Martin K <martin-distlists@nnytech.net> wrote:
> Olin Lathrop wrote:
> > Jamie Lyon wrote:
> > > however as is the challenge of making such things, it's hard to get
> > > the power to the rotating platform, I've built a small one based on
> > > rubbing contacts, and it works fine, but I fancy trying out another
> > > method now. I read a forum posting by Bob Weiss about getting power
> > > to the rotating platform via a custom built generator. It sounded
> > > like a great idea, so here I am.
> > 
> > We had a similar problem for the high end LED blinker that was
> > discussed here a few days ago.  The rotating assembly required
> > about 80W, mostly to power the 288 LEDs.  The solution in this
> > case was a transformer where the primary and secondary could
> > rotate.  The primary was center tapped, so the center was tied
> > to the +170V unregulated rail, and the ends were  alternately
> > chopped to ground with FETs.  It worked rather nicely once we
> > got  everything tweaked right.
> 
> Olin,
> Does the primary rotate about the same access as the secondary 
> (resulting in it being on the main drive shaft)
> I'm assuming either way it's a soft iron core with the air gap being 
> where the device rotates?

Actually a ferrite core in the form of two cup shapes that face each
other. IIRC, we're driving it at about 50 kHz. Here's a cross-section:

                            #  #
             +-------------+#  #+-------------+    
             |   +-----+   |#  #|   +-----+   |    
             |   |  xxx|   |#  #|   |xxx  |   |
             +---+     +---+#  #+---+     +---+
             +---+     +---+#  #+---+     +---+
             |   | sPPP|   |#  #|   |PPPs |   |
             |   | sPPP|   |#  #|   |PPPs |   |
             |   +-----+   |#  #|   +-----+   |    
             +-------------+#  #+-------------+    
                            #  #
                            #  #
                            #  #

The hash marks represent the hollow motor shaft. The bottom core has two
windings, the primary (P) and a nonrotating secondary (s). This core is
fixed to the same structure that holds the motor, and the shaft rotates
freely within it.

The upper core is fixed to the shaft and rotates with it, and carries
another secondary (x) that provides the power to the LEDs and the logic
that drives them.

Olin and I keep joking that everyone's first PIC project is an LED blinker,
and here we are -- still blinking LEDs! Can't seem to get away from it!
But there are some serious engineering problems to be solved in a system
like this.

-- Dave Tweed
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