The basic principle of inductive coupling or inductive power transfer (IPT) is the wireless transfer of power through a medium using magnetic fields generated by inductive coils. It is based on the principle of Ampère’s circuital law, which describes the creation of a magnetic field in a wire where current is flowing and Faraday’s law of induction, which describes the induced current in a wire due to a magnetic field.

There are two main types of IPT systems. One without resonance and one with. Above figure shows the basic schematic of a resonant IPT system. At the primary circuit, the sinusoidal AC current from the grid is first rectified into a stable DC current and then converted back to AC but with a higher frequency thanks to a high-frequency inverter. The high-frequency AC current then enters a primary resonant circuit consisting of a compensation network and a primary coil. This coil generates an alternating magnetic field, which is received by the coil of the secondary circuit. This induces an alternating current in the secondary circuit, which can then go through different filters or rectifiers depending on the load.

The “connection quality” between the coils is called the coupling coefficient k and ranges between 0 and 1. Where 0 means no coupling and 1 perfect coupling. It characterizes the ratio of the flux linking the coils of a pad to the total flux generated by the opposite pad.

$$ k = \frac{M}{\sqrt{L_1 L_2}} $$