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Re: Primary coil as autotransformer?

Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Pete,

Neat experiment!

I hooked up my low-Z amplifier to my small coil primary and input 1VRMS at
50, 100, and 200 kHz into turn 5 and measured the voltages at each turn.
This coil has 13 turns of 1/4 inch copper tubing.  Inner Dia. 6.8 inches.
Pitch 0.4 inches:

Turn	50kHz		100kHz		200kHz
0	0.010806	0.012024	0.010515
1	0.170923	0.171343	0.170347
2	0.359528	0.360721	0.360673
3	0.571709	0.571142	0.570978
4	0.790766	0.789579	0.790747
5	1		1		1
6	1.170923	1.171343	1.171399
7	1.317289	1.318637	1.315457
8	1.448919	1.4499		1.44795
9	1.56778		1.571142	1.567823
10	1.676817	1.680361	1.678233
11	1.777996	1.781563	1.781283
12	1.872299	1.876754	1.878023
13	1.960707	1.966934	1.968454

The graph looks like:


It looks like the graph is sort of two semi-straight lines that are bent at
the tap point.  The autotransformer action is very obvious.  The voltmeter
leads were picking up a little voltage too but it did not seems to affect
the results much.  The frequency is not making any difference at all.



At 11:54 PM 5/11/2001 -0600, you wrote:
>I have a 12KV 60ma NST driving my coil.
>It has a flat primary of 1/4 copper tubing spaced at 1/4 inch.  From the
>start across is 10 1/4 inches, measured at 90 degrees from that it is 10 3/4
>inches.  27 turns (probably excessive but I had the space and the slots)
>tapped at 21.75.  Note that I had some problems with arcing from the end of
>the outside turn to the tubing brought out from the center below the
>windings (more than an inch of clearance).  I suspect that the
>autotransformer effect was causing the breakdown.  The form I wound this on
>is Plexiglas.  I wound the copper tubing working on a card table.  When I
>slid the finished primary off the table, I got zapped with a large static
>discharge.  For a week or more, every time I touched the primary tubing I
>got a shock.
>My question is this:  Is the induced voltage on the outside turns (past the
>tap) proportional to the average voltage per turn of the center tapped
>portion?  Do the center turns have an equal voltage per turn or is that
>proportional to the inductance per turn?  Would the inductance ratio be the
>same as the voltage ratio for the center driven turns versus the outside
>floating turns?  If an experiment was run using 60 Hz low voltage, would it
>compare to Tesla voltage and frequency?
>I think I am better at posing questions than answering them.
>Now that I think about it, here's a test setup.  Wall outlet to variac,
>variac to power transformer (11v at 50 amps output), output of transformer
>to primary coil tapped as is.  Dial up the variac to produce 5 volts output
>to the primary coil (reactance of .1 ohm at 60 Hz), giving 50 amps into
>primary.  Use voltmeter to measure across outside turns.  I'd monitor
>current into the transformer using 5 amp AC meter to make sure that the
>input current stays below about 4.6 amps or so.  Comments?  I'll try it and
>let you know the results.
>Word didn't like my spelling, so this didn't go yet.  Results:
>Tapped at 20.75 turns with 5 Volt input to the primary (60 Hz), input amps
>to transformer are 3.33.
>Inner 5 turns	0.5 v
>Next 5		0.9 v
>Next 5		1.1 v
>Next 5		1.2 v
>17-22 1.2 v
>18-23 1.1 v
>21-26 0.6 v
>Tapped at 21.75 turns with 5 volt input,  3.28 amps into Xfmr.
>Inner 5		0.43 v
>Next 5		0.8 v
>Next 5		1.05 v
>Next 5		1.15 v
>16.75-21.75 1.175 v
>21.75-26.75 0.6 v
>22-27		just less than 0.6 v
>The copper tubing was warm when I finished.  Readings were done on a Radio
>Shack multimeter with 0.05 volt per division on the 5 volt AC scale. (1.175
>was 3 1/2 divisions past 1 volt.)
>1. The voltage does vary from inner turns to outer.
>2. The outer floating turns have voltage induced but are not coupled well
>enough to match the outer driven turns.
>3. Judging from the input current to the transformer, the impedance of the
>primary coil is larger than calculated or other resistance is adding in.
>One hundredth ohm would change the current by nearly 10%.
>That's all for now.  It's too late to think about this any more tonight.