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Variable Mutual Inductance Primary Tuning (VMIPT Sorry :o)))
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.
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.
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.
Theoretically. I could get a tuning range of 1/2 to 4 X that of a single
loop in this way.
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.
----------
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 figure the IGBTs will dissipate a total of 20 watts and the anti-parallel
diodes about 10 watts. So a 780 watt coil loosing only 30 watts to the
gap! Trying for 95% efficiency ?) I am using maybe ten IRG4PF50WD IGBTs
that do 900 volts and 204 amps peak for only $7 each. I'll get ten of them
from DigiKey and see how it goes.
http://www.irf-dot-com/product-info/datasheets/data/irg4pf50wd.pdf
I am trying to locate ten Cornell-Dubilier 940C6W4P7K 4.7uF 600V film
caps. Can't find them yet and stuff like this is hard for a "little guy"
(even with a bottomless credit card) to get in tiny quantities. Any ideas?
Chris B. is trying at his end (hope hope ?) Allied had the higher
voltages, but not these. I think the factory has like 200 in stock so
maybe they sell sample quantities direct.
http://www.cornell-dubilier-dot-com/film/940600.htm
Even though they are "just" metalized 940C types, they will work fine here.
Ten in parallel will handle 168 amps RMS and 4920 amps peak! Easily
enough for our needs. The dV/dT, inductance and series resistance is
trivial. Unfortunately, they may be like $20 each ? Maybe someday, we can
bulk buy them ?
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.
Cheers,
Terry
=============
Mutual Inductance Program V3.1
Copyright (c) 1998, 1999, 2000 by Mark S. Rzeszotarski, Ph.D.
Flat Spiral Primary Coil Geometry
Primary coil inside diameter (inches)= 12.000
Primary coil outside diameter (inches)= 12.100
Number of primary coil turns = 1.000
Wire diameter (inches)= 1.0000
Solenoidal Secondary Coil Geometry
Secondary coil diameter (inches)= 12.000
Secondary coil height (inches)= 1.000
Number of secondary coil turns = 1.000
Secondary coil wire diameter (inches)= 1.0000
Calculational Results
Primary coil inductance in microhenries: Lp = 1.23
Secondary coil inductance in microhenries: Ls = .65
Figure of Merit (square root of Ls/Lp) = .73
Primary coil wire length in feet = 3.15
Secondary coil wire length in feet = 3.14
DC secondary resistance in ohms: .00
Secondary coil distributed capacitance in picofarads: 30.48
(Medhurst formula, assumes one end of secondary is grounded)
Mutual Inductance Results
M = Mutual Inductance in microhenries
K = Coefficient of Coupling: K = M / square root ( Lp x Ls )
Position M K
.000 .550 .6180
.500 .372 .4174
1.000 .292 .3283
1.500 .240 .2692
2.000 .201 .2257
2.500 .171 .1920
3.000 .147 .1651
3.500 .127 .1430
4.000 .111 .1247
4.500 .097 .1093
5.000 .086 .0962
5.500 .076 .0851
6.000 .067 .0755
6.500 .060 .0672
7.000 .053 .0600
7.500 .048 .0537
8.000 .043 .0483