Wireless Power Transfer.

[faisal78]

Hello
I am reading on Wireless Power Transfers (WPT), basically from WPC Qi Standard and WiTricity.
There are subtle difference between the two, Qi being based off magnetic inductive coupling (i.e. transformer), and WiTricity based on inductive resonant circuity.
Reading in details, to make a WPT system efficient, it needs to somewhat be "resonant", and it does so by tuning the Tx Coil inductance using a capacitor.
In the end it seems, both are based on inductive resonant.

Can somebody help shed some light on the difference between these two.?

 

[tpetar]

Hi Faisal78,

This modern todays inductive charging I will call snake oil of mobile device manufacturers or very bad transformer/inductive design. These designs have very high losses, and there is nothing new in this, only new termins and new smaller designs of coils, all others can be called "bad transformer". :smile:

Only one good thing in this is to eliminate usage of cables/connectors, and give possibility to have universal charger for many different devices.

I see some very interesting projects about this, but for me its far from some useful range.


For now only Nikola Tesla successfully used true wireless power tansfer even over several km of distance. He transfer higher AC power on long distance to power some devices such as Teslas light bulbs.



WiTricity Technology: The Basics
**broken link removed**
http://en.wikipedia.org/wiki/WiTricity




What healthy place for everyday life:
**broken link removed**



Qi (inductive power standard)
http://en.wikipedia.org/wiki/Qi_(inductive_power_standard)




Best regards,
Peter

 

[mtwieg]

Practically all wireless power systems will make use of resonant coils, unless it's for extremely short distances (like a few cm). Even if the articles don't mention resonance, it's pretty much always beneficial to do some impedance transformation to get more power through.

Also keep in mind that witricity and Qi shouldn't be taken as hard guidelines for wireless power in general (unless you're working with small consumer electronics, which Qi does deal with).

 

[faisal78]

Hi mtwieg,
Thats where I had got it confused. It seems to me, after a bit of research, all forms of wireless power transfer are based off resonant coils.
If so, what is the biggest difference between the wireless charging pad, usually done by Qi standard, vs Witricity?
I know Qi is only for very short distances, within 1-20mm maybe. Whilst Witriciy operate to the range of 1m-10m.
Same concept, just different range. However, the input power requirements seems similar.

 

[mtwieg]

Hi mtwieg,
Thats where I had got it confused. It seems to me, after a bit of research, all forms of wireless power transfer are based off resonant coils.
If so, what is the biggest difference between the wireless charging pad, usually done by Qi standard, vs Witricity?
I know Qi is only for very short distances, within 1-20mm maybe. Whilst Witriciy operate to the range of 1m-10m.
Same concept, just different range. However, the input power requirements seems similar.

There's no coherent difference between the two. IMO they should be seen as competing marketing brands, not as different technologies. I would also advise you always investigate wireless power with a skeptical attitude, because a great deal of the hype is pure fabrication. For example, Witricity originally started as an impressive demo transmitting power over a distance much larger than the coupled elements, but outside of the lab the only products they've apparently made are basically large charging pads, which are only effective over short distances. And for the foreseeable future, that's going to be the limit of wireless power, outside of a few extreme niche applications.

 

[Myrtonos]

Here's a question, the image shown shows single phase power transfer, is polyphase resonant coupling possible?

 

[chuckey]

No.
Frank

 

[mtwieg]

I don't see how the polyphase concept could apply to wireless power transfer, since in the air all your phases would combine to make one overall "phase." Perhaps one could make a system using mutually decoupled coil pairs, but I don't see the advantage of doing so.

 

[Ice-Tea]

To me it seems the major difference is that one is on the market and widely implemented (kinda) and the other seems parts snake oil and parts wishfull thinking.

But maybe that's just me

 

[tcmtech]

Here's a question, the image shown shows single phase power transfer, is polyphase resonant coupling possible?

Sort of no but if multiple frequencies and related receiving circuits are used then kind of yes.

I have played around with wireless energy transmission for a number of years and it does work and can be made functional at reasonably small scale power levels but unfortunately it is not something I would call easy to do or replicate.

Infact I would say its about 1/3 science 1/3 voodoo and 1/3 pure luck when a design or circuit configuration does work reasonably well even if its just in a laboratory test bench condition. :razz:

 

[Myrtonos]

I don't see how the polyphase concept could apply to wireless power transfer, since in the air all your phases would combine to make one overall "phase." Perhaps one could make a system using mutually decoupled coil pairs, but I don't see the advantage of doing so.

If each transmitter coil is exactly opposite each reciever coil, or at least considerably closest to one of the receiver coils, then I'm sure it would work. If possible a polyphase inductive power transfer would seem more promising because the inductive power transfer can be constant, rather than rising and falling with each half cycle.

 

[mtwieg]

If each transmitter coil is exactly opposite each reciever coil, or at least considerably closest to one of the receiver coils, then I'm sure it would work. If possible a polyphase inductive power transfer would seem more promising because the inductive power transfer can be constant, rather than rising and falling with each half cycle.

Mutually decoupling the the tx/rx pairs would be pretty much impossible, and a waste of space.

A more reasonable idea would be to have multiple receiver coils coupled to a single transmit field, and just put phase shifters on the rx coil outputs. Still not really seeing the advantage though, since usually wireless power is done at very high frequencies where a couple hundred nanoseconds between cycles isn't a problem at all...

 

[Myrtonos]

But I do suppose that a three-phase system could allow a lower frequency to be used, which means that the coupling could occur over greater distances.

 

[tcmtech]

Going from a 1 phase to a 3 phase concept won't really do much for frequency or coupling distance.

The common misconception I see with wireless energy transfer is that although Tesla did do it and it worked the #1 aspect of how is almost always overlooked. Raw sheer power. He didn't do much of the work with small multi watt systems.

Rather, if I am remembering the numbers right, he liked to play in the 10's of Kw or multi megawatt power levels taking advantage of what we today would consider the peculiarities of high energy electrophysics.

To me comparing these modern energy transfer systems to his is sort of like comparing a toy laser pen to what a commercial multi KW metal cutting laser does. Yes both are lasers and both can work at identical wavelength and frequencies but how they do what they do and at what power levels they work at are two very different concepts.

 

[Myrtonos]

As I said, going from single phase from polyphase could allow a lower frequecy to be used, as the power transfer is constant, and a lower frequency increases coupling distance.

 

[mtwieg]

Hold-up time isn't a problem with wireless power systems. Putting in a few uF of capacitance is trivial (compared to a big coil). And lower frequencies generally result in lower transfer efficiency.

 

[tcmtech]

As I said, going from single phase from polyphase could allow a lower frequecy to be used, as the power transfer is constant, and a lower frequency increases coupling distance.

Can you elaborate on this thought considerably more?

I am not following the logic being that once you leave the confines of solid wire and go to electromagnetic coupling at RF frequencies the concept of phasing as such as how solid wire based electrical power transfer works on tends to become pointless if not impossible to use for adding more energy transfer to a electromagnetic field link between two or more coils operating on the same frequency.

Also what frequency ranges are you thinking? 10's? 100's? Killo or mega hertz? Which range?

 

[Myrtonos]

Well it turns out that the relative position of coils does affect the coupling. Coils are coupled if in-line or side by side. I'm not sure what rx and tx are, but if one of the transmitter coils is orthogonal to another, they are mutually decoupled and tuned to the same frequency and synchronised they could transmit two out of phase suppiles to another pair of coils, each in the same plane as one of the transmitter coils.