Re: Variable Mutual Inductance Primary Tuning (VMIPT Sorry :o)))

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Darren Freeman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <free0076-at-flinders.edu.au>

At 07:18 AM 27/07/2002, you wrote:
>Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
>
>Hi All,
>
>I don't know if this has been done, thought of, or discussed here before but.
>
>My Off-Line coil project will have a pretty fixed primary caps size, fixed
>primary inductor size, fixed secondary inductance, and reasonably fixed
>secondary capacitance.  So there is a problem with tuning.
>
>Here is the idea.

Sounds like a good idea but you made a mistake:

>There will be two primary coils (actually two BIG single turn loops of
>copper).  They will be movable so that the two loops are either very close
>together or maybe up to 6 inches away from each other.  They will be wired
>in parallel.
>
>When they are close together, they will have high mutual inductance between
>each other so the "total" inductance will tend to increase.  If one roughly
>assumes the inductance is proportional to N^2, I would get about 4X the
>inductance of a single loop when they are close together.

Proportional to N^2 if the current flows sequentially through all turns.

You will actually have a parallel configuration with half the current in 
each loop =(..

At a coupling of zero, 1/2 the inductance of a single turn on its own.

At max coupling, the induced EMF would be that of a single turn as the same 
total Amp-turns is present in the combination.

Hence the effective inductance has doubled, bringing us back to that of a 
single loop.

Think of it as a single loop of litz wire containing 2 strands...


>Now if I move them apart so the mutual inductance is low, they will tend to
>act as two magnetically separate inductors wired in parallel and the
>inductance will be roughly 1/2 that of a single loop.

Yep.

>Theoretically.  I could get a tuning range of 1/2 to 4 X that of a single
>loop in this way.

See above argument. Range of 1/2 to 1.0

>Does that seem at all reasonable??  I really would not need a great range
>of tuning so if it just worked a little it would be fine.  I ran a test
>case of this on MandK, realizing that it was never made to do this type of
>stuff, but I got the results attached at the end of this post.
>
>These coils have to be at least 1.5 inches apart where k=0.27 (M=0.24uH).
>At say 6 inches, K=0.0755 (M=0.067uH).  The coils naturally have an
>inductance of 1.23uH (12 inches diameter copper tube 1 inch thick).  The
>program gives the secondary (identical to the primary in this case) an
>inductance of 0.65uH.
>
>I suppose this would be like having two size full primary coils in which
>you vary the distance to tune them, which I think has been discussed
>before.  But with just single turn loops, it may work better.
>
>Of course, I would have to make up and test something like this before I
>got too carried away, but I thought I would bring it up for ideas and 
>comments.

In principle it would work fine.. Don't worry about insulating between them 
if it is truly wired in parallel.. If they are symmetrically placed there 
shouldn't be many volts between them.

>----------
>Everything else is going very well.  I ordered up parts for the AC control
>cabinet which in this case will be pretty hollow and weigh about two pounds
>;-)  Basically just big switches rectifiers lights and fuses.  But it
>interfaces the coil directly to the 240VAC line so it has to be pretty
>good.  It only needs to handle about 5 amps so it's not to bad at all.
>Still very simple compared to a normal TC controller.  Anyone who does not
>like hauling heavy stuff will love these Off-Line coils :o))  There just
>isn't any iron in them!

I'm curious to see how it goes! Sounds like you are doing something pretty 
rare and taking a risk =) Hope it goes well!

After all, MMCs were thought of as pretty dodgy until recently.. Now 
everybody uses them ;)

---8<-- snip --8<--

>Not sure about the secondary yet until I get the primary inductance thing
>figured out.  I have worked some on the zero crossing detection and control
>stuff but that should all be very conventional "bunch of ICs" stuff in a
>heavily shielded box.
>
>In general, the Off-Line coil is physically very simple with little real
>hardware to it.  Lots of "completely different" ideas to it, but the goal
>is all the same.  Very strange not dealing with transformers or high
>voltages, but just really high currents and very low resistances.  Aside
>from the problem finding the caps, all the parts are easy off-the-shelf
>stuff.  Anyone should be able to make one.  You can't be quite as creative
>in just using any old parts, but you don't need many.  The primary coils
>and circuits will probably be about 10 pounds if I use a heavy frame.  The
>cost is say $50 for the secondary and top terminal, $200 for caps, $150 for
>IGBTs and control stuff.  So about $400 right now.  There will be all kinds
>of opportunities for improvements, but I am trying to keep is super simple,
>cheap.


I'm wondering if my SSTC could be run in stupidly-high-pulsed mode by 
discharging a cap through the H-bridge at an appropriate firing voltage. 
The bridge would be running at resonant frequency the whole time.

Of course, simply slamming an IGBT into conduction has gotta be more 
efficient than running a bridge..

I wonder if a beefy SCR could be used in the same application with 
continuous gate current. I saw a guy on the Web use hockey-puck SCRs on his 
Gauss-gun at several kA pulsed...


>Cheers,
>
>         Terry

Have fun,
Darren Freeman