The frequency is usually set by something other then the power transfer coil. This then isolates the frequency from any effects to the power transfer coil. Of course you could design a circuit that uses the power transfer coil as the frequency determining item, but the frequency will then shift all over the place.
If you go for a fixed frequency working, then resonance of the power transfer coil will increase the currents in it and hence its magnetic field and the received power.
Frank
(1) Yes, what goes in comes out if its direct magnetic coupling. Definitely the same frequency and same sine waveform.
(2) As the coupling is purely magnetic, what amplitude comes out depends on both spacing and orientation. In fact you can twist the receiving coil about and very likely find an orientation where no energy at all couples, even at short range. The frequency stays the same, what goes in comes out, but how much recoverable energy comes out is the variable.
(3) Nothing in the transmission path can impact the received frequency, only the received amplitude.
(4) The problem with resonance in the secondary is that although it will build up energy in the winding and increase the circulating voltages and currents, you cannot draw that built up energy off in the form of power.
It works in a radio receiver because the magnified voltage created by resonance can be used by a sensitive amplifier to increase receiver sensitivity at the resonant frequency.
But where you are transferring true watts of power into a load, as soon as you try to extract that circulating power from the resonant circuit it damps down the resonance.
Tuned circuits are not over unity devices for power transfer.
What goes in comes out, and no more.
But resonance can have an advantages for increasing sensitivity where there is minimal loading on the tuned resonant circuit.
It would be rather different if you were just trying to transfer some data to a sensitive receiver. Coupling real power into a load is a different game.
My general suggestion is to model the link as non-ideal transformer, coupled inductors with coupling factor < 1. A realistic model would also include a series resistance of each coil, comprising DC resistance and proximity/skin effect losses. Eddy current losses generated by metallic parts in the vicinity and losses of a possibly used magnetic core could be modelled by an additional parallel resistor.
You should at least stay below 150 kHz not to collide with general EMC regulations. Permitted stray field strength of inductive power transmission system is still a matter of regulations under development. Several bands are occupied by existing services, e.g. still used DECCA navigation or RFID. Inductive power guys argue that the regulations are only applicable to information transmitting systems.
But the coil will have interwinding capacitance and a Self Resonant frequency, so you will care and what frequency you choose and what losses you have.
Skin Effect losses increase with frequency. Ferrite core losses increase with frequency
But coupling losses reduce with rising frequency where the gap must be < diameter and gap losses reduce with gap size.
A Ferrite Bar is useful for increasing the coupling factor.
A step up transformer for sending and step-down transformer winding for receiving improves impedance from very low ESR battery<<1 Ohm loads to the optimum impedance for free-space in the 150 Ohm range.
Therefore plan on using step-up autotransformer windings with a flat ferrite bar just below SRF for optimal power transfer in the license free industrial noise band designated for your area. in the short wave band or below 150kHZ
there is a push for operation in the higher frequency ISM bands of 6.78 MHz and 13.56 MHz where resonant systems allow high spatial freedom at high efficiency. At these high frequencies, traditional MOSFET technology is approaching its capability limit.
Mismatched impedance transformer coupling results in significant reflected power.
For biological power transfer, there is a tradeoff between gap losses , high F absorption and low frequency coupling losses, where something in the 1GHz range is still ideal for charging internal biological battery systems.
The problem can be split into two separate parts.
Firstly we need to generate sufficient power at the required frequency to drive the transmitting coil. Frank has summed up pretty well what is required in #7.
The transmitter coil should be carefully tuned to resonance, and correctly impedance matched to the power amplifier driving it.
That should all be fairly straightforward.
The next consideration is the size and geometry of the transmitter coil.
This depends on the distance and how accurate the placement is going to be.
If this is for a human biomedical implant, presumably the distance will be small and the placement accuracy fairly high.
If its to work with unrestrained animals or a wearable device by a human exercising, the situation could be highly variable.
In the first case, it may be preferable to focus the transmitted magnetic field with a flux concentrator such as a large ferrite U core. This would hugely increase the coupling at very close range, but greatly reduce the far field intensity.
For working at greater ranges the transmitter coil should be large and air cored.
An example might be the dinner plate sized search coils typically used with metal detectors. These can generate a surprisingly large far reaching magnetic field, although the coupling of significant actual useful power with something like that, will still be problematic.
If its to be used within a room, one solution might be to run a multi core cable around the room, and connect all the conductors in series. A 25 core cable then becomes a large area 25 turn coil. Flat ribbon cable may be of interest here too if its to go under a carpet.
Something like that might offer your best chance if the receiving coil is moving around unpredictably within an enclosed space.
A rule of thumb might be to make the transmitting coil diameter at least as large as the expected distance to the receiver coil, and preferably surrounding the receiver coil completely if you can manage to do that.
At the receive end, the problem is quite different. As we are trying to transfer power, not just detect a weak signal, the current induced in the receive coil will produce its own magnetic field which opposes the field that produced it.
What happens is the surrounding exciting field will try to flow AROUND the receiving coil. It will always take the easy path, rather than try to exert itself driving power into your load.
For that reason a flux concentrator at the receiver coil will be of very great benefit.
A ferrite rod is recommended, the physically larger the better.
This is common practice with transistor radios that work at lower frequencies.
The magnetic field will find a much easier path through the ferrite rod than through the surrounding air, even if the receiver coil is heavily loaded, which it will be.
Trying to tune a highly resonant peak in the receiver coil is not likely to happen because of the very heavy loading and resultant low Q.
But balancing out the inductive reactance with capacitive reactance will bring the voltage and current into phase which should give a broad peak in output power, which is a requirement for transferring maximum true power.
Its rather difficult unless you can give us some much better specifics about what you are actually trying to do.
Required amount of power to transfer, distances involved and the application and any limitations we can only guess about ?
Are we talking about hundreds of milliwatts, or microwatts of power transmission ?
Millimeters of distance through skin, tens of meters of range outdoors, or what ?
With data transmission it would be really helpful to know the required data rate is it a few hertz or megahertz of bandwidth ?
Yes exactly.So can I send a sine signal with what ever frequency I want in the primary side of the data coil, to get the same freuqncy sine wave at the secondary side data coil, then use comparator to process and generate a square wave at the same frequency?
That helps a lot !!
Close to skin means very short range, so a flux concentrator at the transmitter coil will help a very great deal. A ferrite U core would be my first choice.
The magnetic field strength bridging the end poles of the U core could be made quite intense so both power and data transmissin becomes very easy.
The receiver coil could be quite small and very few turns should be required.
Your best bet might be to generate a sinewave frequency that can shift back and forth between two very close frequencies.
This is called frequency shift keying and is a very effective way to transmit data.
That would provide continuous uninterrupted power transmission, and at the receiver the change in frequency would be very easy to detect and turn back into a digtal data stream.
It all then becomes fairly simple to implement with minimum parts and complication.
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