Induction heater
Encyclopedia
An induction heater is a key piece of equipment used in all forms of induction heating
. Typically an induction heater operates at either medium frequency (MF) or radio frequency (RF) ranges. Three main components form the basis of a modern induction heater including the power unit (power inverter), the work head (transformer
) and the coil (inductor
). Induction heating is a non contact method of heating a conductive body by utilising a strong magnetic field
. Supply (mains) frequency 50/60 Hz induction heaters incorporate a coil directly fed from the electricity supply, typically for lower power industrial applications where lower surface temperatures are required.
.
consisting of a number of turns of copper tube wound around a mandrel, to a precision item machined from solid copper, brazed and soldered together. As the inductor is the area where the heating takes place, coil design is one of the most important elements of the system and is a science in itself.
an engine valve.
The term "MF induction" is traditionally used to describe induction generators designed to work in the frequency range from 1 to 10 kHz. The output range typically incorporates 50 to 500 kW. Induction heaters operating within these ranges are normally utilised on medium to larger components and applications such as the induction forging
of a shaft.
as early as 1831. Faraday's work involved the use of a switched DC supply provided by a battery
and two windings of copper wire wrapped around an iron core. It was noted that when the switch was closed a momentary current
flowed in the secondary winding, which could be measured by means of a galvanometer
. If the circuit remained energized then the current ceased to flow. On opening the switch a current again flowed in the secondary winding, but in the opposite direction. Faraday concluded that since no physical link existed between the two windings, the current in the secondary coil must be caused by a voltage that was induced from the first coil, and that the current produced was directly proportional to the rate of change of the magnetic flux
.
Initially the principles were put to use in the design of transformer
s, motors
and generators where undesirable heating effects were controlled by the use of a laminated core.
Early in the 20th century engineers started to look for ways to harness the heat-generating properties of induction
for the purpose of melting steel. This early work used motor generators to create the medium frequency (MF) current, but the lack of suitable alternators and capacitors of the correct size held back early attempts. However, by 1927 the first MF induction melting system had been installed by EFCO
in Sheffield, England.
At around the same time engineers at Midvale Steel
and The Ohio Crankshaft Company in America were attempting to use the surface-heating effect of the MF current to produce localized surface case hardening
in crankshafts. Much of this work took place at the frequencies of 1920 and 3000 Hz as these were the easiest frequencies to produce with the equipment available. As with many technology-based fields it was the advent of World War II which led to huge developments in the utilization of induction heating in the production of vehicle parts and munitions.
Over time, the technology advanced and units in the 3 to 10 kHz frequency range with powers outputs to 600 kW became common place in induction forging
and large induction hardening
applications. The motor generator would remain the mainstay of MF power generation until the advent of high voltage semiconductors in the late 1960s and early 1970s.
Early in the evolutionary process it became obvious to engineers that the ability to produce a higher radio frequency range of equipment would result in greater flexibility and open up a whole range of alternative applications. Methods were sought to produce these higher RF power supplies to operate in the 200 to 400 kHz range.
Development in this particular frequency range has always mirrored that of the radio transmitter and television broadcasting industry and indeed has often used component parts developed for this purpose. Early units utilised spark gap
technology, but due to limitations the approach was rapidly superseded by the use of multi-electrode thermionic triode
(valve) based oscillators. Indeed, many of the pioneers in the industry were also very involved in the radio and telecommunications industry and companies such as Phillips
, English Electric
and Redifon were all involved in manufacturing induction heating equipment in the 1950s and 1960s.
The use of this technology survived until the early 1990s at which point the technology was all but replaced by power MOSFET
and IGBT solid state
equipment. However there are still many valve
oscillators still in existence, and at extreme frequencies of 5 MHz and above they are often the only viable approach and are still produced.
. The unit consists of three basic elements:
) power supply consists of a standard air or water cooled step-up transformer and a high voltage rectifier unit capable of generating voltages typically between 5 and 10 kV to power the oscillator. The unit needs to be rated at the correct kilovolt-ampere (kVA) to supply the necessary current to the oscillator. Early rectifier systems featured valve rectifiers such as GXU4 (high power high voltage half wave rectifier) but these were ultimately superseded by high voltage solid state rectifiers.
(tank coil) and a capacitance
(tank capacitor) and an oscillator valve. Basic electrical principles dictate that if a voltage is applied to a circuit containing a capacitor and inductor the circuit will oscillate in much the same way as a swing which has been pushed. Using our swing as an analogy if we do not push again at the right time the swing will gradually stop this is the same with the oscillator. The purpose of the valve is to act as a switch which will allow energy to pass into the oscillator at the correct time to maintain the oscillations. In order to time the switching, a small amount of energy is fed back to the grid of the triode
effectively blocking or firing the device or allow it to conduct at the correct time. This so called grid bias can be derived, either capacitively, conductively or inductively depending on whether the oscillator is a Colpitts, Hartley oscillator
, Armstrong tickler or a Meissner.
power control which works by means of a full wave AC (alternating current
) drive varying the primary voltage to the input transformer. More traditional methods include three phase variacs (autotransformer
) or motorised Brentford type voltage regulators to control the input voltage. Another very popular method was to use a two part tank coil with a primary and secondary winding separated by an air gap. Power control was affected by varying the magnetic coupling of the two coils by physically moving them relative to each other.
was used extensively for the production of MF power up to 10 kHz. While it is possible to generate multiples of the supply frequency such as 150 Hz using a standard induction motor driving an AC generator, there are limitations. This type of generator featured rotor mounted windings which limited the peripheral speed of the rotor due to the centrifugal forces on these windings. This had the effect of limiting the diameter of the machine and therefore its power and the number of poles which can be physically accommodated, which in turn limits the maximum operating frequency.
To overcome these limitations the induction heating industry turned to the inductor-generator. This type of machine features a toothed rotor constructed from a stack of punched iron laminations. The excitation
and AC windings are both mounted on the stator, the rotor is therefore a compact solid construction which can be rotated at higher peripheral speeds than the standard AC generator above thus allowing it to be greater in diameter for a given RPM. This larger diameter allows a greater number of poles to be accommodated and when combined with complex slotting arrangements such as the Lorenz gauge condition
or Guy slotting which allows the generation of frequencies from 1 to 10 kHz.
As with all rotating electrical machines, high rotation speeds and small clearances are utilised to maximise flux variations. This necessitates that close attention is paid to the quality of bearings utilised and the stiffness and accuracy of rotor. Drive for the alternator is normally provided by a standard induction motor for convention and simplicity. Both vertical and horizontal configurations are utilised and in most cases the motor rotor and generator rotor are mounted on a common shaft with no coupling. The whole assembly is then mounted in a frame containing the motor stator
and generator stator. The whole construction is mounted in a cubicle which features a heat exchanger and water cooling systems as required.
The motor-generator became the mainstay of medium frequency power generation until the advent of solid state
technology in the early 1970s.
In the early 1970s the advent of solid state switching technology saw a shift from the traditional methods of induction heating power generation. Initially this was limited to the use of thyristors for generating the 'MF range of frequencies using discrete electronic control systems.
State of the art units now employ SCR (silicon-controlled rectifier
), IGBT or MOSFET technologies for generating the 'MF' and 'RF' current. The modern control system is typically a digital microprocessor
based system utilising PIC, PLC (programmable logic controller
) technology and surface mount manufacturing techniques for production of the printed circuit boards. Solid state now dominates the market and units from 1 kW to many megawatts in frequencies from 1 kHz to 3 MHz including dual frequency units are now available.
A whole range of techniques are employed in the generation of MF and RF power using semiconductors, the actual technique employed depends often on a complex range of factors. The typical generator will employ either a current or a voltage fed topology. The actual approach employed will be a function of the required power, frequency, individual application, the initial cost and subsequent running costs. Irrespective of the approach employed however, all units tend to feature four distinct elements:
converts the DC supply to a single phase AC output at the relevant frequency. This features the SCR, IGBT or MOSFETS and in most cases is configured as an H-bridge
. The H-bridge has four legs each with a switch, the output circuit is connected across the centre of the devices. When the relevant two switches are closed current flows through the load in one direction, these switches then open and the opposing two switches close allowing current to flow in the opposite direction. By precisely timing the opening and closing of the switches, it is possible to sustain oscillations in the load circuit.
technology, the majority of advanced systems now feature digital control.
on the input to the inverter and a series resonant output circuits. The voltage-fed system is extremely popular and can be used with either SCR's up to frequencies of 10 kHz, IGBT's to 100 kHz and MOSFETS up to 3 MHz. A voltage-fed inverter with a series connection to a parallel load is also known as a third order system. Basically this is similar to solid state, but in this system the series connected internal capacitor and inductor are connected to a parallel output tank circuit. The principal advantage of this type of system is the robustness of the inverter due to the internal circuit effectively isolating the output circuit making the switching components less susceptible to damage due to coil flashovers or mismatching.
(100-500) which are heated below the Curie temperature
of that material.
Induction heating
Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...
. Typically an induction heater operates at either medium frequency (MF) or radio frequency (RF) ranges. Three main components form the basis of a modern induction heater including the power unit (power inverter), the work head (transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
) and the coil (inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
). Induction heating is a non contact method of heating a conductive body by utilising a strong magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. Supply (mains) frequency 50/60 Hz induction heaters incorporate a coil directly fed from the electricity supply, typically for lower power industrial applications where lower surface temperatures are required.
Power unit
Often referred to as the inverter or generator. This part of the system is used to take the mains frequency and increase it to anywhere between 1 and 400 kHz. Typical output power of a unit system is from 2 to 500 kWKw
kw or KW may refer to:* Kuwait, ISO 3166-1 country code** .kw, the country code top level domain for Kuwait* Kilowatt* Self-ionization of water Kw* Cornish language's ISO 639 code* Kitchener–Waterloo, Ontario, Canada...
.
Work head
This contains a combination of capacitors and transformers and is used to mate the power unit to the work coil.Work coil
Also known as the inductor, the coil is used to transfer the energy from the power unit and work head to the work piece. Inductors range in complexity from a simple wound solenoidSolenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...
consisting of a number of turns of copper tube wound around a mandrel, to a precision item machined from solid copper, brazed and soldered together. As the inductor is the area where the heating takes place, coil design is one of the most important elements of the system and is a science in itself.
Definition
The term "RF induction" is traditionally used to describe induction generators designed to work in the frequency range from 100 kHz up to 10 MHz, in practical terms however the frequency range tends to cover 100 to 200 kHz. The output range typically incorporates 2.5 to 40 kW. Generally, the induction heaters in this range are used for smaller components and applications such as induction hardeningInduction hardening
Induction hardening is a form of heat treatment in which a metal part is heated by induction heating and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the part...
an engine valve.
The term "MF induction" is traditionally used to describe induction generators designed to work in the frequency range from 1 to 10 kHz. The output range typically incorporates 50 to 500 kW. Induction heaters operating within these ranges are normally utilised on medium to larger components and applications such as the induction forging
Induction forging
Induction forging refers to the use of an induction heater to pre-heat metals prior to deformation using a press or hammer. Typically metals are heated to between and to increase their malleability and aid flow in the forging die.-Process:...
of a shaft.
History
The basic principle involved in induction heating was discovered by Michael FaradayMichael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....
as early as 1831. Faraday's work involved the use of a switched DC supply provided by a battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
and two windings of copper wire wrapped around an iron core. It was noted that when the switch was closed a momentary current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
flowed in the secondary winding, which could be measured by means of a galvanometer
Galvanometer
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...
. If the circuit remained energized then the current ceased to flow. On opening the switch a current again flowed in the secondary winding, but in the opposite direction. Faraday concluded that since no physical link existed between the two windings, the current in the secondary coil must be caused by a voltage that was induced from the first coil, and that the current produced was directly proportional to the rate of change of the magnetic flux
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
.
Initially the principles were put to use in the design of transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s, motors
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
and generators where undesirable heating effects were controlled by the use of a laminated core.
Early in the 20th century engineers started to look for ways to harness the heat-generating properties of induction
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
for the purpose of melting steel. This early work used motor generators to create the medium frequency (MF) current, but the lack of suitable alternators and capacitors of the correct size held back early attempts. However, by 1927 the first MF induction melting system had been installed by EFCO
EFCO
EFCO is a civil company based in Des Moines, Iowa. It was founded in 1934 and is today one of the leading providers of concrete formwork systems in the USA and the world. EFCO has an extensive subsidiary network in North America, South America, Asia, and Europe....
in Sheffield, England.
At around the same time engineers at Midvale Steel
Midvale Steel
Midvale Steel was a succession of steel-making corporations whose flagship plant was the Midvale Steel Works at Nicetown, Philadelphia, Pennsylvania, which operated from 1867 until 1976...
and The Ohio Crankshaft Company in America were attempting to use the surface-heating effect of the MF current to produce localized surface case hardening
Case hardening
Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy...
in crankshafts. Much of this work took place at the frequencies of 1920 and 3000 Hz as these were the easiest frequencies to produce with the equipment available. As with many technology-based fields it was the advent of World War II which led to huge developments in the utilization of induction heating in the production of vehicle parts and munitions.
Over time, the technology advanced and units in the 3 to 10 kHz frequency range with powers outputs to 600 kW became common place in induction forging
Induction forging
Induction forging refers to the use of an induction heater to pre-heat metals prior to deformation using a press or hammer. Typically metals are heated to between and to increase their malleability and aid flow in the forging die.-Process:...
and large induction hardening
Induction hardening
Induction hardening is a form of heat treatment in which a metal part is heated by induction heating and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the part...
applications. The motor generator would remain the mainstay of MF power generation until the advent of high voltage semiconductors in the late 1960s and early 1970s.
Early in the evolutionary process it became obvious to engineers that the ability to produce a higher radio frequency range of equipment would result in greater flexibility and open up a whole range of alternative applications. Methods were sought to produce these higher RF power supplies to operate in the 200 to 400 kHz range.
Development in this particular frequency range has always mirrored that of the radio transmitter and television broadcasting industry and indeed has often used component parts developed for this purpose. Early units utilised spark gap
Spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...
technology, but due to limitations the approach was rapidly superseded by the use of multi-electrode thermionic triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
(valve) based oscillators. Indeed, many of the pioneers in the industry were also very involved in the radio and telecommunications industry and companies such as Phillips
Philips
Koninklijke Philips Electronics N.V. , more commonly known as Philips, is a multinational Dutch electronics company....
, English Electric
English Electric
English Electric was a British industrial manufacturer. Founded in 1918, it initially specialised in industrial electric motors and transformers...
and Redifon were all involved in manufacturing induction heating equipment in the 1950s and 1960s.
The use of this technology survived until the early 1990s at which point the technology was all but replaced by power MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
and IGBT solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
equipment. However there are still many valve
Valve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...
oscillators still in existence, and at extreme frequencies of 5 MHz and above they are often the only viable approach and are still produced.
Valve oscillator based power supply
Due to its flexibility and potential frequency range, the valve oscillator based induction heater was until recent years widely used throughout industry. Readily available in powers from 1 kW to 1 MW and in a frequency range from 100 kHz to many MHz, this type of unit found widespread use in thousands of applications including soldering and brazing, induction hardening, tube welding and induction shrink fittingInduction shrink fitting
Induction shrink fitting refers to the use of induction heater technology to pre-heat metal components between and thereby causing them to expand and allow for the insertion or removal of another component. Typically the lower temperature range is used on metals such as aluminium and higher...
. The unit consists of three basic elements:
High voltage DC power supply
The DC (direct currentDirect current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
) power supply consists of a standard air or water cooled step-up transformer and a high voltage rectifier unit capable of generating voltages typically between 5 and 10 kV to power the oscillator. The unit needs to be rated at the correct kilovolt-ampere (kVA) to supply the necessary current to the oscillator. Early rectifier systems featured valve rectifiers such as GXU4 (high power high voltage half wave rectifier) but these were ultimately superseded by high voltage solid state rectifiers.
Self exciting class 'C' oscillator
The oscillator circuit is responsible for creating the elevated frequency electrical current, which when applied to the work coil creates the magnetic field which heats the part. The basic elements of the circuit are an inductanceInductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
(tank coil) and a capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
(tank capacitor) and an oscillator valve. Basic electrical principles dictate that if a voltage is applied to a circuit containing a capacitor and inductor the circuit will oscillate in much the same way as a swing which has been pushed. Using our swing as an analogy if we do not push again at the right time the swing will gradually stop this is the same with the oscillator. The purpose of the valve is to act as a switch which will allow energy to pass into the oscillator at the correct time to maintain the oscillations. In order to time the switching, a small amount of energy is fed back to the grid of the triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
effectively blocking or firing the device or allow it to conduct at the correct time. This so called grid bias can be derived, either capacitively, conductively or inductively depending on whether the oscillator is a Colpitts, Hartley oscillator
Hartley oscillator
The Hartley oscillator is an electronic oscillator circuit that uses an inductor and a capacitor in parallel to determine the frequency. Invented in 1915 by American engineer Ralph Hartley, the distinguishing feature of the Hartley circuit is that the feedback needed for oscillation is taken from...
, Armstrong tickler or a Meissner.
Means of power control
Power control for the system can be achieved by a variety of methods. Many latter day units feature thyristorThyristor
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
power control which works by means of a full wave AC (alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
) drive varying the primary voltage to the input transformer. More traditional methods include three phase variacs (autotransformer
Autotransformer
An autotransformer is an electrical transformer with only one winding. The auto prefix refers to the single coil acting on itself rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where...
) or motorised Brentford type voltage regulators to control the input voltage. Another very popular method was to use a two part tank coil with a primary and secondary winding separated by an air gap. Power control was affected by varying the magnetic coupling of the two coils by physically moving them relative to each other.
Solid state power supplies
In the early days of induction heating, the motor-generatorMotor-generator
A motor-generator is a device for converting electrical power to another form. Motor-generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electrical power supply line...
was used extensively for the production of MF power up to 10 kHz. While it is possible to generate multiples of the supply frequency such as 150 Hz using a standard induction motor driving an AC generator, there are limitations. This type of generator featured rotor mounted windings which limited the peripheral speed of the rotor due to the centrifugal forces on these windings. This had the effect of limiting the diameter of the machine and therefore its power and the number of poles which can be physically accommodated, which in turn limits the maximum operating frequency.
To overcome these limitations the induction heating industry turned to the inductor-generator. This type of machine features a toothed rotor constructed from a stack of punched iron laminations. The excitation
Excitation (magnetic)
An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred...
and AC windings are both mounted on the stator, the rotor is therefore a compact solid construction which can be rotated at higher peripheral speeds than the standard AC generator above thus allowing it to be greater in diameter for a given RPM. This larger diameter allows a greater number of poles to be accommodated and when combined with complex slotting arrangements such as the Lorenz gauge condition
Lorenz gauge condition
In electromagnetism, the Lorenz gauge or Lorenz gauge condition is a partial gauge fixing of the electromagnetic vector potential. The condition is that \partial_\mu A^\mu=0...
or Guy slotting which allows the generation of frequencies from 1 to 10 kHz.
As with all rotating electrical machines, high rotation speeds and small clearances are utilised to maximise flux variations. This necessitates that close attention is paid to the quality of bearings utilised and the stiffness and accuracy of rotor. Drive for the alternator is normally provided by a standard induction motor for convention and simplicity. Both vertical and horizontal configurations are utilised and in most cases the motor rotor and generator rotor are mounted on a common shaft with no coupling. The whole assembly is then mounted in a frame containing the motor stator
Stator
The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...
and generator stator. The whole construction is mounted in a cubicle which features a heat exchanger and water cooling systems as required.
The motor-generator became the mainstay of medium frequency power generation until the advent of solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
technology in the early 1970s.
In the early 1970s the advent of solid state switching technology saw a shift from the traditional methods of induction heating power generation. Initially this was limited to the use of thyristors for generating the 'MF range of frequencies using discrete electronic control systems.
State of the art units now employ SCR (silicon-controlled rectifier
Silicon-controlled rectifier
A silicon-controlled rectifier is a four-layer solid state device that controls current. The name "silicon controlled rectifier" or SCR is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W...
), IGBT or MOSFET technologies for generating the 'MF' and 'RF' current. The modern control system is typically a digital microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
based system utilising PIC, PLC (programmable logic controller
Programmable logic controller
A programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...
) technology and surface mount manufacturing techniques for production of the printed circuit boards. Solid state now dominates the market and units from 1 kW to many megawatts in frequencies from 1 kHz to 3 MHz including dual frequency units are now available.
A whole range of techniques are employed in the generation of MF and RF power using semiconductors, the actual technique employed depends often on a complex range of factors. The typical generator will employ either a current or a voltage fed topology. The actual approach employed will be a function of the required power, frequency, individual application, the initial cost and subsequent running costs. Irrespective of the approach employed however, all units tend to feature four distinct elements:
AC to DC rectifier
This takes the mains supply voltage and converts it from the supply frequency of 50 or 60 Hz and also converts it to 'DC'. This can supply a variable DC voltage, a fixed DC voltage or a variable DC current. In the case of a variable systems, they are used to provide overall power control for the system. Fixed voltage rectifiers need to be used in conjunction with an alternative means of power control. This can be done by utilising a switch mode regulator or a by using a variety of control methods within the inverter section.DC to AC inverter
The inverterInverter (electrical)
An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
converts the DC supply to a single phase AC output at the relevant frequency. This features the SCR, IGBT or MOSFETS and in most cases is configured as an H-bridge
H-bridge
An H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards...
. The H-bridge has four legs each with a switch, the output circuit is connected across the centre of the devices. When the relevant two switches are closed current flows through the load in one direction, these switches then open and the opposing two switches close allowing current to flow in the opposite direction. By precisely timing the opening and closing of the switches, it is possible to sustain oscillations in the load circuit.
Output circuit
The output circuit has the job of matching the output of the inverter to that required by the coil. This can in it simplest form be a capacitor or in some cases will feature a combination of capacitors and transformers.Control system
The control section monitors all the parameters in the load circuit, the inverter and supplies switching pulses at the appropriate time to supply energy to the output circuit. Early systems featured discrete electronics with variable potentiometers to adjust switching times, current limits, voltage limits and frequency trips. However with the advent of microcontrollerMicrocontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
technology, the majority of advanced systems now feature digital control.
The voltage-fed inverter
The voltage-fed inverter features a filter capacitorCapacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
on the input to the inverter and a series resonant output circuits. The voltage-fed system is extremely popular and can be used with either SCR's up to frequencies of 10 kHz, IGBT's to 100 kHz and MOSFETS up to 3 MHz. A voltage-fed inverter with a series connection to a parallel load is also known as a third order system. Basically this is similar to solid state, but in this system the series connected internal capacitor and inductor are connected to a parallel output tank circuit. The principal advantage of this type of system is the robustness of the inverter due to the internal circuit effectively isolating the output circuit making the switching components less susceptible to damage due to coil flashovers or mismatching.
The current-fed inverter
The current-fed inverter is different from the voltage-fed system in that it utilizes a variable DC input followed by a large inductor at the input to the inverter bridge. The power circuit features a parallel resonant circuit and can have operating frequencies typically from 1 kHz to 1 MHz. As with the voltage-fed system, SCRs are typically used up to 10 kHz with IGBTs and MOSFETs being used at the higher frequencies.Suitable materials
Suitable materials are those with high permeabilityPermeability (electromagnetism)
In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...
(100-500) which are heated below the Curie temperature
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
of that material.
See also
- Induction forgingInduction forgingInduction forging refers to the use of an induction heater to pre-heat metals prior to deformation using a press or hammer. Typically metals are heated to between and to increase their malleability and aid flow in the forging die.-Process:...
- Induction shrink fittingInduction shrink fittingInduction shrink fitting refers to the use of induction heater technology to pre-heat metal components between and thereby causing them to expand and allow for the insertion or removal of another component. Typically the lower temperature range is used on metals such as aluminium and higher...
- Induction hardeningInduction hardeningInduction hardening is a form of heat treatment in which a metal part is heated by induction heating and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the part...
- Induction heatingInduction heatingInduction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...
External links
- frequently asked questions about MF & RF induction heaters with process with examples of induction heating applications
- Sheffield University undertakes fundamental and applied research on enabling induction heater technologies - University of Sheffield
- Induction soldering using induction heater technology example from TWI