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Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600
For abstract knowledge I was trying to work out whether it's possible to make wireless power work in any direction.
I figured if you use slightly different frequencies for 3 orthogonal coils, then provided the bandwidth of the receiving coil is sufficiently wide, then it will accept power for any/all of them.
Then I started wondering whether you need different frequencies, whether you could just phase the coils.
If you had 2 coils at 90 degrees, phased at 90 degrees for example then that gives a rotating field, that would allow you to have the receiver coil in anything except the plane of rotation. Adding a third coil and running at 120 degrees phase helps, but there's the diagonal line where the receiver coil is coaxial you get no reception because the phases cancel along it.
It looks like if you add a fourth transmitter coil along that line that it's possible to get omindirectional reception, but I'm not absolutely completely certain.
Anyone have any deep insights? It's just a bit of fun, dunno what it's good for.
If you had 2 coils at 90 degrees, phased at 90 degrees for example then that gives a rotating field, that would allow you to have the receiver coil in anything except the plane of rotation.
That looks like a sound idea to me. A magnetic far field can be described by a vector, i.e. a dipole moment. The field it creates is strongest in the plane perpendicular to the vector. A rotating field, like you describe it, would correspond to a rotating vector. The plane of strongest field rotates with it and sweeps all of the space around it. The receiving loop should not lie in the plane of rotation, but perpendicular to it.
Edit: The orthogonal of the plane the receiving loop lies in should not point into the direction of the source of the magnetic field.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
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Uspring wrote ...
BigBad wrote:
If you had 2 coils at 90 degrees, phased at 90 degrees for example then that gives a rotating field, that would allow you to have the receiver coil in anything except the plane of rotation.
That looks like a sound idea to me. A magnetic far field can be described by a vector, i.e. a dipole moment. The field it creates is strongest in the plane perpendicular to the vector. A rotating field, like you describe it, would correspond to a rotating vector. The plane of strongest field rotates with it and sweeps all of the space around it. The receiving loop should not lie in the plane of rotation, but perpendicular to it.
Edit: The orthogonal of the plane the receiving loop lies in should not point into the direction of the source of the magnetic field. [/quote]
Not quite; if it points directly away or towards from the centre of the transmitter, then the magnitude of the normal flux is maximum when the peaks/troughs of the spinning field are 45 degrees either side of the coil.
It turns out that, over time, the total flux density going through any point in a rotating field is constant, but the direction constantly rotates around.
I'm pretty sure that with more coils the directions can be forced to keep changing giving omnidirectional response.
In my first post I mixed up static dipole fields with radiation fields. Radiation fields don't have any longitudinal components but static fields can have them.
With a third coil you can make the dipole moment point in any direction. With some phase shift in the third coil, the plane of rotation will be different, but you will still have planar rotation unless you change the phase over time. A phase change involves the use of 2 frequencies, though.
Planar rotation will give you some power everywhere, although 4 times as much in the plane of rotation than perpendicular to it.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
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Yes, I don't think three are enough for a single frequency, although it is better than two.
I think if you have four coils placed at the corners of a pyramid with their axes pointing towards the centre, and wire them as two phase, I think the magnetic field wobbles as you do a single cycle and that gives you omnidirectional reception for a flat coil.
They would cancel at the middle of the pyramid, but not anywhere else.
I'm not sure whether that's a practical design though, the coupling might be a bit much for the two driving tank circuits.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
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Oh wait, perhaps you're right, maybe there is a 90 degree thing if each of the two sets of coils are on different phase. So each phase doesn't have to be 90 degrees, but the two phases are 90 degrees to each other? Hmm.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
It's late and I've had a drink or three, but can you use six coils, with opposing pairs 180 degrees out of phase, all three pairs at the same frequency, but different phases?.....ie, a 'three phase' system? (actually 6 phase, I think)
Increasing the number of coils beyond 2 won't help, if you consider the far field dipole moment and a single frequency with fixed phases and amplitudes. The dipole moment is the sum of the moments of all coils and will always trace out an ellipse in space. This is also possible with just 2 coils.
Near fields are more complex, they can have quadrupole and higher moments. These will die out faster with distance.
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Omnidirectional resonant inductive wireless power?
