be created.
2.Development direction of the induction melting technology
To look back on the development history of the induction melting technology during the period of more than 100 years from the induction furnace(channel type and crucible type)with the mains frequency power supply,the induction furnace with the doubling frequency power supply,the IF induction furnace to the present VF induction furnace,the technology development was carried on around the following three goals:
a.To adapt in the casting and melting process requests in foundry,which being unceasingly developed and improved;
b.To reduce the melting power consumption and to raise the productivity;
c.To reduce the labor intensity and to simplify the operation as well as to satisfy
the society request to the humanities and the environmental protection.
The above mentioned three goals are also the next direction of the induction melting technology development,which may appear concretely in following several aspects.
1)More advanced temperature control technology
The temperature control of the induction melting mainly represents at the control of the tapping temperature.The traditional control method is that in the progress of raising the temperature after all solid charge have been melted up completely,it is needed to visually estimate the charge temperature as long as the identification of the tapping temperature is found then to measure the temperature several times using the quick thermocouple until the tapping temperature has met the requirement,thus the metal liquid may pour out.However this method has following malpractice:
a.Experienced operator is needed;
b.Large consumption of the quick thermocouple;
c.In case of carelessness,the metal liquid may be heated continually and causes possible overheat to burn the partial chemical composition in the
charge or even burn out the furnace lining.
The update temperature control method is a control technology based on the energy balance principle,it may be a kind of computer software,which is a simple
energy computational procedure embedded in CPU of controlling the IF power supply or may be a complex software programmed by the high level language and installed in the melting management computer.Its basic function is to monitor continually the output power of the IF power supply and to count the energy fed into the furnace,meanwhile to measure the ambient temperature and both the outlet temperature and the flow rate of the cooling water,therefore the waste energy consumption of the furnace can be found out because the difference between the energy fed in and the waste energy consumption is just the energy received by the furnace charge and thus the enthalpy in the furnace charge could be got.Due to there is a fixed relations between the furnace charge temperature and its enthalpy,so that this furnace charge temperature may be calculated out by using a proper mathematical model.
Usually,this kind of software has an error correction function,which will adjust own mathematical model according to the difference between the calculated and the actually measured,thus to make the result calculated approach to the actually measured.In practice a good mathematical model may obtain the computation precision of smaller than±15℃(when the molten iron at 1450℃).
The melting process becomes extremely simple when such temperature control technology is used,thus neither needs to judge the temperature manually,no needs temperature measurements many times,also the overheat danger of the furnace charge could be avoided.While the furnace charge temperature has reached to the requested tapping temperature,the control software reduces the output power of the IF power supply to the holding power automatically,and then the operator shall be prompted that the charge temperature has already reached and is waiting for next process.
2)Constant power control technology
During a complete melting process the furnace impedance between the induction coil and furnace charge is not the constant,which will be changed with both
the temperature and the dense degree of the furnace charge.Usually the lower the furnace charge temperature or the denser the charge feeding,then the smaller the furnace impedance.If the rated tapping temperature in full charge condition is taken as the datum,thus during the melting process the variation range for the mentioned furnace impedance may be about±35%
The variation of furnace impedance has two limitations,one lowest limitation occurred when the full cold and loose charge to be fed and the power is just fed into the furnace,which causes a low electrical resistivity,thus the furnace impedance shows a smallest amount and another limitation occurred while the furnace charge temperature reaches to the Curie point at which the iron begins to lose the magnetic conductivity due to the iron has not been melted up yet also the furnace charge dense degree has not been improved,so that the furnace impedance shows a biggest amount.Then along with temperature rises the gap between the solid charge material will be gradually filled up by the molten iron liquid,thus the furnace impedance approaches to a stable value.
For the IF power supply with series resonance the furnace impedance may be regulated by means of adjusting the working frequency of the IF power supply.The change of the working frequency directly changes the furnace impedance,if the furnace impedance is too low the working frequency may be increased properly,conversely if the furnace impedance is too big then the working frequency may be reduced properly.By such adjustment the furnace impedance may basically be kept of consistent in the entire melting process.
For the IF power supply with parallel resonance,its inverting system is controlled by a micro processor.There is an artificial intelligence control procedure in the inverter system,in which the inverting angle of the inverter may be changed in a small range(inside the allowed range of shutdown time of the thyristor)if it is required,therefore enable the inverter with parallel resonance to have the ability to adjust the equivalent DC resistance of the load in a small range,which coordinates with electric current remainder of the rectifier.Then this kind of thyristor inverter with parallel resonance may have the ability near to output a constant power(can’t fully reached).So you can have an concept that the IF power supply with series resonance have good performance regarding to the feature of constant power output during whole melting period,which leads higher productivity.
3)Research in load matching
The IF power supply and the induction furnace together with the compensation capacitors have constituted an organism,which is indivisible.Whether appropriately for load change or the load impedance matching may affect the rated power of the power supply directly.Also whether the working frequency can achieve the design goal may also affect the efficiency of the inductive heating directly.It is a very complex task to design and calculate the inductive coil(load);therefore it is not an easy matter to design a satisfactory load coil.
Currently,less important secondary parameters are ignored in the design procedure or the revised experienced formula may be used for the load coil designing,but it could cause bigger error.
Henceforth,in order to facilitate the designing aspect,it is needed to establish the precise arithmetical model under the instruction of the relevant theory,especially to apply the simulation technology by computer so as to adapt the computation precision at the more wide range for all kind of load.In overseas,especially in USA,the dedicated software used for the computer aided design and simulation about the inductive coil(load)have been developed yet,which are worth us to learn.
4)High power factor and low harmonics
The diode rectifier is used for the IGBT IF power supply with series resonance,which has over 0.95 of the power factor under any operation power state,and also the thyristor IF power supply with series resonance may have over 0.9 of the power factor.However for the IF power supply with parallel resonance the power factor is lower,which can only reach to 0.8~0.9 even under the full output power condition.
Moreover the inevitable harmonics may cause pollution to the electric power network and the bigger the mains input is,the more prominent harmonic pollution to power network is.
The new generation of IF power supply must be of high power factor and low harmonics.Up to now,the multi-rectification,the full-controlled power transistor plus matrix control technology or PWM control technology,and the series connected circuit as well as the chopping technology are being developed.Simultaneously it has also expedited the development and production about the harmonics elimination device used for filtering off the harmonics and the power factor compensating device of the power supply.
3.Energy saving and carbon emission reducing
Generally,the conversion efficiency of well designed modern IF power supply(regardless of thyristor or IGBT)may obtain over 98%.The electrical efficiency of the inductor-furnace charge system is only decided by the system itself;it has nothing to do with the output voltage of the IF power supply and the inverter form.The electrical efficiency of well designed IF induction furnace may generally reach over 78%,therefore it is not easy to obtain the bigger improvement of the melting efficiency for the IF induction furnace under the existing technical idea.In order to have a large scale of energy saving and carbon emission reducing,it must focus on the inductor-furnace charge system,need to breaks through the existing frame and to establish an entirely new design model,
so as to elevate the electrical efficiency to over 90%.
Above discussed technology and idea on which now some have initially been realized and some are also in research or on trial.Along with technical progress,more advanced and more ideal method will be invented certainly and then to come into use,which must bring about the induction melting technology to developing continually in the direction of energy saving and carbon emission reducing as well as highly effective.
Present article was published on<MW Metal Working>2011 No.13 in China.