What Is Inductive Power Transmission?

Inductive power transmission is a method of transferring electrical energy between two circuits through a shared magnetic field, without a direct metallic connection. One coil, connected to an AC power source, produces a time-varying magnetic flux that links to a second coil, where it induces a voltage. The arrangement is functionally equivalent to a transformer in which the primary and secondary are physically separated by an air gap rather than a shared iron core, allowing power to flow across a spatial gap, through a barrier, or to a moving load. The field draws from transformer design theory, power electronics, and magnetics, and it finds application wherever conductive contacts are impractical or unreliable.

The technique is sometimes described by the related term inductive power transfer, or IPT. It encompasses a broad range of power levels and coupling distances, from milliwatt medical implant recharging through kilowatt electric vehicle charging pads. The underlying physics are common to all scales, though the engineering tradeoffs, particularly between operating frequency, coil geometry, and compensation topology, shift considerably with power level.

Inductors, Coils, and the Magnetic Coupling Model

The transmitting and receiving coils function as the primary and secondary windings of a loosely coupled transformer. The degree of coupling is expressed by the coupling coefficient, a dimensionless number between zero and one that captures how much of the flux produced by the transmitter links the receiver. In contactless power systems, coupling coefficients range from roughly 0.1 to 0.9, far lower than the near-unity values seen in wound transformers with shared ferrite cores. Ferrite magnetic cores or shields are often incorporated in the coil assemblies to direct flux and reduce stray fields, but the coils themselves must accommodate the gap. Research on inductively coupled wireless power transfer systems from the University of Michigan describes the coil geometry, coupling, and efficiency relationships that govern practical designs.

Resonant Compensation and Efficiency

Because loosely coupled coils present significant leakage inductance, raw inductive power transfer is inefficient without compensation. Adding capacitors to form series or parallel resonant circuits on the transmitter, the receiver, or both cancels the reactive power associated with the leakage inductance and allows the link to appear as a purely resistive load to the power electronics. Series-series compensation is one of the most widely analyzed topologies; IEEE Xplore papers on resonant inductive wireless power transfer document designs achieving efficiencies above 90% at power levels of several kilowatts and operating frequencies around 85 kHz, which is the frequency specified in the SAE J2954 standard for wireless EV charging. The choice of compensation network also affects how the output voltage and current respond to changes in coupling and load, which is important for systems where alignment varies during operation.

Control and the Transmitter-Receiver Interface

Coordinating power delivery between a stationary transmitter and a potentially moving or variably coupled receiver requires a communication channel. The fundamentals of inductively coupled systems, including compensation network selection and coil design methodology, are surveyed in the IntechOpen chapter on inductively coupled wireless power transfer. Back-channel communication, using load modulation or a separate wireless link, allows the receiver to report battery state, request power changes, or signal faults to the transmitter. Sensorless methods that infer receiver state from changes in transmitter current and voltage are used in simpler systems, paralleling the observer-based estimation approaches seen in sensorless motor drives. The transmitter-side inverter, typically a half-bridge or full-bridge topology switching at the resonant frequency, must regulate both the output power and the switching frequency to maintain resonance as component values shift with temperature.

Applications

Inductive power transmission has applications in a wide range of fields, including:

• Wireless charging pads for electric and plug-in hybrid vehicles
• Power delivery to implantable medical devices, including cochlear implants and neural stimulators
• Contactless power for rotating machine parts and slip-ring replacements
• Automated manufacturing lines where connectors introduce wear or contamination
• Underwater and sealed-enclosure systems where electrical penetrations are hazardous