Re: acmi error found? - Measuring techniques?

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Ross and all,

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. This question also came up
recently on teh Tesla-2 list.

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. 

Now, remove the secondary and physically move it so that it's FAR away
(>6 feet) from the primary. Remove the grounds and short that were
previously attached to the primary. In the case of a disruptive coil,
connect the tank cap across the primary by bridging your main spark gap
with a shorting wire. Remove the HV connections going between your NST
protection circuitry and the main spark gap - this is especially
important if you are using older style LC or RLC protection networks to
prevent spurious resonant points. In the case of a Vacuum Tube Tesla
Coil (VTTC), the tank capacitor will normally already be connected in
parallel with the primary winding. Now measure the resonant frequency of
the primary LC circuit. Adjust the primary tap (or tank capacitance)
until the primary circuit's resonant frequency is the same as that
previously measured for the secondary (Fo). Reconnect the HV lines to
the spark gap, and remove the short across the main spark gap (in a
disruptive system). Now reinstall the secondary. This should put your
system in proper "instrument tune". 

One more note with respect to VTTC tuning/operation: 
If you have independently instrument tuned both the primary and
secondary to Fo, your system will actually operate at a different
frequency! Most VTTC's operate at coupling coefficients that are much
greater than critical. The "double hump" frequency response results in
TWO possible preferred frequencies during coil operation, neither of
which is Fo. The primary LC circuit never becomes "disconnected" as it
does in a disruptive system when the gap quenches. Since the two LC
circuits are ALWAYS coupled together, your VTTC will actually prefer to
oscillate at the sideband frequency having the highest peak (usually the
lower one) and not Fo. However, it's quite possible that your coil may
start operating at one sideband frequency, then suddenly shift to the
other with changes in proximity or secondary loading. This is normal and
should be expected. Your system IS in proper tune - it simply won't be
running at Fo...  :^)

Hope this helped! 

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "Ross Overstreet by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <ross-o-at-mindspring-dot-com>
> 
> This brings up a question that I have had for some time now... How should we
> be measuring primary and secondary resonant freqs?  Should we measure the
> freqs with the coil in "operating" configuration or should we separate the
> primary and secondary.  I'm guessing that we should separate when we are
> comparing the resonant freq to values calculated using computer programs and
> we should measure it in place when trying to verify that the secondary and
> primary freqs are the same.
> 
> This is really important to me right now since I'm trying to get my big tube
> coil running.
> 
> Happy Holidays,
> Ross-o
> Austin, TX
> http://users.better-dot-org/roverstreet/
> 
<SNIP>