Induction Heating
Induction Heating: Theoretical Foundations
Principles of Induction Heating
Professor, what's the difference between induction heating and high-frequency hardening?
The principle is the same (Joule heating by eddy currents), but the applications differ. Induction heating is used for a wide range of purposes such as melting, forging heating, brazing, and cooking (IH cooking heaters). Heat generation:
IH cooking heaters use the same principle, right?
Yes. They induce eddy currents in the pot bottom at 20–100 kHz. Aluminum pots have high conductivity but $\mu_r \approx 1$, making them difficult to heat. Iron pots have a large $\mu_r$ and can be heated efficiently. All-metal compatible IH heaters use higher frequencies to work with aluminum as well.
Summary
- Joule heating by eddy currents — $Q = J^2/\sigma$
- More efficient for magnetic materials — Larger $\mu_r$ leads to smaller $\delta$ and current concentration
- Non-contact, rapid heating — Energy efficiency 80–90%
Solving the Mystery of IH Stoves: "Why is the pot bottom hot but the unit itself isn't?"
If you've ever used an IH (induction heating) cooking heater, you might have wondered, "Why does the pot bottom get so hot, yet the appliance body remains cool enough to touch with bare hands?" The answer is because eddy currents are generated "only in the pot bottom." The alternating magnetic flux created by the IH's heating coil passes through the electrically conductive pot bottom (iron or stainless steel), causing eddy currents to flow within the pot bottom and generate Joule heat. Glass or wood do not conduct electricity, so no eddy currents flow and they are not heated. This selective heating is the essence of IH, achieving thermal efficiency over 90%. CAE induction heating analysis is precisely the technology that predicts "which material heats up and by how much."
Computational Methods for Induction Heating
Electromagnetic-Thermal Coupled Analysis
How do you set up a simulation for induction heating?
It's a coupling of electromagnetic fields and heat conduction. The heat generation distribution is obtained from frequency-domain eddy current analysis and passed to thermal analysis.
Since material properties ($\mu$, $\sigma$, $k$, $c_p$) are all temperature-dependent, the weak coupling method, which alternately calculates electromagnetic fields and heat, is standard.
Do you consider convection in melting simulations?
To handle stirring by Lorentz force in molten metal (electromagnetic stirring), three-way coupling of electromagnetic-thermal-fluid is required. This is where COMSOL Multiphysics excels.
Summary
- Electromagnetic-Thermal Coupling — Using $Q_{eddy}$ as a heat source
- Temperature dependence of materials — $\mu(T)$, $\sigma(T)$ are particularly important
- Electromagnetic-Thermal-Fluid — Necessary for melting simulation
The "Nonlinear Loop" of Induction Heating Analysis—Magnetization and Temperature Interfere with Each Other
Induction heating numerical analysis is difficult because electromagnetic fields, heat, and material properties interfere with each other in a three-way struggle. As temperature rises, electrical resistivity increases, changing the eddy current distribution. When iron exceeds the Curie temperature (770°C for pure iron) where it transitions from ferromagnetic to paramagnetic, permeability changes drastically, completely altering the magnetic flux distribution. On the other hand, if the eddy current distribution changes, the heat generation pattern also changes, which in turn changes the temperature distribution. To properly solve this three-way coupling of "electromagnetic ⇔ thermal ⇔ nonlinear material," iterative convergence calculation is essential. Many troubles like "analysis not converging" or "temperature diverging" stem from how these nonlinear material properties are handled.