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how to shield a coil from lossy core ? wireless power transfer

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maniana

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Hi, I have to wind a coil on a metal rod that is lossy. I measure Q of air coil as 30, and when I insert the rod (6 inch diameter), the Q drops to 2.5 and the inductance changes only slightly from 6uH to 6.7uH. the coil has to be on top of that rod and it is used as a receive coil in wireless power transfer. So I need a good Q.

My question is how to shield the coil from the rod, what materials to use to improve the Q factor ?

I can't change the material for a rod, it has to be what it is, but I can put anything on top of it (it has to be chip and easy to make).
 

Why do you need that rod? What is the frequency?

You can shield the rod by putting copper foil around it (thickness >> skin depth). It acts like a long single short-circuited turn. Inside the long shorted turn, AC flux will be zero. If your winding is close to the copper, the inductance of your winding will drop significantly. Actually you have a transformer with single turn secondary that is short-circuited.

Using the foil will only help you when there is sufficient clearance between the copper around the metal rod and the winding (so Drod+copper << Dwinding.innerside).
 

The frequency is around 100kHz, and the coil is a receive antenna for power tranfser to the electronics that are build inside the rod. I need to deliver about 200mW this way.
 

There isn't much to say in general about it. The "good conucting" shield as suggested by WimRFP will prevent losses in the rod, but also displace the magnetic field from the shield. So without sufficient clearance you won't receive much. A ferromagnetic shield would be possble in principle, but must have some thickness to be effective.

The best way to evaluate possible coil configurations is a field simulation. For a rotationally symmetric geometries, 2.5-D tools like Quickfield can work.
 

200mW at 100 kHz seems not impossible, provided you have some clearance, and you have sufficient power (in fact magnetic field) available from the transmitter coil so that you can tolerate some loss.

If you have the right tools and materials, magnetic shielding as suggested by FvM would be better then the copper foil shielding. Copper shielding will result in reduction of inductance and magnetic shielding will increase it. Though magnetic shielding requires some thickness (depending on ur of shielding material and ur of rod), but doesn't require clearance between the shielding material and the winding.

magnetic shielding will work when ur(shielding magnetic material) >> ur(rod), of course both measured at 100 kHz.
 

As your system load (electronics to which you transfer the power by induction at 100 kHz)is located in the core of your coil, then it is correct that such load drives down the coil Q as you described.

I think that you should simply tune the transmitting coil not for a high Q but for a maximum power delivered to the load. As the core contains the electronics as the load, its receiving coil should also be tuned for the best power-transmission efficiency.
Coaxial coils tuned to the same frequency are tightly coupled, so you may find that some degree of decoupling (finding the best point on-axis) can give you the best efficiency.
 

I was talking about unloaded Q. Because the Q is about 2, there is not much to be tuned. The transmit side uses Roer converter, hence it finds its own frequency automaticaly. I want to increase the Q of the receive coil to maximize the efficiency, which is bad in my case anyway, because my transmit coil has to be flat, I can not make a loop around the receive coil. It looks like a rotating shaft above a table and the transmit coil is build in the table. 200mW in this case is not that easy to transfer without a good Q as it seems..
 

First you wrote that your coil had a lossy core, now that the coil is flat.
Any coil in air would have a high Q; when its field hits any lossy body, the Q decreases by that loss.

If you use a conductive screen to separate coil field and a lossy body, then you cannot couple the power from the coil to or through the lossy body where your load is located.

I think you must change your geometry to allow the power from the active coil to get into the desired load, not tothe losy body.
 

A drawing would be in fact helpful. If the shaft is perpendicular to the table, the receiving should be placed on the end face of the shaft, possibly using a magnetic shield. That's a usual RFID geometry.
 

Here is a drawing of a product that is very similar to what we are building:
https://www.magtrol.com/torque/torqueflangesensors.html
"Measuring flange" is made of metal with a coil winded around it. "HF Transmitter" is a coil that generates electromagnetic field that is then coupled to the coil on "Measuring flange" providing power to the electronics build inside the "Measuring flange".
As you can see "HF Transmitter" is coupled to the coil on the flange on a very short length compared to the whole perimeter of the "Measuring flange", hence small efficiency of the power transfer. But the system has to look more or less as it is on the picture (look also at the datasheet).
We also have to build a datalink "Measuring flange" => "HF Receiver" using probably a separate coil placed next to the power coil. In this case we don't want to have a coil with high Q due to high data rate of the datalink.
 

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