Why doesn't magnetic power transfer use ferromagnetic cores in the coils?

Question

One well-known fact is that putting a ferromagnetic material in an inductor will increase the magnetic flux and thus the inductance value.

Wireless power transfer (WPT) happens through the magnetic near-field. It seems logical that if the magnetic field strength generated by the transmitter is increased so is the field at the receiver. It therefore seems very desirable.

On the other hand all the WPT solutions I have seen use air cores and only a ferromagnetic shield. The high permeability of the shield makes it equivalent to a much thicker air gap, and thus the field at its other end will not be weakened by any metals.

The situation seems to be similar on the receiver side. There are ferrite rod antennas for far-field reception. The connection between two ferrite core coils at a given air gap seems to be the same as if we would take one ferrite rod, wind the two coils on it, and just cut the ferrite rod in the middle and pull it apart a little. The magnetic field should be concentrated in and around the ferrite cores, shouldn't it? Why aren't ferrite cores used in WPT? What are their drawbacks?

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PS: an image search has not resulted with any coil using a ferrite core, but the black pad below the planar coil winding is a ferromagnetic material, which concentrates the magnetic flux density. This way the coils can be placed on conductive surfaces, such as the battery of a mobile phone, without great losses.

Answer 1

It is all about the coupling factor between the supply coil and the receiver coil.

A ferrite can funnel the magnetic field through the receiver coil, improving the coupling, but this is not always necessary.

Image a cell phone charger. The phone coil can be very wide - several cm in diameter - and close to the supply coil, maybe only as much as 1 cm away. In this case, even the rapidly dispersing magnetic field of the air coil will essentially pass through the receiving coil with close to 100% efficiency, especially if the supply coil has a similar diameter.

However, if the two coils are further apart the coupling factor rapidly drops for air coils. In this case, field concentration with ferrites can improve the coupling strongly, while the added losses are rather small.

Answer 2

I think they just don't do it because the price for these devices has to be really competitive and that would increase the manufacturing cost.

First, I agree with you, a ferrite piece in the center would increase power transfer. My guess on why the don't have it is more on the economical side. You need the ferrite sheet in order to direct the returning flux and shield the electronics on the other side. A sheet is easy and cheap to make. If now we add a cylinder in the center to direct the power transfer, we have to options:

1. We design it as part of the ferrite sheet, similar to a planar ER core. Now instead of a cheap sheet, we have a sintered ferrite core, much more expensive.
2. We design it as a different piece. Now we have to attach that to sheet, increasing the assembly costs, plus the extra piece cost.

Both are bad for business, while not really increasing the value of the product for the average user (non-Engineers), as the charger works fine without that.

Answer 3

It is not all about the coupling factor between the supply coil and the receiver coil.

A high coupling factor avoids currents in the supply coil that don't have an effect in the receiver coil but instead create an external field. But an external field does not mean an energy loss as long as its energy is recycled (in a more honestly literal meaning of the word than common) by the supply coil when it collapses. Energy leaves the supply coil for good in the form of electromagnetic radiation. When the supply coil dimensions are much smaller than the wavelength of the AC running the coil, comparatively little EM will actually depart.

With household electricity transformers, you want to minimize stray inductivities in order to have high power factors and not waste energy on line losses for the reactive energy components of apparent energy.

But if you mostly supply your currents using a resonant circuit, without an energy tap nearby there will not be much of a power loss since you can provide the reactive power driving the stray inductances with a capacitive circuit. A magnetic core will actually cause energy losses through magnetisation. Instead of using a core for increasing energy transfer through magnetic flux, you can increase the operating frequencies, and in that manner lower the idle currents and increase induced voltages.

One such energy tap can actually be a ferrite core... It makes it harder to idle efficiently. High quality passive crossovers for speakers will actually use coreless inductivities (partly of humongous size) because magnetic cores have nonlinear losses and saturation that air cored coils are not prone to.

The disadvantage is, of course, that air core coils are much more prone to interact with other circuitry and nearby metal because a core can do a lot to confine the field.