Inverter
Work coil
Workpiece
Process
Resonant tank
Operating point
Component stress
Thermal
Pick a formula
Inputs
Result
What this tool does and does not do
Results carry a confidence tag. Treat them differently.
| Tag | Meaning | How much to trust it |
|---|---|---|
| exact | A definition or an exact circuit relation (series RLC identities, Fourier fundamental of a square wave, energy balance). | Exact, given the inputs and the first-harmonic assumption. |
| std | A standard published approximation, e.g. Wheeler's solenoid inductance. | A few percent when used inside its stated validity range. |
| est | First-order physical estimate: workpiece coupling, coil AC resistance. | Within a factor of about two. Use it to choose a starting point and a safety margin, then measure. |
The coupling model
The workpiece is treated as a shorted single turn whose current flows in
a shell one reference depth thick, referred through N²k².
This is the classical transformer analogue. It ignores the Bessel-function
field solution, end effects, flux concentrators, and the temperature
dependence of ρ and μr. It is only
sane while d/δ is comfortably above 4.
Permeability is not a constant
For ferromagnetic steel μr depends on field
strength and temperature and collapses to 1 at the Curie point
(~770 °C). A single-point calculation cannot represent a heating
cycle that crosses it. Run the cold case and the hot case separately and
expect the truth to lie between them, with the absorbed power dropping
sharply once the surface goes non-magnetic.
Deliberately not modelled
- Switching loss and dead time. No turn-on/turn-off energy, no reverse-recovery, no dead-time or shoot-through analysis. Operate slightly above resonance so the tank looks inductive and the bridge turns on at zero voltage; this tool assumes you do but does not verify it.
- Off-resonance operation. Everything is computed exactly at resonance. Real drives sweep and track.
- Control loop, PLL, start-up and protection.
- Rectifier, DC link ripple and bus capacitor sizing. Only the ideal DC input current is shown.
- Thermal design of the semiconductors, busbars and cooling.
- EMC, creepage and clearance, and any safety case.
- Parallel and LCL tanks. Series only, so far.
Before you build
Verify the tank against a simulator (LTspice, PLECS or similar), then
against a low-power bench measurement of the real coil and workpiece.
An LCR measurement of the loaded coil at the working frequency will beat
any of these estimates. These are lethal voltages and currents; the
capacitor and coil see Q times the applied tank voltage.