[Prev][Next][Index][Thread]
Re: Pri-Sec Phasing
---------- Forwarded message ----------
Date: Sat, 04 Oct 1997 22:18:56 -0700
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Pri-Sec Phasing
Tesla List wrote:
>
> ---------- Forwarded message ----------
> Date: Fri, 03 Oct 1997 15:46:31 +0000
> From: Greg Leyh <lod-at-pacbell-dot-net>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Pri-Sec Phasing
>
> Malcolm, Bert, All,
>
> When the pri and sec are physically close to each other, the relative
> polarities of the pri and sec should be aligned, in order to minimize
> the voltage difference between them.
> Given all of the possible 'minus-sign' errors, and the fact that the
> two coils are 90 deg out of phase, should the coils have the same or
> opposite 'handedness' when wound?
> I am no longer completely sure of the correct answer, and would much
> appreciate a confirmation. If possible, a concise explanation of how
> the answer was obtained would be far preferable to anecdotal evidence.
>
> Thanks in advance --GL
Greg and all,
This is REALLY tough to visualize, especially if you attempt to
simultaneously take into account coil "handedness", voltage polarities,
and current directions. For coaxial coils wound in the same sense, the
sign of the mutual inductance, M, will be positive. Although this helps
a bit, voltages and currents are still 90 degrees apart in both the
primary and secondary, and it gets much more confusing when you throw in
dual-resonance energy transfers! I took some Excedrin, put on a pot of
coffee, and decided to simplify the problem by ignoring transmission
line/helical resonator effects. After trying in vain to simultaneously
use the right hand rule, then the left hand rule, my fingers ended up
all tied up in knots... some experiments were clearly in order!
I made a couple of small coils, each with about a 2" long single-layer
winding of #21 AWG on 3.5" long PVC forms (about 65 turns each). The
smaller diameter secondary winding was on a 1/2" Schedule 40 PVC (about
13/16" OD), and the primary on 1" Schedule 40 PVC (about 1 5/16" OD).
Both coils were carefully wound in the same direction. The smaller
secondary coil fit nicely inside the primary, and the degree of coupling
could be easily adjusted by sliding the secondary to various depths. Two
types of experiments were run - simple transformer and dual-tuned
circuits under transient conditions.
Simple Transformer Case:
========================
In the first case, the bottom of each coil was connected to ground. The
primary was connected to a B&K 3011 function generator, with the signal
coming in the top of the primary winding. The top primary was connected
to channel 1, and the top of the secondary winding tied to channel 2 of
a Tektronix 434 Storage Scope. A 150 Ohm loading resistor was also added
across the secondary. In all cases, and with any degree of coupling, the
primary and secondary voltages were in phase. This phasing held
independently of any secondary current flow (with/without the 150 ohm
loading resistor). This implies that identical coil handedness would be
the desired mode for lowest secondary:primary voltage stress. As
expected, this phase relationship held irrespective of whether the
driving function was sinusoidal, triangular, or square-wave.
Dual Resonant Mode:
==================
Lumped capacitance was added to the primary and secondary, and the
resonant frequency of each circuit was measured independently (i.e.,
with NO coupling). A 0.047 uF cap was connected directly across the
secondary's terminals, and the resonant frequency of this combination
was found to be about 123 kHz. Similarly, a pair of 0.047 uF caps were
connected in series, and also in series with the primary coil. The
isolated resonant frequency of this was found to be about 120 kHz -
close enough! The signal generator was adjusted provide a low duty cycle
rectangular pulse signal, through the primary caps and the primary.
With the secondary uncoupled from the primary, I adjusted the
pulse-width and duty cycle of the pulse so that it would "shock excite"
the primary on every edge transition, causing the primary to ring and
then die out before the next transition. Fortunately, although the
relatively high source impedance of the function generator makes for a
very lousy primary Q, the relative phasing can still be observed. Since
the primary and secondary are "tuned" to the same frequency, the
behavior appears to be similar to a 2-coil system. While the
primary-to-secondary interactions are more complex, using the same coil
handedness is still preferred, particularly so for higher coupling
coefficients.
Hope this makes sense...
-- Bert --