Re: acmi error found? - Measuring techniques?

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "R.E.Burnett by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <R.E.Burnett-at-newcastle.ac.uk>
> 
> Hi Bert and all,
> 
> Well it's rare that I disagree with you Bert ;-)  but I think something
> seems wrong in the method you described below:
> 
> > Original poster: "Bert Hickman by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>
> >
> > Once the primary and secondary LC circuits are significantly coupled to
> > each other, it is difficult to independently measure the primary or
> > secondary resonant frequencies. The following process permits proper
> > measurement and setup for the primary and secondary by preventing their
> > interaction as two tuned LC circuits....
> >
> > The loaded secondary's resonant frequency should be measured with the
> > secondary/topload installed at its intended operating position within
> > the primary. However, while measuring the resonant frequenmcy, both the
> > inner lead and tap point of the primary should be shorted together and
> > tied to ground. This will prevent the primary LC circuit from affecting
> > the measurement. The primary is still "there" with respect to capacitive
> > coupling to the secondary, but its no longer a tuned circuit. Measuring
> > the resonant frequency of the secondary/topload in this fashion will
> > measure it's self-resonant frequency (Fo) when in its operational
> > environment. Record this value.
> 
> If you short circuit the primary,  it would seem to me that this short
> circuit is reflected to a portion of the secondary coil determined by the
> coupling coefficient.  In a way you are "shorting-out" the inductance
> which is common to both pri and sec.    

Comment:  Shorting the primary screws up any measurements on a coupled
secondary.  Likewise, shorting the secondary screws up measurements on
the primary.  As long as the coils are coupled, accurate measurements
cannot be made on either one, AT LEAST AT RF anywhere near the resonant
frequency of the secondary.  The resonant frequency of the OPEN primary
is so high it can't affect any secondary measurements significantly.. 
Since the resonant frequency of the secondary is large compared to, say,
1 kHz, measurement of the primary circuit can be made without
significant error.  Measurement of the mutual inductance made by passing
a known 60 Hz current through the primary and measuring the secondary
voltage should be valid, whether or not the secondary is loaded. 
However, in this sort of measurement it is important to measure ONLY THE
60 Hz COMPONENT of the secondary voltage.  If the input current has
harmonic content, either due to using some sort of non-linear current
limiting or distortion of the line voltage due to non-linear loads, the
measured mutual inductance will be too high because the mutual reactance
will be higher at the harmonic frequencies, giving more voltage from a
given primary current than would be calculated for 60 Hz excitation.  I
think even a few percent harmonic distortion would be significant if one
is attempting to measure with the precision discussed here.  Note that,
in order to get the correct mutual inductance value, both the primary
ammeter and the secondary voltmeter must be accurate to at least the
desired accuracy, at the particular current being used and the
particular voltage being measured.  I don't have my electrical
measurements texts handy, but am almost sure there are ways to measure
mutual inductance with bridges which probably don't have this source of
errors.

For this reason I would expect the
> secondary to resonate at a higher than normal frequency due to its
> coupling with the shorted primary coil.  The higher the coupling the more
> the secondary frequency will be shifted above its natural Fres ???

	Absolutely.

> 
> (I thought you could actually use this method to find the K factor ???)

	You can. Look it up.
> 
> I would agree with leaving the primary winding in place with _ONE END_
> grounded,  and all other connections removed.  But, I don't beleive it
> should be shorted out ???

	You're right.  As stated above, it must be OPEN.
> 
> This should give an accurate Fo value for the secondary below breakout.  I
> think Fo drops by 5% or so during breakout though !

	Think that effective streamer capacitance would be a function of
streamer size, which is inturn a function of power input, so such a
simple fraction would be at best an approximation.

> 
> Does anyone else have any thoughts on this ???

	See above.
> 
> I Hope everyone had a good Christmas,  and I wish you all the best for the
> new year,
>                                                 Cheers,
> 
>                                                 -Richie,

Ed