Applications of Electromagnetic Induction
3. Induction Heating
Learning outcomes
- I can explain how induction heating produces thermal energy.
- I can describe the role of eddy currents in induction heating.
- I can explain why induction heating is efficient.
- I can identify industrial applications of induction heating.
- I can compare induction heating with conventional heating methods.
Induction Heating
Induction heating is a method of heating electrically conductive materials using electromagnetic induction. Instead of transferring thermal energy from a flame or hot heating element, an alternating current produces a changing magnetic field that induces electric currents directly inside the material being heated.
The basic process is:
alternating current → changing magnetic field → induced eddy currents → electrical resistance → thermal energy
Induction heating is widely used in cooking, manufacturing, metalworking, heat treatment, welding, melting, and other industrial processes.
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Induction heating begins with Faraday's law of electromagnetic induction.
A changing magnetic flux produces an induced EMF:
ε = −N ΔΦ/Δt
where:
- ε = induced EMF
- N = number of turns
- ΔΦ = change in magnetic flux
- Δt = time interval
In an induction heater, alternating current flows through a coil.
Because the current continually changes:
changing current → changing magnetic field
A conductive object placed within this changing magnetic field experiences changing magnetic flux.
This produces induced electrical currents inside the object.
The Induction Coil
The coil that produces the changing magnetic field is often called the induction coil or work coil.
Alternating current flows through the coil.
As the current changes direction:
- the magnetic field changes strength
- the magnetic field reverses direction
- magnetic flux through the nearby workpiece changes
The workpiece therefore experiences continuous electromagnetic induction.
Eddy Currents
The changing magnetic field induces circulating currents inside a conducting material.
These are called eddy currents.
Unlike current flowing through a wire in a simple circuit, eddy currents circulate through paths within the bulk material.
Their direction follows Lenz's law.
The induced currents produce magnetic effects that oppose the change in magnetic flux that created them.
From Eddy Currents to Thermal Energy
Real materials have electrical resistance.
When eddy currents flow through the resistance of the workpiece, electrical energy is converted into thermal energy.
The heating effect is related to:
P = I²R
where:
- P = thermal power produced
- I = current
- R = electrical resistance
In induction heating, large induced currents can therefore produce substantial heating inside the workpiece.
The Energy Conversion
The overall energy pathway can be represented as:
electrical energy
↓
alternating current in coil
↓
changing magnetic field
↓
induced eddy currents
↓
electrical resistance in workpiece
↓
thermal energy
The magnetic field transfers energy from the induction coil to the workpiece without requiring direct electrical contact between them.
Lenz's Law
Lenz's law determines the direction of the induced currents.
It states that:
the induced effect opposes the change in magnetic flux that produced it
The induced eddy currents therefore produce their own magnetic fields.
These fields interact with the applied changing magnetic field.
Energy must continually be supplied by the electrical source to maintain the changing field and induced currents.
This is consistent with conservation of energy.
Why the Metal Gets Hot but the Coil Does Not Heat It Directly
The induction coil does not need to touch the workpiece.
Instead:
- the coil produces a changing magnetic field
- the field penetrates the workpiece
- currents are induced inside the material
- electrical resistance converts electrical energy into thermal energy
The workpiece can therefore be heated directly by currents within itself.
This is one of the major differences between induction heating and many conventional heating methods.
Joule Heating
Heating caused by current flowing through electrical resistance is called Joule heating or resistive heating.
Electrical power converted to thermal energy can be expressed as:
P = I²R
It can also be written:
P = VI
or:
P = V²/R
depending on the quantities known.
For induction heating, the I²R relationship is particularly useful for understanding how induced currents produce heat.
Worked Example: Resistive Heating
Suppose an induced current of:
I = 50 A
flows through an effective resistance of:
R = 0.020 Ω
The thermal power is:
P = I²R
P = (50)²(0.020)
P = 2500 × 0.020
P = 50 W
Electrical energy is being converted into thermal energy at a rate of 50 J each second.
Energy and Heating Time
Energy transferred can be calculated using:
E = Pt
where:
- E = energy in joules
- P = power in watts
- t = time in seconds
If an induction process delivers:
P = 2000 W
for:
t = 30 s
then:
E = (2000)(30)
E = 60,000 J
or:
E = 60 kJ
In a real system, not all input electrical energy necessarily becomes useful thermal energy in the workpiece.
Why Alternating Current Is Important
A constant current would produce an approximately constant magnetic field after the initial change.
A constant magnetic field would not continuously induce eddy currents in a stationary workpiece.
Induction heating therefore uses a changing current, commonly AC at a chosen frequency.
The changing current produces continuously changing magnetic flux.
Therefore:
AC → changing B → changing Φ → induced EMF → eddy currents
Frequency
The frequency of the alternating current is an important part of induction-heating design.
Frequency affects:
- how rapidly magnetic flux changes
- distribution of induced current
- heating depth
- heating rate
- efficiency for a particular workpiece
Different applications therefore use different operating frequencies.
Skin Effect
At sufficiently high frequencies, alternating current tends to become concentrated closer to the surface of a conductor.
This is called the skin effect.
As frequency increases, induced currents can become increasingly concentrated near the surface.
Therefore:
higher frequency → generally shallower current penetration
and:
lower frequency → generally deeper penetration
This allows induction-heating systems to be designed for different heating depths.
Surface Heating
Suppose a manufacturer wants to harden the surface of a steel component while leaving its interior relatively unchanged.
A suitable induction frequency can concentrate heating near the surface.
The surface can rapidly reach the required temperature.
It can then be cooled rapidly to produce desired material properties.
This process is called induction hardening.
Deeper Heating
Some applications require heating throughout a larger volume of material.
Lower operating frequencies can allow induced currents to penetrate more deeply.
The appropriate frequency depends on factors including:
- material
- workpiece size
- geometry
- required heating depth
- desired heating rate
Induction heating is therefore highly controllable.
Magnetic Materials
Ferromagnetic materials such as many steels can experience additional heating associated with changing magnetization.
As the alternating magnetic field changes direction, magnetic domains repeatedly respond to the changing field.
This can produce hysteresis losses.
Therefore, heating in suitable ferromagnetic materials may involve both:
- eddy-current heating
- magnetic hysteresis losses
Eddy currents remain a central mechanism in induction heating.
Material Properties
Not every material responds to induction heating in the same way.
Important properties include:
- electrical conductivity
- electrical resistivity
- magnetic permeability
- geometry
- thickness
- temperature
Metals are especially suitable because they conduct electricity.
Ferromagnetic materials can couple particularly strongly to magnetic fields under suitable conditions.
Coil Geometry
The shape of the induction coil affects where the magnetic field is strongest.
Coils can therefore be designed to match particular workpieces.
Examples include:
- cylindrical coils around rods
- flat spiral coils
- coils surrounding pipes
- specially shaped coils for individual components
This allows energy to be concentrated where heating is required.
Distance from the Coil
Magnetic coupling generally becomes weaker as the workpiece moves farther from the coil.
Placing the workpiece appropriately relative to the coil can improve energy transfer.
Industrial induction systems therefore carefully control:
- coil position
- workpiece position
- air gap
- coil geometry
Good magnetic coupling improves heating performance.
Why Induction Heating Can Be Efficient
Conventional heating often requires heat to travel from an external source into the object.
For example:
flame → hot gases → surface → interior
Some energy heats:
- surrounding air
- furnace walls
- nearby equipment
Induction heating can instead generate thermal energy directly within the conductive workpiece.
This can reduce unnecessary heating of the surroundings.
Localized Heating
One major advantage is the ability to heat a selected region.
A carefully designed coil can concentrate the changing magnetic field around a particular area.
This allows manufacturers to heat:
- a gear tooth
- a bearing surface
- a pipe joint
- the end of a metal rod
- a selected section of a component
without necessarily heating the entire object to the same temperature.
Rapid Heating
Because thermal energy is generated within the workpiece, induction heating can be very rapid.
Rapid heating can provide advantages such as:
- shorter processing times
- increased production rates
- reduced heat exposure of surrounding components
- precise process control
This is especially valuable in automated manufacturing.
Precise Control
The heating process can be controlled electronically.
Engineers can adjust:
- electrical power
- frequency
- heating time
- coil position
- workpiece movement
This allows repeatable heating cycles.
In manufacturing, repeatability is important because products should receive consistent treatment.
Induction Cooktops
An induction cooktop is a familiar application.
An alternating current flows through a coil beneath the cooking surface.
The changing magnetic field interacts with suitable cookware.
Currents are induced in the cookware, producing heating.
The energy pathway is:
electrical energy → magnetic field → induced currents in cookware → thermal energy
Why Suitable Cookware Is Needed
Induction cooktops work best with cookware that couples effectively to the magnetic field.
Many suitable pans contain ferromagnetic materials such as certain steels or cast iron.
Cookware made only from some nonmagnetic materials may not work well with a typical induction cooktop unless it contains a suitable induction-compatible layer.
This is why cookware is often labelled as induction compatible.
Induction Cooking vs Conventional Electric Cooking
A conventional electric cooktop typically heats an element first.
The sequence is approximately:
electrical energy → heating element → thermal transfer → cookware
An induction cooktop instead produces much of the heating within the cookware:
electrical energy → changing magnetic field → induced currents → cookware heats
This can provide faster response and reduce unnecessary heating of the cooktop and surroundings.
Induction Cooking vs Gas Cooking
Gas cooking uses combustion.
The energy pathway is approximately:
chemical energy → flame → thermal energy → cookware
Some thermal energy escapes into the surrounding air.
Induction cooking transfers electromagnetic energy directly into suitable cookware, where it becomes thermal energy.
Potential advantages include:
- rapid heating
- precise control
- reduced waste heat around the cooking area
- no open flame at the cooking surface
Induction Melting
Industrial induction furnaces can heat metals until they melt.
Applications include melting:
- steel
- iron
- copper alloys
- aluminium alloys
- precious metals
The electromagnetic field induces large currents within the metal.
These currents produce intense thermal energy.
Advantages of Induction Melting
Induction melting can provide:
- controlled heating
- rapid temperature increase
- reduced direct contamination from fuel combustion
- precise process control
- efficient energy transfer
- compatibility with automated systems
It is widely used in foundries and metal-processing industries.
Induction Hardening
Induction hardening is used to change the properties of the surface of certain metal components.
The process generally involves:
- rapidly heating the selected surface
- reaching a suitable temperature
- rapidly cooling the material
Applications can include:
- gears
- shafts
- bearings
- machine components
The goal is often to create a hard, wear-resistant surface while maintaining useful properties in the interior.
Induction Brazing
Brazing joins materials using a filler metal that melts at a lower temperature than the main components.
Induction heating can heat the joint region precisely.
Advantages can include:
- localized heating
- rapid processing
- repeatability
- reduced heating of surrounding material
- easy automation
Induction Soldering
The same general principle can be used for some soldering processes.
The joint area is heated electromagnetically.
This can be useful when:
- controlled heating is important
- direct flame is undesirable
- repeatable production is required
Induction Welding
Induction heating can contribute to certain welding processes.
For example, rapidly induced currents can heat the edges of metal components before they are pressed together.
This is used in some industrial production processes, including manufacturing certain pipes and tubes.
Shrink Fitting
Induction heating can also assist in shrink fitting.
Suppose a metal ring must fit tightly around another component.
The ring is heated.
Thermal expansion increases its size slightly.
It is placed into position and allowed to cool.
As it contracts, it forms a tight fit.
Induction heating allows the selected component to be heated quickly and accurately.
Industrial Heat Treatment
Heat treatment changes the properties of materials by carefully controlling temperature and cooling.
Induction heating can be used in processes requiring:
- surface hardening
- annealing
- tempering
- stress relieving
The exact process depends on the material and desired properties.
Advantages in Manufacturing
Induction heating can offer several advantages:
- rapid heating
- localized heating
- precise control
- repeatability
- easy automation
- reduced direct contact
- clean working environment
- reduced unnecessary heating
- high production rates
These features make it useful in modern manufacturing.
Induction Heating and Conventional Furnaces
A conventional furnace heats the environment around the workpiece.
Thermal energy then moves into the workpiece by:
- conduction
- convection
- radiation
Induction heating generates thermal energy directly within the electrically conductive workpiece.
This fundamental difference can make induction heating much faster for suitable applications.
Comparison of Heating Methods
| Induction Heating | Conventional Heating |
|---|---|
| Heating generated within conductive material | Heat usually supplied from an external source |
| Uses electromagnetic induction | May use combustion or resistive elements |
| Can be highly localized | Often heats a larger surrounding region |
| Rapid response | May require longer warm-up |
| Easily electronically controlled | Control depends on heating system |
| No direct electrical contact with workpiece required | Heat source may be in direct or indirect thermal contact |
| Requires suitable conductive material | Can heat a wider variety of materials directly |
Neither method is automatically best for every situation.
Limitations of Induction Heating
Induction heating also has limitations.
These can include:
- specialized electrical equipment
- carefully designed coils
- high initial equipment cost
- dependence on material properties
- difficulty directly heating nonconductive materials
- cooling requirements for high-power equipment
- electromagnetic shielding requirements in some installations
The best heating method depends on the application.
Heating Nonconductive Materials
Materials such as many:
- plastics
- ceramics
- glasses
do not readily support the eddy currents required for direct induction heating.
However, they may sometimes be heated indirectly.
For example, induction can heat a conductive container or susceptor, which then transfers thermal energy to the nonconductive material.
Efficiency
Heating efficiency can be expressed as:
efficiency = useful thermal energy / electrical energy input × 100%
or using power:
efficiency = useful heating power / electrical input power × 100%
Efficiency depends on the complete system.
It can be affected by:
- coil design
- material properties
- frequency
- distance between coil and workpiece
- power electronics
- heating time
- unwanted electrical and thermal losses
Worked Example: Heating Efficiency
An induction heater receives:
E_input = 500 kJ
The workpiece gains:
E_useful = 425 kJ
Efficiency:
efficiency = 425/500 × 100%
efficiency = 85%
Energy not transferred usefully:
500 − 425
E_loss = 75 kJ
Worked Example: Useful Heating Power
An induction system receives:
P_input = 20 kW
and operates at:
90% efficiency.
Useful heating power:
P_useful = 0.90(20 kW)
P_useful = 18 kW
Therefore:
2 kW
is being transferred into other forms or locations.
Heating a Metal Object
The thermal energy required to change an object's temperature can be estimated using:
Q = mcΔT
where:
- Q = thermal energy
- m = mass
- c = specific heat capacity
- ΔT = temperature change
This equation can be combined with induction-heating power calculations.
Worked Example: Heating Time
A 2.0 kg metal component has:
c = 500 J/(kg·°C)
It must be heated from:
20°C to 220°C
Temperature change:
ΔT = 220 − 20
ΔT = 200°C
Required thermal energy:
Q = mcΔT
Q = (2.0)(500)(200)
Q = 200,000 J
Suppose useful induction heating power is:
P = 5000 W
Using:
t = Q/P
t = 200,000/5000
t = 40 s
In this simplified calculation, the component requires approximately 40 seconds to reach the target temperature.
Including Efficiency
Suppose the heater in the previous example is:
80% efficient.
The workpiece requires:
Q = 200,000 J
Therefore, required electrical input energy is:
E_input = Q/0.80
E_input = 250,000 J
At an electrical input power of:
5000 W
time is:
t = 250,000/5000
t = 50 s
Accounting for efficiency increases the predicted heating time.
Induction Heating vs Eddy-Current Braking
Both technologies use eddy currents.
In induction heating, the thermal energy produced by eddy currents is useful.
In eddy-current braking, the magnetic force opposing motion is useful, while the resulting thermal energy is usually a consequence of braking.
The same electromagnetic phenomenon can therefore be used for different purposes.
Induction Heating vs Transformers
Induction heating also has similarities to transformer operation.
In a transformer:
changing primary current → changing magnetic flux → induced secondary voltage/current
In induction heating:
changing coil current → changing magnetic flux → induced currents in workpiece
The conductive workpiece can be thought of, in a simplified sense, as behaving somewhat like a short-circuited secondary in which large currents are induced.
Those currents produce useful heating.
Resonance in Induction Heating Systems
Some induction-heating systems use capacitors and inductors in resonant circuits.
Resonance can help power electronics efficiently produce the large alternating currents required in the induction coil.
The resonant frequency depends on:
f₀ = 1/(2π√(LC))
This connects induction heating with the principles of LC circuits and resonance.
Energy Conservation
Induction heating does not create thermal energy from nothing.
The electrical power supply provides energy.
That energy passes through several stages:
electrical input
↓
electromagnetic field
↓
induced electrical currents
↓
thermal energy
Some energy may also become:
- thermal energy in the coil
- thermal energy in electronic components
- electromagnetic losses
- cooling-system losses
The total energy remains conserved.
A Complete Heating Problem
An industrial induction heater has an electrical input power of:
P_input = 30 kW
Its overall efficiency in heating the workpiece is:
85%
Useful heating power:
P_useful = 0.85(30)
P_useful = 25.5 kW
Suppose the process operates for:
40 s
Useful energy transferred:
E = Pt
E = (25,500)(40)
E = 1,020,000 J
or:
E = 1.02 MJ
The energy not usefully transferred to the workpiece is:
30,000 × 40 − 1,020,000
= 1,200,000 − 1,020,000
E_loss = 180,000 J
or:
180 kJ
Choosing Between Heating Methods
Engineers consider several factors when selecting a heating method:
- material being heated
- required temperature
- required heating depth
- heating speed
- energy efficiency
- equipment cost
- production rate
- precision
- safety
- environmental conditions
- required level of automation
Induction heating is especially attractive when rapid, precise, and localized heating of conductive materials is required.
Common Mistakes
Thinking the induction coil must touch the workpiece
Energy is transferred through the changing electromagnetic field. Direct electrical contact is not required.
Thinking the magnetic field itself is simply "hot"
The changing magnetic field induces currents. Resistive and, in some materials, magnetic losses then produce thermal energy.
Thinking eddy currents are always undesirable
Eddy currents cause unwanted losses in transformers but are deliberately produced in induction heating.
Thinking a constant magnetic field continuously produces induction
Continuous induction requires changing magnetic flux.
Thinking all materials heat equally well
Electrical conductivity, magnetic properties, geometry, frequency, and other factors affect heating.
Thinking higher frequency always means better heating
Different frequencies are appropriate for different materials, dimensions, and required heating depths.
Thinking induction heating has no energy losses
Real induction systems are efficient in many applications but are never perfectly efficient.
Confusing temperature with thermal energy
Temperature describes the thermal state of a material. Thermal energy transferred depends on factors such as mass, specific heat capacity, and temperature change.
Key Terms
Induction heating — heating of electrically conductive material using currents produced by electromagnetic induction.
Electromagnetic induction — production of an EMF due to changing magnetic flux.
Induction coil — coil carrying alternating current that produces the changing magnetic field used for induction heating.
Workpiece — material or component being heated.
Eddy currents — circulating currents induced within a bulk conductor by changing magnetic flux.
Joule heating — conversion of electrical energy into thermal energy when current flows through resistance.
Lenz's law — principle stating that an induced effect opposes the change in magnetic flux that produced it.
Skin effect — tendency of high-frequency alternating current to become concentrated nearer the surface of a conductor.
Penetration depth — characteristic depth over which significant induced current and heating occur.
Induction hardening — process using induction heating and controlled cooling to harden selected regions of a component.
Induction furnace — furnace that uses electromagnetic induction to heat or melt conductive materials.
Induction brazing — use of induction heating to heat a joint so that brazing filler material can join components.
Hysteresis loss — energy transferred into thermal energy as magnetic materials undergo repeated magnetization changes.
Coupling — effectiveness with which the induction coil's magnetic field interacts with the workpiece.
Heating efficiency — proportion of input energy transferred usefully into heating the desired material.
Key Takeaways
- Induction heating uses electromagnetic induction to produce thermal energy in conductive materials.
- Alternating current in an induction coil produces a changing magnetic field.
- The changing magnetic field produces changing magnetic flux in a nearby conductive workpiece.
- Changing magnetic flux induces EMF and eddy currents.
- Eddy currents encounter electrical resistance and produce Joule heating.
- The basic energy pathway is electrical energy → magnetic field → induced currents → thermal energy.
- Lenz's law determines the direction of induced currents.
- Induction heating does not require direct electrical contact between the coil and workpiece.
- Frequency influences the distribution and penetration depth of induced currents.
- Higher frequencies generally concentrate heating closer to the surface.
- Induction heating can provide rapid, localized, and precisely controlled heating.
- Suitable coil geometry can concentrate heating in selected regions.
- Induction heating is used for cooking, melting, hardening, brazing, soldering, welding, and other industrial processes.
- Eddy currents that are undesirable in transformers can be deliberately useful in induction heating.
- Ferromagnetic materials may also experience heating associated with hysteresis.
- Induction heating can reduce unnecessary heating of surrounding equipment compared with some conventional methods.
- Conventional furnaces generally transfer thermal energy from an external heat source into the workpiece.
- Induction heating generates much of the useful thermal energy directly within the conductive workpiece.
- Real induction systems have energy losses and efficiencies below 100%.
- Induction heating demonstrates a practical conversion of electrical energy into electromagnetic energy and ultimately thermal energy.
Check Your Understanding
- Explain how alternating current in an induction coil can cause a nearby metal object to become hot.
- What are eddy currents, and why are they important in induction heating?
- Explain how Lenz's law applies to the currents produced inside an induction-heated workpiece.
- Why is alternating current normally required for continuous induction heating?
- An induced current of 80 A flows through an effective resistance of 0.025 Ω. Calculate the rate at which thermal energy is produced using P = I²R.
- A heater transfers useful thermal power of 4.0 kW to a workpiece for 45 s. Calculate the useful energy transferred.
- Explain how operating frequency affects the depth at which induction heating occurs.
- Why might high-frequency induction heating be useful for surface hardening?
- Compare the energy-transfer process in induction heating with the energy-transfer process in a conventional furnace.
- An induction heater receives 12 kW of electrical power and transfers 10.2 kW to the workpiece. Calculate its efficiency.
- A 3.0 kg metal component with a specific heat capacity of 450 J/(kg·°C) is heated through 200°C. Calculate the thermal energy gained by the component.
- Describe three industrial applications of induction heating and explain why localized or rapid heating is useful in each.
- Explain why eddy currents are deliberately encouraged in induction heating but reduced in transformer cores.
- Compare induction heating with conventional heating in terms of speed, localization, control, materials, and energy efficiency.
- An engineer must heat only the outer surface of a steel shaft without excessively heating its interior. Explain how the engineer could use coil design, electromagnetic induction, frequency, eddy currents, and heating time to achieve this goal.