The other day I somehow ran into a wireless power transmission video on YouTube:
It's a bit too technical for you but I wanted to understand a bit more about the subject so I started googling and youtubing the terms inductance and capacitance because I know little about them. Then I ran into a few other cool videos:
Inductance:
And then practical experiments:
So I'm wondering because the experiments look quite simple yet the theory, specially in the original Alanis lab video sounds much more complex.
My doubt is, am I mixing different things/concepts?
Is wireless power transmission similar to RF transmission in terms of the basic concept? Radio frequency transmission of data is movement of energy waves from one place to another. Is this the same basis for wireless power transmission?
In the videos of inductance they talk about a coil of wire around a magnetic material generating a force that delays the current flow yet that concept is not exactly what happens in wireless power transmission because there is no magnetic material inside the coil.
Sometimes they mention the resonating frequency which makes me think if the while RF transmission bit. Are tuning and resonant frequency related to this concept of wireless power transmission?
The main difference between wireless power and radio is that radio used electromagnetic waves where as wires power just uses a magnetic coupling.
Basically all it is is a transformer, it is a technique that is as old as the hills but has been given a new gloss. It is very inefficient as the distance increases as a magnetic field drops off as the cube of the distance.
A tuned and a resonant frequency in this context are the same thing.
Technically this is the difference between far-field (RF comms) and near-field (wireless power).
The equations are the same, Maxwell's equations, the solutions have different boundary conditions
(in particular there is always far- and near-field, but in RF comms you arrange that the near-field
isn't damped/absorbed by nearby objects so that the amplitude builds up high enough (antenna
resonance is normally relied on) to radiate significantly. With wireless power you try to absorb all
of the near-field power in the load, by having it couple efficiently to the transmitting coil, just like
a transformer.
In terms of the E and B fields (electric and magnetic), you aren't interested in the E-field at all
for wireless power, you can shield that out to reduce far-field significantly. For far-field radiation
the E and B field are in a fixed ratio, E field dominating B.
Or put another way the near field can be thought of as separate E and B fields, whereas far field
is a precise combination of the two needed for the propagating wave solution to Maxwell's equations.
In theory you could use the near E-field for wireless power, and this is already commonplace - its
called a capacitor! However the amount of capacitance available from a wireless charging pad would be
measured in pF and only efficient for transfering significant amounts of power at very high frequencies
and would leak lots of far-field (hence isn't safe).
It's all transmission of energy waves whether you take those waves and interpret them into digital signals or take their actual energy and use it to power something.
The concept is magnetic inductive coupling. Whether it factors in resonant frequencies of the Tx and Rx determines the efficiency in longer range transmissions.
It's similar to transformers in that they are composed of two coils which are slightly separated from one another. In the case of wireless that space can grow a bit more and is very inefficient. In the case of transformers they are very efficient because they are so close together and from what I understand they usually are contained in the metal buckets filled with oil, right?
I'm currently trying to understand how inductive and capacitative couping differ, how this relates to F=qvB, the frequency bit, and how the transistor in most tutorials for wireless power plays a part.
Marciokoko:
Pwillard
So our making those 2 little copper wire coils is what makes the power alternate?
Or did you mean that it's just because the literature refers to ac power?
In the video, it looks like the transmit side has a component oscillator (the silver package on the battery side of the breadboard) driving some sort of amplifier (the transistor etc) providing the AC current to the transmit side coil. I didn't watch the full video, so it's not clear if he specifies the frequency at which the circuit is running, but it would be set by the oscillator.
The receive side looks to consist of the coil in a series loop with a capacitor which would have some resonant frequency, presumably designed to match the transmit side.
Whether it factors in resonant frequencies of the Tx and Rx determines the efficiency in longer range transmissions.
No, resonance is almost irrelevant here. What is important is the coupling. At resonance the voltage coupling is highest but that is not power coupling.
It's similar to transformers
No it is identical to a transformer, in other words it is a transformer.
from what I understand they usually are contained in the metal buckets filled with oil, right?
No not usually.
In any transformer you get losses in the core, due to eddy currents, this shows up as heat. An oil filled transformer can be used so the oil conducts away the heat. This only applies with very high powers like you get in local sub stations. In a wireless charger there are no oil filled transformers.
It's all transmission of energy waves
There is no such thing as energy waves. Waves of all types can carry energy but the waves themselves are not energy. Think about a water wave in the ocean, they are not energy waves they are water waves. They carry energy which can be extracted with tidal power systems like floats.
Thanks. I didn't mean to imply they were energy waves
OK the original video is more complex. So he does have an oscillator and yes I saw it on the diagram and in the video he talks a out resonance and so it immediately became apparent that it was too complex for me to understand at first.
Luckily I found the next few videos which are much simpler in that they don't talk about resonance and don't use capacitors or oscillators. They only use the transistor.
So how does it work in those circuits where it's just the 2 coils, the transistor, the resistor and the led? Is this still the same concept?
A transformer is basically the result of close coupling 2 coils. A coil operates by storing current in the "coils of wire" which creates an electric field. The electric field can quickly reach a static state if fed DC. When the current is removed... the charge stored in the electric field of the coil is released back into the the circuit.
The coupling of the coils means that the field that is created in one side is induced in the other side "while it is in transition" but not when in a steady state... so even if the power source is chopped by a transistor and not really an true AC Signal... there is still the avoidance of the DC steady state. This "chopping" of the signal allows the charge and discharge of the coupled coils and the result is the induction of a voltage in the other side of the transformer.
I probably blew that explanation... but it is an attempt.
When a magnetic field line "cuts" a conductor it induces a current to flow in the conductor. Cutting means that the field the conductor sees changes.
It does not matter if the field changes from zero to a positive value or from a positive value through zero to a negative value.
A coil is just a collection of conductors, each turn of the coil having a current induced in it. So a coil with lots of turns has lots of current induced. If there is no circuit, that is the ends of the coil don't connect to anything then this current ends up producing a voltage at the ends.
is this still the same concept?
Yes. The transistor, the resistor and the led are just the load on the coil, somewhere for the induced current to go.
Ok so the last bit would be putting it all together. So when we coil up the Tx wire (which is called Tx because it has the power source, right?), what is happening is that the coil produces a magnetic field which produces a current on the other coil which then flows to the LED. So there is no flow of electrons "thru the air" from the Tx to the Rx coil, right?
Hey, btw, so in an RF module, thats what the coils are! And thats what that flat silver disc structure is...the oscillator, because in that case the resonant frequency is important. Why does one of the units have 2 coils on it instead of 1?
So there is no flow of electrons "thru the air" from the Tx to the Rx coil, right?
Correct.
Why does one of the units have 2 coils on it instead of 1?
Coils exhibit the property of inductance as well as radiating power, there are many reasons why you would have several in a circuit. Power supply decoupling, radiating, forming a resonant circuit, creating a magnetic field for a permanent magnate to react with in a loud speaker, making a motor spin, making a stepping motor hold still ..... these are all reasons to have coils.
I did not watch more than the first minute of that video as I thought 35mins was a bit too much of my life to loose watching about something I already knew.
It's obviously too technical for you but not us , being as you are asking US the questions.
FYI,
The term "wireless power transmission " is a BS term used by people to BS people who they believe do not understand the most basic concept of magnetic coupling .
It brings to mind the image of sending power from point A to point B but they somehow neglect to mention that the distance between point A and point B is not more than a few mms. The reason the fellow in the video was able to get the led to turn on at a couple of inches is that he is not using any series resistor with the led and due to POV (persistance of vision) it is not possible to tell that the led is probably actually oscillating as opposed to on continuously and the current is probably only a few mA at a couple of volts. I will say the gentleman was clear about the coupling coefficient being very weak (0.1) as compared to a low frequency high efficiency (0.98) iron core transformer. I didn't get the impression he was trying to BS anyone, though I would take exception to his use of the title "wireless power transfer " as opposed to something more accurate like "inductive mutual coupling".
Actually, I think it is an excellent tutorial for someone who want to learn about inductance. He covers a lot of material and goes into great detail and I didn't see any BS in his presentation.
We all use transformers in equipment all over our house but we don't go around telling people we can transfer power wirelessly, which implies something new or revolutionary.
Where I work we built a power converter that converts 13 kv 3-phase power to low voltage (48Vdc)
at about 185A using a transformer but it was only a prototype and currently we are not pursuing it as a product but I think that deserves the term "wireless power transfer" a bit more than driving an led but we're not calling any press conferences. It's still just mutual inductive coupling, no matter how you pitch it.
If I can give some advice, if are not an electronics student (and maybe even if you are), it's generally a good idea to watch tutorial videos like the one you linked more than once because it is very easy to get fixated on something the speaker is saying and miss something else. I find that if you watch them several times, by the third time you have grasped the main concepts and can focus on the smaller details.
and wireless phone chargers.