A company I previously worked for used this for dynamic industrial fan 
balancing.  The rotating ring had 4 chambers with liquid, opposite 
chambers connected by a tube (two tubes).  Each chamber had heaters, and 
if you heated one chamber, fluid flowed to the opposite side changing 
the balance of the thing.

The heaters required quite a bit of power, and AFAIR they used an 
inductive ring to send both power and control signals to the circuitry 
located on the rotating assembly.  The fans these things balanced were 
usually huge (12 foot diameter) fans used in cement plants and other 
processes where the fan would continually build up an unbalanced coating 
of some material or another.

There are patents galore covering this technology.  I suspect the main 
issues are the fact that the core is split and you need a specially 
shaped core to get the best possible, but still low, efficiency.

-Adam

Josh Koffman wrote:

>I remember that article! I really wanted to build one...but as you
>mentioned, parts were a bit dear. Of course, at that point my skills
>aren't what they are now, and I probably would have ruined the diode
>anyways.
>
>I really want to do this on a product I've been thinking of. The
>"head" of the unit would need to rotate continuously. I've thought
>about brushes, but I am worried about the cost/lifetime issue.
>Induction would be super sweet, I worry about it interfering with
>outside devices though. If I was really clever (which I'm not) I could
>make the induction part of the switching power supply. That would
>reduce the size of the transfer coils needed, right? Any idea if I
>could get reasonable efficiency trying to transfer ~200W of power like
>this?
>
>Josh
>  
>
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