I wanted to show you how to make an induction heater that works great using a single mosfet. The circuit is in two stages. a signal generator is created first a coil and a mosfet are used to amplify this signal The circuit is my design. I know you are open to some improvements. Since the emphasis is on the best result with the minimum amount of hardware, the circuit is shared in this way. Thank you in advance, do not hide your comments and likes from me. see you soon in the next project
Inductive heating el homemade circuit. Simple induction heating. How to make an induction heater with your own hands according to the scheme: the price of materials is not high
Now we will learn how to make an induction heater with our own hands, which can be used for various projects or just for fun. You can instantly melt steel, aluminum or copper. You can use it to weld, melt and forge metals. You can also use a homemade induction heater for casting.
Step 1: Components
The main components of high frequency induction heating to heat metal with electricity are inverter, driver, coupling transformer and RLC oscillation circuit. You will see the diagram a little later. Let's start with the inverter. It is an electrical device that converts direct current into alternating current. For a powerful module, it must work stably. On top is a guard used to protect the MOSFET gate driver from any accidental overvoltage. The random drops cause noise, which leads to swinging to high frequencies. This leads to overheating and failure of the MOSFET.
The high current lines are at the bottom of the PCB. Many layers of copper are used to allow them to carry more than 50 A of current. We don't need overheating. Also note the large water-cooled aluminum heatsinks on both sides. This is necessary to dissipate the heat generated by the MOSFETs.
Initially I used fans, but to handle that power I installed small water pumps that circulate water through aluminum radiators. As long as the water is clear, the tubes do not conduct current. I also installed thin mica plates under the MOSFETs to ensure there is no conduction through the drains.
Step 2: Inverter Schematic
This is the inverter circuit. The circuit is actually not that complicated. The inverted and non-inverted driver increases or decreases the voltage by 15 V to adjust the alternating signal in the transformer (GDT). This transformer isolates the chips from the mosfets. The diode at the output of the mosfet acts to limit the spikes, and the resistor minimizes the oscillation.
Capacitor C1 absorbs any manifestation of direct current. Ideally, you want the fastest voltage drops on the circuit because they reduce heat. Resistance slows them down, which seems counterintuitive. However, if the signal does not fade, you get overloads and oscillations which destroy the mosfets. More information can be obtained from the damping circuit.
Diodes D3 and D4 help protect the MOSFETs from reverse currents. C1 and C2 provide open paths to pass current during switching. T2 is the current transformer that allows the driver, which we will talk about next, to get the output current back.
Step 3: Pilot
This circuit is really big. In general, you can read about a simple low-power inverter. If you need more power, you need the right driver. This speaker will stop itself at the resonant frequency. Once your metal has melted, it will stay locked to the correct frequency with no tuning required.
If you've ever built a simple PLL chip induction heater, you probably remember the process of adjusting the frequency to heat metal. You observed the movement of the waveform on the oscilloscope and adjusted the trigger frequency to maintain this sweet spot. You won't have to do it again.
This circuit uses an Arduino microprocessor to monitor the phase difference between the inverter voltage and the capacitor capacitance. Using this phase, it calculates the correct frequency using the "C" algorithm.
The capacitor capacitance signal is located to the left of the LM6172. This is a high speed which converts the signal into a nice clean square wave. This signal is then isolated using a FOD3180 optical isolator. These insulators are the key!
Additionally, the signal enters the PLL through the PCAin input. It is compared to the signal on PCBin which controls the inverter via VCOout. The Arduino carefully controls the PLL clock using a 1024 bit pulse modulated signal. A two-stage RC filter converts the PWM signal to a simple analog voltage which passes to VCOin.
How does Arduino know what to do? Magic? Guess? No. It receives information about the phase difference between PCA and PCB from PC1out. R10 and R11 limit the voltage to 5 voltages for Arduino, and a two-stage RC filter cleans the signal of any noise. We need strong, clean signals because we don't want to pay extra money for expensive mosfets after they blow up from noisy inputs.
Step 4: Take a break
It was a huge amount of information. You might be wondering if you need such a fancy system? It depends on you. If you want automatic tuning, the answer is yes. If you want to set the frequency manually, the answer is no. You can create a very simple driver with just the NE555 timer and use an oscilloscope. You can improve it a bit by adding a PLL (zero phase loop)
However, let's continue.
Step 5: LC circuit
There are several approaches for this part. If you need a powerful radiator, you will need a network of capacitors to control current and voltage.
First, you need to determine the operating frequency you will be using. Higher frequencies have more skin effect (less penetration) and are suitable for small objects. Lower frequencies are more suitable for larger objects and have greater penetration. Higher frequencies have more switching losses, but less current will flow through the tank. I chose a frequency around 70 kHz and went up to 66 kHz.
My capacitor array has a capacitance of 4.4uF and can handle over 300A. My coil is around 1uH. I also use switching film capacitors. They are self-healing metallized polypropylene axial wire and have high voltage, high current and high frequency (0.22uF, 3000V). Model number 224PPA302KS.
I used two copper bars and drilled appropriate holes on each side. I soldered the capacitors to these holes with a soldering iron. I then attached copper tubing to each side for water cooling.
Don't buy cheap capacitors. They will break and you will pay more money than if you bought good ones right away.
Step 6: Transformer Assembly
If you read the article carefully, you will ask yourself the question: how to control the LC circuit? I have already talked about the inverter and the circuit, without mentioning their relationship.
The connection is made via a coupling transformer. Mine is from Magnetics, Inc. The part number is ZP48613TC. Adams Magnetics is also a good choice when choosing ferrite cores.
The one on the left has a 2mm wire. It is good if your input current is less than 20A. The wire will overheat and burn if the current is higher. For high power, you need to buy or craft a litz wire. I made it myself, weaving 64 strands of 0.5mm yarn. Such a wire can easily withstand a current of 50A.
The inverter I showed you earlier takes high voltage direct current and changes it to variable high or low values. This alternating square wave passes through the coupling transformer through the mosfet switches and DC coupling capacitors on the inverter.
A copper tube of a capacitance capacitor passes through it, making it a single-turn transformer secondary winding. This, in turn, allows the discharged voltage to pass through the capacitive capacitor and the work coil (LC circuit).
Step 7: Make a working coil
One of the questions I've often been asked is, "How do you make such a curved coil?" The answer is sand. The sand will prevent the tube from breaking during the bending process.
Take a copper tube from a 9mm refrigerator and fill it with clean sand. Before doing this, cover one end with tape and also cover the other after filling with sand. Dig a pipe of the appropriate diameter into the ground. Measure the length of your coil tube and start winding it slowly around the tube. Once you've done one trick, the rest will be easy to do. Continue winding the tubing until you get the number of turns you want (usually 4-6). The second end should line up with the first. This will make it easier to connect to the capacitor.
Now remove the plugs and take an air compressor to blow out the sand. It is advisable to do this outdoors.
Please note that the copper tube is also used for water cooling. This water circulates through a capacitive condenser and through the work coil. The work coil generates a lot of heat from the current. Even if you use ceramic insulation inside the coil (to retain heat), you will still have extremely high temperatures in the workspace that will heat up the coil. I will start with a large bucket of ice water and after a while it will get hot. I advise you to prepare a lot of ice cream.
Step 8: Presentation of the project
The video shows a 12 kW induction furnace in action. The main difference is that it has a microprocessor-controlled driver, larger MOSFETs, and heatsinks. The 3kW unit operates on 120V AC; the 12 kW unit uses 240V.
The induction furnace was invented a long time ago, in 1887, by S. Farranti. The first industrial factory was commissioned in 1890 by Benedicks Bultfabrik. For a long time, induction furnaces were exotic in industry, but not because of the high cost of electricity, so it was no more expensive than it is today. There was still a lot of incomprehensibility in the processes taking place in induction furnaces, and the element base of electronics did not allow creating effective control circuits for them.
In the field of induction furnaces, a revolution has literally taken place before our eyes today, thanks to the appearance, on the one hand, of microcontrollers, whose computing power exceeds that of personal computers ten years. Secondly, thanks to ... mobile communications. Its development required the appearance on the market of inexpensive transistors capable of delivering several kW of power at high frequencies. They, in turn, were created on the basis of semiconductor heterostructures, for the research of which the Russian physicist Zhores Alferov was awarded the Nobel Prize.
In the end, induction cookers have not only completely changed in the industry, but also widely entered daily life. Interest in the topic has given rise to many homemade products, which, in principle, could be useful. But most of the authors of designs and ideas (there are many more descriptions in the sources than realizable products) have a poor idea of both the basics of the physics of induction heating and the potential danger of illiterate designs . This article aims to clarify some of the more confusing points. The material is built on the consideration of specific structures:
An industrial channel furnace for melting metal and the ability to create it yourself.
Induction type crucible furnaces, the easiest to make and the most popular among craftsmen.
Induction hot water boilers, quickly replacing boilers with heating elements.
Home induction cooking appliances that compete with gas stoves and outperform microwaves on many parameters.
Note: all the devices considered are based on the magnetic induction created by the inductor (inductor), and therefore are called induction. Only electrically conductive materials, metals, etc. can be melted/heated there. There are also electric induction capacitive ovens based on electric induction in the dielectric between the plates of the capacitor; they are used for "soft" melting and electrical heat treatment of plastics. But they are much less common than those of inductors, their consideration requires a separate discussion, so let's leave that for now.
Principle of operation
The principle of operation of the induction furnace is shown in fig.on the right. It is basically an electrical transformer with a shorted secondary winding:
The alternating voltage generator G creates an alternating current I1 in the inductor L (heating coil).
Capacitor C forms together with L an oscillating circuit tuned to the operating frequency, which in most cases increases the technical parameters of the installation.
If the generator G is self-oscillating, then C is often excluded from the circuit, using the inductor's own capacitance instead. For the high frequency inductors described below, this is several tens of picofarads, which is just within the operating frequency range.
The inductor, in accordance with Maxwell's equations, creates in the surrounding space an alternating magnetic field of intensity H. The magnetic field of the inductor can either be closed by a separate ferromagnetic core or exist in free space.
The magnetic field, penetrating into the part (or the fusion charge) W placed in the inductor, creates a magnetic flux F in it.
Ф, if W is electrically conductive, induces a secondary current I2 there, then the same Maxwell equations.
If Ф is sufficiently massive and strong, then I2 closes inside W, forming an eddy current, or eddy current.
Eddy currents, according to the Joule-Lenz law, give off the energy received by the inductor and the magnetic field of the generator, heating the part (load).
From the point of view of physics, the electromagnetic interaction is quite strong and has a fairly high long-range action. Therefore, despite the multi-stage energy conversion, the induction furnace is able to show up to 100% efficiency in air or vacuum.
Note: in a non-ideal dielectric medium with permittivity >1, the potentially achievable efficiency of induction furnaces decreases, and in a medium with magnetic permeability >1, it is easier to achieve high efficiency.
channel oven
The channel induction melting furnace is the first used in the industry. It is structurally similar to a transformer, see fig. on the right:
The primary winding, supplied with power frequency current (50/60 Hz) or increased (400 Hz), consists of a copper tube cooled from the inside by a liquid coolant;
Secondary winding shorted - melting;
An annular crucible made of heat-resistant dielectric in which the molten pool is placed;
Composition of transformer steel magnetic core plates.
Channel furnaces are used to remelt duralumin, special non-ferrous alloys and produce high quality cast iron. Industrial channel furnaces require melt seeding, otherwise the "secondary" will not short-circuit and there will be no heating. Or arc discharges will occur between the crumbs of the charge, and the whole melt will simply explode. Therefore, before starting the furnace, a little molten material is poured into the crucible, and the remelted part is not completely poured. Metallurgists say that the channel furnace has a residual capacity.
A duct furnace with a power of up to 2-3 kW can also be made from a power-frequency welding transformer. In such a furnace, up to 300-400 g of zinc, bronze, brass or copper can be melted. It is possible to melt duralumin, only the casting must age after cooling, from several hours to 2 weeks, depending on the composition of the alloy, in order to gain strength, toughness and elasticity.
Note: duralumin was usually invented by accident. The developers, angry that it was impossible to alloy aluminum, threw another "no" sample into the lab and went into a frenzy of grief. Sobered up, returned - but none changed color. Checked - and it gained strength almost steel, remaining light as aluminum.
The "primary" of the transformer is left as standard, it is already designed to operate in the short-circuit mode of the secondary with a welding arc. The "secondary" is removed (it can then be put back in place and the transformer can be used for its intended purpose), and an annular crucible is put in place. But trying to convert a welding RF inverter into a channel oven is dangerous! Its ferrite core will overheat and break into pieces due to the dielectric constant of ferrite >> 1, see above.
The problem of residual capacity in a low-power furnace disappears: a wire of the same metal, bent into a ring and with twisted ends, is placed in the charge for seeding. Wire diameter – from 1 mm/kW furnace power.
But there is a problem with the ring Residual capacity irritated metallurgists - expensive alloys melted. Therefore, as soon as sufficiently powerful radio tubes appeared in the 20s of the last century, an idea immediately arose: throw a magnetic circuit on (we will not repeat the professional idioms of hard men), and put an ordinary crucible directly in the inductor, see fig.
You can't do this at power frequency, a low frequency magnetic field without a magnetic circuit concentrating it will propagate (this is called the stray field) and give up its energy anywhere but not into mass fondue. The stray field can be compensated by increasing the frequency to a high frequency: if the diameter of the inductor is proportional to the wavelength of the operating frequency and the whole system is in electromagnetic resonance, then up to 75% or more of its electromagnetic field energy will be concentrated inside the "coreless" coil. The efficiency will be corresponding.
However, already in the laboratories it turned out that the authors of the idea overlooked the obvious circumstance - the melting in the inductor, although diamagnetic, but electrically conductive, due to its own magnetic field from the currents of Foucault, changes the inductance of the heating coil. The initial frequency had to be tuned under the cold load and changed as it melted. Moreover, in the wider limits, the larger the room: if for 200 g of steel you can get by with a range from 2 to 30 MHz, then for a blank with a railway tank, the initial frequency will be around 30 to 40 Hz, and working frequency will be up to several kHz.
It is difficult to achieve proper automation on the lamps, to “pull” the frequency behind a blank - a highly skilled operator is needed. Moreover, at low frequencies, the stray field manifests itself in the strongest way. The molten mass, which in such a furnace is also the core of the coil, to some extent collects a nearby magnetic field, but all the same, in order to achieve an acceptable yield, it was necessary to surround the entire furnace with a powerful shield ferromagnetic.
Nevertheless, due to their outstanding advantages and unique qualities (see below), crucible induction furnaces are widely used both in industry and by DIY enthusiasts. Therefore, we will dwell in more detail on how to do it correctly with your own hands.
A bit of theory
When designing a homemade "induction", you must firmly remember that minimum power consumption does not correspond to maximum efficiency, and vice versa. The stove will take the minimum power from the network when operating at the main resonant frequency, Pos. 1 in fig. In this case, the blank/charge (and at lower pre-resonance frequencies) operates as a single shorted coil, and a single convective cell is observed in the melt.
I wanted to show you how to make an induction heater that works great using a single mosfet. The circuit is in two stages. a signal generator is created first a coil and a mosfet are used to amplify this signal The circuit is my design. I know you are open to some improvements. Since the emphasis is on the best result with the minimum amount of hardware, the circuit is shared in this way. Thank you in advance, do not hide your comments and likes from me. see you soon in the next project
Inductive heating el homemade circuit. Simple induction heating. How to make an induction heater with your own hands according to the scheme: the price of materials is not high
Now we will learn how to make an induction heater with our own hands, which can be used for various projects or just for fun. You can instantly melt steel, aluminum or copper. You can use it to weld, melt and forge metals. You can also use a homemade induction heater for casting.
Step 1: Components
The main components of high frequency induction heating to heat metal with electricity are inverter, driver, coupling transformer and RLC oscillation circuit. You will see the diagram a little later. Let's start with the inverter. It is an electrical device that converts direct current into alternating current. For a powerful module, it must work stably. On top is a guard used to protect the MOSFET gate driver from any accidental overvoltage. The random drops cause noise, which leads to swinging to high frequencies. This leads to overheating and failure of the MOSFET.
The high current lines are at the bottom of the PCB. Many layers of copper are used to allow them to carry more than 50 A of current. We don't need overheating. Also note the large water-cooled aluminum heatsinks on both sides. This is necessary to dissipate the heat generated by the MOSFETs.
Initially I used fans, but to handle that power I installed small water pumps that circulate water through aluminum radiators. As long as the water is clear, the tubes do not conduct current. I also installed thin mica plates under the MOSFETs to ensure there is no conduction through the drains.
Step 2: Inverter Schematic
This is the inverter circuit. The circuit is actually not that complicated. The inverted and non-inverted driver increases or decreases the voltage by 15 V to adjust the alternating signal in the transformer (GDT). This transformer isolates the chips from the mosfets. The diode at the output of the mosfet acts to limit the spikes, and the resistor minimizes the oscillation.
Capacitor C1 absorbs any manifestation of direct current. Ideally, you want the fastest voltage drops on the circuit because they reduce heat. Resistance slows them down, which seems counterintuitive. However, if the signal does not fade, you get overloads and oscillations which destroy the mosfets. More information can be obtained from the damping circuit.
Diodes D3 and D4 help protect the MOSFETs from reverse currents. C1 and C2 provide open paths to pass current during switching. T2 is the current transformer that allows the driver, which we will talk about next, to get the output current back.
Step 3: Pilot
This circuit is really big. In general, you can read about a simple low-power inverter. If you need more power, you need the right driver. This speaker will stop itself at the resonant frequency. Once your metal has melted, it will stay locked to the correct frequency with no tuning required.
If you've ever built a simple PLL chip induction heater, you probably remember the process of adjusting the frequency to heat metal. You observed the movement of the waveform on the oscilloscope and adjusted the trigger frequency to maintain this sweet spot. You won't have to do it again.
This circuit uses an Arduino microprocessor to monitor the phase difference between the inverter voltage and the capacitor capacitance. Using this phase, it calculates the correct frequency using the "C" algorithm.
The capacitor capacitance signal is located to the left of the LM6172. This is a high speed which converts the signal into a nice clean square wave. This signal is then isolated using a FOD3180 optical isolator. These insulators are the key!
Additionally, the signal enters the PLL through the PCAin input. It is compared to the signal on PCBin which controls the inverter via VCOout. The Arduino carefully controls the PLL clock using a 1024 bit pulse modulated signal. A two-stage RC filter converts the PWM signal to a simple analog voltage which passes to VCOin.
How does Arduino know what to do? Magic? Guess? No. It receives information about the phase difference between PCA and PCB from PC1out. R10 and R11 limit the voltage to 5 voltages for Arduino, and a two-stage RC filter cleans the signal of any noise. We need strong, clean signals because we don't want to pay extra money for expensive mosfets after they blow up from noisy inputs.
Step 4: Take a break
It was a huge amount of information. You might be wondering if you need such a fancy system? It depends on you. If you want automatic tuning, the answer is yes. If you want to set the frequency manually, the answer is no. You can create a very simple driver with just the NE555 timer and use an oscilloscope. You can improve it a bit by adding a PLL (zero phase loop)
However, let's continue.
Step 5: LC circuit
There are several approaches for this part. If you need a powerful radiator, you will need a network of capacitors to control current and voltage.
First, you need to determine the operating frequency you will be using. Higher frequencies have more skin effect (less penetration) and are suitable for small objects. Lower frequencies are more suitable for larger objects and have greater penetration. Higher frequencies have more switching losses, but less current will flow through the tank. I chose a frequency around 70 kHz and went up to 66 kHz.
My capacitor array has a capacitance of 4.4uF and can handle over 300A. My coil is around 1uH. I also use switching film capacitors. They are self-healing metallized polypropylene axial wire and have high voltage, high current and high frequency (0.22uF, 3000V). Model number 224PPA302KS.
I used two copper bars and drilled appropriate holes on each side. I soldered the capacitors to these holes with a soldering iron. I then attached copper tubing to each side for water cooling.
Don't buy cheap capacitors. They will break and you will pay more money than if you bought good ones right away.
Step 6: Transformer Assembly
If you read the article carefully, you will ask yourself the question: how to control the LC circuit? I have already talked about the inverter and the circuit, without mentioning their relationship.
The connection is made via a coupling transformer. Mine is from Magnetics, Inc. The part number is ZP48613TC. Adams Magnetics is also a good choice when choosing ferrite cores.
The one on the left has a 2mm wire. It is good if your input current is less than 20A. The wire will overheat and burn if the current is higher. For high power, you need to buy or craft a litz wire. I made it myself, weaving 64 strands of 0.5mm yarn. Such a wire can easily withstand a current of 50A.
The inverter I showed you earlier takes high voltage direct current and changes it to variable high or low values. This alternating square wave passes through the coupling transformer through the mosfet switches and DC coupling capacitors on the inverter.
A copper tube of a capacitance capacitor passes through it, making it a single-turn transformer secondary winding. This, in turn, allows the discharged voltage to pass through the capacitive capacitor and the work coil (LC circuit).
Step 7: Make a working coil
One of the questions I've often been asked is, "How do you make such a curved coil?" The answer is sand. The sand will prevent the tube from breaking during the bending process.
Take a copper tube from a 9mm refrigerator and fill it with clean sand. Before doing this, cover one end with tape and also cover the other after filling with sand. Dig a pipe of the appropriate diameter into the ground. Measure the length of your coil tube and start winding it slowly around the tube. Once you've done one trick, the rest will be easy to do. Continue winding the tubing until you get the number of turns you want (usually 4-6). The second end should line up with the first. This will make it easier to connect to the capacitor.
Now remove the plugs and take an air compressor to blow out the sand. It is advisable to do this outdoors.
Please note that the copper tube is also used for water cooling. This water circulates through a capacitive condenser and through the work coil. The work coil generates a lot of heat from the current. Even if you use ceramic insulation inside the coil (to retain heat), you will still have extremely high temperatures in the workspace that will heat up the coil. I will start with a large bucket of ice water and after a while it will get hot. I advise you to prepare a lot of ice cream.
Step 8: Presentation of the project
The video shows a 12 kW induction furnace in action. The main difference is that it has a microprocessor-controlled driver, larger MOSFETs, and heatsinks. The 3kW unit operates on 120V AC; the 12 kW unit uses 240V.
The induction furnace was invented a long time ago, in 1887, by S. Farranti. The first industrial factory was commissioned in 1890 by Benedicks Bultfabrik. For a long time, induction furnaces were exotic in industry, but not because of the high cost of electricity, so it was no more expensive than it is today. There was still a lot of incomprehensibility in the processes taking place in induction furnaces, and the element base of electronics did not allow creating effective control circuits for them.
In the field of induction furnaces, a revolution has literally taken place before our eyes today, thanks to the appearance, on the one hand, of microcontrollers, whose computing power exceeds that of personal computers ten years. Secondly, thanks to ... mobile communications. Its development required the appearance on the market of inexpensive transistors capable of delivering several kW of power at high frequencies. They, in turn, were created on the basis of semiconductor heterostructures, for the research of which the Russian physicist Zhores Alferov was awarded the Nobel Prize.
In the end, induction cookers have not only completely changed in the industry, but also widely entered daily life. Interest in the topic has given rise to many homemade products, which, in principle, could be useful. But most of the authors of designs and ideas (there are many more descriptions in the sources than realizable products) have a poor idea of both the basics of the physics of induction heating and the potential danger of illiterate designs . This article aims to clarify some of the more confusing points. The material is built on the consideration of specific structures:
An industrial channel furnace for melting metal and the ability to create it yourself.
Induction type crucible furnaces, the easiest to make and the most popular among craftsmen.
Induction hot water boilers, quickly replacing boilers with heating elements.
Home induction cooking appliances that compete with gas stoves and outperform microwaves on many parameters.
Note: all the devices considered are based on the magnetic induction created by the inductor (inductor), and therefore are called induction. Only electrically conductive materials, metals, etc. can be melted/heated there. There are also electric induction capacitive ovens based on electric induction in the dielectric between the plates of the capacitor; they are used for "soft" melting and electrical heat treatment of plastics. But they are much less common than those of inductors, their consideration requires a separate discussion, so let's leave that for now.
Principle of operation
The principle of operation of the induction furnace is shown in fig.on the right. It is basically an electrical transformer with a shorted secondary winding:
The alternating voltage generator G creates an alternating current I1 in the inductor L (heating coil).
Capacitor C forms together with L an oscillating circuit tuned to the operating frequency, which in most cases increases the technical parameters of the installation.
If the generator G is self-oscillating, then C is often excluded from the circuit, using the inductor's own capacitance instead. For the high frequency inductors described below, this is several tens of picofarads, which is just within the operating frequency range.
The inductor, in accordance with Maxwell's equations, creates in the surrounding space an alternating magnetic field of intensity H. The magnetic field of the inductor can either be closed by a separate ferromagnetic core or exist in free space.
The magnetic field, penetrating into the part (or the fusion charge) W placed in the inductor, creates a magnetic flux F in it.
Ф, if W is electrically conductive, induces a secondary current I2 there, then the same Maxwell equations.
If Ф is sufficiently massive and strong, then I2 closes inside W, forming an eddy current, or eddy current.
Eddy currents, according to the Joule-Lenz law, give off the energy received by the inductor and the magnetic field of the generator, heating the part (load).
From the point of view of physics, the electromagnetic interaction is quite strong and has a fairly high long-range action. Therefore, despite the multi-stage energy conversion, the induction furnace is able to show up to 100% efficiency in air or vacuum.
Note: in a non-ideal dielectric medium with permittivity >1, the potentially achievable efficiency of induction furnaces decreases, and in a medium with magnetic permeability >1, it is easier to achieve high efficiency.
channel oven
The channel induction melting furnace is the first used in the industry. It is structurally similar to a transformer, see fig. on the right:
The primary winding, supplied with power frequency current (50/60 Hz) or increased (400 Hz), consists of a copper tube cooled from the inside by a liquid coolant;
Secondary winding shorted - melting;
An annular crucible made of heat-resistant dielectric in which the molten pool is placed;
Composition of transformer steel magnetic core plates.
Channel furnaces are used to remelt duralumin, special non-ferrous alloys and produce high quality cast iron. Industrial channel furnaces require melt seeding, otherwise the "secondary" will not short-circuit and there will be no heating. Or arc discharges will occur between the crumbs of the charge, and the whole melt will simply explode. Therefore, before starting the furnace, a little molten material is poured into the crucible, and the remelted part is not completely poured. Metallurgists say that the channel furnace has a residual capacity.
A duct furnace with a power of up to 2-3 kW can also be made from a power-frequency welding transformer. In such a furnace, up to 300-400 g of zinc, bronze, brass or copper can be melted. It is possible to melt duralumin, only the casting must age after cooling, from several hours to 2 weeks, depending on the composition of the alloy, in order to gain strength, toughness and elasticity.
Note: duralumin was usually invented by accident. The developers, angry that it was impossible to alloy aluminum, threw another "no" sample into the lab and went into a frenzy of grief. Sobered up, returned - but none changed color. Checked - and it gained strength almost steel, remaining light as aluminum.
The "primary" of the transformer is left as standard, it is already designed to operate in the short-circuit mode of the secondary with a welding arc. The "secondary" is removed (it can then be put back in place and the transformer can be used for its intended purpose), and an annular crucible is put in place. But trying to convert a welding RF inverter into a channel oven is dangerous! Its ferrite core will overheat and break into pieces due to the dielectric constant of ferrite >> 1, see above.
The problem of residual capacity in a low-power furnace disappears: a wire of the same metal, bent into a ring and with twisted ends, is placed in the charge for seeding. Wire diameter – from 1 mm/kW furnace power.
But there is a problem with the ring Residual capacity irritated metallurgists - expensive alloys melted. Therefore, as soon as sufficiently powerful radio tubes appeared in the 20s of the last century, an idea immediately arose: throw a magnetic circuit on (we will not repeat the professional idioms of hard men), and put an ordinary crucible directly in the inductor, see fig.
You can't do this at power frequency, a low frequency magnetic field without a magnetic circuit concentrating it will propagate (this is called the stray field) and give up its energy anywhere but not into mass fondue. The stray field can be compensated by increasing the frequency to a high frequency: if the diameter of the inductor is proportional to the wavelength of the operating frequency and the whole system is in electromagnetic resonance, then up to 75% or more of its electromagnetic field energy will be concentrated inside the "coreless" coil. The efficiency will be corresponding.
However, already in the laboratories it turned out that the authors of the idea overlooked the obvious circumstance - the melting in the inductor, although diamagnetic, but electrically conductive, due to its own magnetic field from the currents of Foucault, changes the inductance of the heating coil. The initial frequency had to be tuned under the cold load and changed as it melted. Moreover, in the wider limits, the larger the room: if for 200 g of steel you can get by with a range from 2 to 30 MHz, then for a blank with a railway tank, the initial frequency will be around 30 to 40 Hz, and working frequency will be up to several kHz.
It is difficult to achieve proper automation on the lamps, to “pull” the frequency behind a blank - a highly skilled operator is needed. Moreover, at low frequencies, the stray field manifests itself in the strongest way. The molten mass, which in such a furnace is also the core of the coil, to some extent collects a nearby magnetic field, but all the same, in order to achieve an acceptable yield, it was necessary to surround the entire furnace with a powerful shield ferromagnetic.
Nevertheless, due to their outstanding advantages and unique qualities (see below), crucible induction furnaces are widely used both in industry and by DIY enthusiasts. Therefore, we will dwell in more detail on how to do it correctly with your own hands.
A bit of theory
When designing a homemade "induction", you must firmly remember that minimum power consumption does not correspond to maximum efficiency, and vice versa. The stove will take the minimum power from the network when operating at the main resonant frequency, Pos. 1 in fig. In this case, the blank/charge (and at lower pre-resonance frequencies) operates as a single shorted coil, and a single convective cell is observed in the melt.
No comments:
Post a Comment