Over the last few years Wireless Power has made rapid advances towards becoming a mainstream technology and is often the case, marketing departments become the source of many new inventions. Perhaps the biggest marketing “invention” to date is something called “Magnetic Resonance” (related to Resonant Inductive Coupling) when everyone else is just doing stone age “Inductive Power.”
It’s a term many have now adopted, as if it was some space-age technological leap from “Inductive Power.” When in fact Magnetic Resonance and Inductive Power are EXACTLY the same thing.
Lets dig into this a little more …
I’m sure all EEE majors will remember that Inductive Power uses Magnetic Resonance.
Any Inductive Power system has to have resonance, even the WPC which requires complete alignment between the transmitter and receiver coils uses resonance. Yes, tightly coupled systems, like loosely coupled systems, do use resonance! This is accepted science since The University of Auckland started researching modern day wireless power 20 years ago.
Originally, before induction, wireless power could be achieved by effectively taking a transformer and separating the primary and secondary coils (i.e. a split transformer).
To increase the power efficiency it was worked out a long time ago that we need to use resonant coupling. This is just a fancy way of saying that by adding capacitors on both coils, a resonant circuit is created between the inductance of coil and capacitor. At the resonance frequency, the reactance cancels out and you are left with only the parasitic effects of finite winding resistance, AC resistance (proximity effect) and dielectric losses.
If you had a perfect AC source and drive the resonant circuit you have no losses. Losses are solely limited by your parasitics.
“Magnetic Resonance” is just sticking some capacitors in place. It was great to see Marin Soljacic, the inventor of WiTricity confirm this in the IEEE publication, A Critical Look at Wireless Power. “Resonance enables efficient energy transfer…. …. it’s not a new idea: Tesla’s eponymous coils use that very same principle.”
To summarize in non-technical speak, all Inductive Power systems use magnetic resonance and its certainly not the difference between tightly coupled and loosely coupled systems.
Like most new technologies you need to get underneath the marketing spin to understand the features and benefits that each vendor can deliver to those who matter most – our customers!
When we talk about wireless power, one of the first design questions we consider is… “at what resonant frequency should the system transfer its power?”. Selecting the system’s resonant frequency helps increase the distance for power transfer and improves the efficiency of the wireless power solution. But what are the health implications? and are wireless charging products being accurately measured?
Resonant frequency refers to the frequency at which an object naturally vibrates or oscillates. It is at this frequency that objects are able to achieve their maximum amplitude – whereas it is difficult to get them to vibrate at other frequencies. In the case of inductive power transfer (IPT), the principle of resonance is applied to induce an electrical current between coils of the same frequency over greater distances.
Naturally the resonant frequency will depend on the problem or application, however there are other factors which influence this decision. As with all electronic products, the foremost issue is safety.
Usually safety is determined by how much radio frequency (RF) exposure can be applied without being harmful to human health. Guidleines do exist which provide reference to what are acceptable and safe limits of exposure…or so we think.
Most would consider the 1998 and the 2009 ICNIRP (International Council on Non-Ionizing Radiation Protection) guidelines as a reference to those limits. Yet the guidelines lack any specific mention or reference to the behaviour of wireless power systems. What this means is that the “limits” identfied may not accurately apply to RF radiation created by wireless charging.
That is not to say that wirelss power systems operating under these guidleines are unsafe – electromagnetic fields created by this technology are considered to be at the lower end of the exposure spectrum. But surely more specific and conclusive measurements of IPT applications are required to understand the appropriate exposure limits?
Otherwise how do we know if limits are too harsh? (impacting the quality of solutions), or conversely, not harsh enough?