Wireless power transfer delivers energy across a gap through electromagnetic fields. A familiar charging pad uses magnetic coupling, while other approaches use electric-field coupling or radiated energy. The method, separation and receiver determine how the system behaves.
The gold standard follows power from the stated input to useful output and explains the physical link between them. This guide concentrates on magnetic coupling and the interpretation of performance figures, using supplied examples to build the reasoning.
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Follow the field interaction, calculate a simple induced signal and then account for the complete energy pathway.
Full chapter index · Practice and transfer · Education root · How X Works
Full chapter index
Understand the transfer · Chapters 1–2
Calculate the interaction · Chapters 3–4
Read performance · Chapters 5–7
Compare and practise · Chapters 8–10
Connect and investigate · Chapters 11–12
Magnetically coupled systems use a changing magnetic field linking transmitter and receiver. Capacitive approaches use an electric-field coupling structure. Radiative approaches deliver energy using propagating electromagnetic radiation and suitable receivers.
These categories answer different design questions. A statement about one arrangement’s separation or performance should not be copied to every wireless system. Begin with the actual method and operating conditions.
The source still supplies the energy. Removing a direct electrical contact across the gap changes the transfer arrangement; it does not remove the need to account for input power, losses and output.
A practical system can include source conversion, the coupling link, receiver conversion, control and a load. Some reported efficiencies cover only the link; others include more of this pathway.
Write the reference points on the diagram before using a percentage. ‘Input’ could mean a supply’s electrical output or an earlier mains measurement, and ‘output’ could mean receiver power or stored battery energy.
A complete comparison also states the operating load and whether auxiliary power is included. Two correct efficiency measurements can differ because their boundaries differ, even when neither instrument is wrong.
CHAPTER 3 OF 12 · Calculate the interaction
3. Changing magnetic flux induces a voltage
Mutual inductance describes how one circuit’s current links magnetic flux with another. For a fixed geometry and a linear model with constant mutual inductance M, a contribution to the receiver’s induced emf is ε₂ = −M dI₁/dt.
OpenStax derives this relationship from Faraday’s law. The minus sign expresses the opposition associated with induction; the equation connects a changing current with an induced voltage.
A voltage estimate alone does not establish delivered load power. The receiver circuit, its load and the interaction with the transmitter matter. An open-circuit reading and a useful loaded output are different measurements.
CHAPTER 4 OF 12 · Calculate the interaction
4. Worked example: an induced-voltage contribution
Use an illustrative M = 10 microhenries and a current-change rate dI₁/dt = 20,000 A/s. The magnitude of this mutual-induction contribution is (10 × 10⁻⁶) × 20,000 = 0.20 V.
Doubling the change rate doubles this modelled contribution to 0.40 V if M stays fixed. That conditional result does not supply an efficiency or a power value.
The calculation assumes fixed geometry and the stated constant-M model. It is a paper example, not a specification for a charging device. Explaining the assumptions is part of the solution.
CHAPTER 5 OF 12 · Read performance
5. Coupling and resonance describe different features
Coupling concerns the electromagnetic interaction between the structures. Resonance concerns an oscillating system’s response near its characteristic conditions. Resonant arrangements can support useful energy exchange, but losses, loading and control still determine performance.
The 2007 primary research on strongly coupled magnetic resonances is one demonstration of a particular arrangement. ORNL’s vehicle-charging research provides other examples with different hardware and operating conditions.
Do not interpret resonance as an energy source or a promise of perfect transfer. It changes a system’s response; the power balance must still close.
Suppose a fictional system has three successive stage efficiencies: 90%, 80% and 95%. If their boundaries join consistently and each loss is counted once, total efficiency is 0.90 × 0.80 × 0.95 = 0.684, or 68.4%.
At 100 W input, the useful output is 68.4 W under these steady conditions. Adding the three percentages or averaging them would not describe the cascade.
This example also shows why a high link efficiency may coexist with a lower complete-system efficiency. The link is only one stage.
| Reported quantity | Needed context | What it does not establish alone |
|---|---|---|
| Link efficiency | Power measured on each side of the defined link | Whole-system energy efficiency |
| Output power | Measurement point and operating load | Energy delivered over an unspecified period |
| Energy efficiency | Matched input and output intervals | Performance at every alignment or load |
CHAPTER 7 OF 12 · Read performance
7. Power and delivered energy use different denominators
If the example’s 68.4 W output remains constant for two hours, delivered energy is 136.8 Wh. Input energy is 200 Wh, so energy efficiency is again 68.4% for this deliberately steady case.
If output varies, add or integrate the power over time. A device that briefly reaches a high power may deliver less energy than one that runs steadily at a lower power.
State whether the useful output is electrical energy at a receiver, energy supplied to a battery-management system or a measured change in stored energy. Those boundaries can include different losses and behaviours.
CHAPTER 8 OF 12 · Compare and practise
8. Alignment and operating conditions must travel with the result
Relative position and orientation affect magnetic coupling. Load, temperature, material surroundings and control choices can also change performance. A single best-position reading cannot establish performance over a whole operating envelope.
ORNL’s research includes work on couplers and misalignment. The useful lesson is methodological: evaluate the condition relevant to the claim and report how the result changes away from it.
A fair comparison records the test arrangement and a suitable range of conditions. Otherwise, an apparent improvement may come from changing the setup rather than changing the technology.
CHAPTER 9 OF 12 · Compare and practise
9. Separate a transfer claim from a complete service claim
A successful laboratory demonstration establishes what was demonstrated under its conditions. Product compatibility, reliable operation and deployment require additional evidence specific to the intended system.
Likewise, wireless communication and wireless power transfer have different primary outputs. A communication signal carries information; a power-transfer claim must quantify useful received energy. Some systems do both, but one measurement does not automatically validate the other.
Ask whether a proposed advantage concerns convenience, power, efficiency, alignment tolerance or another outcome. A single percentage cannot rank every aspect of a system.
A receiver delivers 60 W from a defined 75 W link input. Link efficiency is 60/75 = 80%. If the upstream supply takes 90 W at a wider boundary, the efficiency to that same receiver output is 60/90 ≈ 66.7%. Explain why the figures are compatible.
For one hour the useful output is 40 W, followed by one hour at 60 W. Delivered energy is 100 Wh. If the measured input energy over those two hours is 150 Wh, energy efficiency is about 66.7%.
For transfer, compare a phone pad with a vehicle-charging demonstration. Identify the method, power boundary, separation, alignment and load conditions. Keep the comparison limited to evidence that is actually supplied.
Physical and electronic foundations
- Electromagnetism — connect fields and induction
- Energy — follow rates, transfers and losses
- Electronics — understand conversion, control and loads
- Physics — build model and measurement reasoning
- Measurement — define reference points and uncertainty
Systems and focused applications
- Electromagnetic Compatibility — consider interactions within an operating system
- Instrumentation Engineering — connect performance claims with measurements
- Wireless Communication — compare information and energy transfer
- Wireless charging and alignment — extend the coupling question
- Induction and heating control — compare a related use of changing magnetic fields
CHAPTER 12 OF 12 · Connect and investigate
12. Questions, sources and the next comparison
Frequently asked questions
Does wireless transfer eliminate losses? No; account for the full pathway. Does an induced voltage prove useful power delivery? The loaded circuit must be considered. Can a link-efficiency figure be compared directly with a whole-system figure? Their boundaries must first be aligned.
Choose two documented performance reports and rewrite their claims using the same measurement vocabulary. List any missing condition that prevents a fair numerical comparison. Use the paper calculations here to check units and denominators.
Primary learning and research sources
- OpenStax: mutual inductance — the induction relationship and its assumptions
- Wireless power transfer via strongly coupled magnetic resonances — the original 2007 research record
- ORNL: charging research — a specific vehicle-transfer demonstration