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Re: Guide 60HzMatch
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> From: Tesla List <tesla-at-poodle.pupman-dot-com>
> To: Tesla-list-subscribers-at-poodle.pupman-dot-com
> Subject: Re: Guide 60HzMatch
> Date: Wednesday, January 29, 1997 11:10 PM
>
> Subscriber: Esondrmn-at-aol-dot-com Wed Jan 29 21:02:07 1997
> Date: Wed, 29 Jan 1997 15:25:54 -0500 (EST)
> From: Esondrmn-at-aol-dot-com
> To: tesla-at-pupman-dot-com
> Subject: Re: Guide 60HzMatch
>
> In a message dated 97-01-29 02:04:19 EST, you write:
>
> << All,
> I need feedback on the following possible entry for the Guide. (It is no
> longer the Idiot's Guide. Just the Guide).
>
> Critique style, tone, syntax, grammar, but especially home in on
content,
> useability, etc. For all I know, this may be totally wrong.
> If you have ANY relevant material, please e-mail to me Re:Guide
60HzMatch.
>
> *** Beginning of article
> MATCHING THE IMPEDANCE OF THE TRANSFORMER AND THE CAPACITOR
>
> This section discusses material that affects the efficiency of
operation.
> It also affects the quality of the spark produced at the spark gap.
>
> By carefully matching the impedance of the transformer and the
capacitor,
> the efficiency of the circuit can be improved. Matching will allow us
to:
> a) Get maximum delivered power from the transformer.
> b) Use the smallest capacitor necessary to store the power delivered by
the
> transformer.
>
> The transformer secondary winding has a characteristic impedance at
50/60Hz
> that will limit the maximum current that the transformer can deliver.
You
> can model the transformer as a perfect voltage source in series with an
> inductor and a resistor. The resistance is caused by
> the length and diameter of wire used in the construction of the
> transformer's secondary. The inductor represents the inductance of the
> transformer secondary, which causes its AC resistance at 50/60Hz. {Fig
??
> Eq ??}
>
> Methods for measuring actual transformer secondary impedance are given
in
> {Ref Transformer Sec ??}
>
> The current available at any given instant from a given transformer is
> limited by the transformer's instantaneous voltage and total effective
> resistance at that instant.
>
> If a capacitor and an inductor are connected in series, and if the
> inductive reactance and the capacitive reactances are made to be equal,
> then the effective total reactance equals ZERO. For a given inductance
and
> capacitor, this will occur at only one particular frequency, known as
the
> resonant frequency. {Eq ??}
>
> Note: Some authors prefer to refer to this as the anti-resonant
frequency,
> because at this frequency the overall Series Circuit resistance is at
its
> Minimum. In a Parallel Resonant Circuit the resistance is Maximum at
> Resonance.
>
> Note: Do not confuse this 50/60Hz resonant frequency with the resonant
> frequency of the RF portion of the Tank circuit, or the self-resonant
> frequency of the secondary.
>
> You might object that it doesn't LOOK like the transformer and capacitor
> are in series, but they are. That is because we can consider the circuit
AS
> IF the transformer was a perfect voltage source in series with a
resistor
> and an inductive reactance, and the capacitor as if it were a perfect
> energy storage device in series with a resistance and a capacitive
> reactance.
>
> At Resonance the circuit reverts to acting like a perfect voltage
source, a
> resistance, and a perfect energy storage device connected in series,
> because at resonance the reactances cancel. At resonance the transformer
is
> able to transfer the maximum current possible to the capacitor.
>
> If the Spark Gap were to fire at the instant the above mentioned Maximum
> voltage was reached across the capacitor, then the system would be
firing
> at a point where optimum energy useage was being achieved.
>
> It has been shown that for optimal efficiency (and operation) the power
> capacitor's capacitive reactance should be equal to the inductive
reactance
> of the transformer. When this is done the capacitor's capacitance rating
> will be the smallest value possible that will allow that particular
> transformer to transfer the maximum possible charge within a given half
> cycle. {Eq ??}
>
> If the inductive reactance of the transformer is greater than the
> capacitive reactance of the capacitor, then this results in a lower
maximum
> voltage appearing across the capacitor, with resultant reduction in the
> total charge transferred to the capacitor during the charging time. The
> capacitor is under-utilized. This means that the capacitor is actually
> larger and more expensive than it has to be. The Tesla coil will work,
but
> efficiency is lower than it could be.
>
> If the inductive reactance of the transformer is less than the
capacitive
> reactance of the capacitor, then the transformer will cause the
capacitor
> to reach its maximum voltage early in the charging cycle. In this case
the
> transformer is under-utilized. As we shall see later, this can create
> excessive flame arcing at the Spark Gap. {Ref Spark Gaps}
>
> {Prog ??: Transformer and Capacitor Matching}
>
> Additional Remarks:
>
> If RF chokes are introduced into the transformer circuit, then their
> inductive reactance at 50/60Hz may have some effect on the total
inductive
> reactance, though it is generally small compared to the inductive
reactance
> of the transformer's secondary.
>
> The RF impedance of chokes does not enter into this
> discussion of matching, because in matching we are only concerned with
the
> charge cycle, not the discharge cycle.
>
> The inductive reactance of a transformer's secondary may vary. This can
be
> due to such things as Saturation of the core, or the inclusion of
> resistances and/or inductances in the PRIMARY circuit. This is
especially
> the case with pole pigs, as they are often fitted with things like arc
> welders and heating coils for use as ballast. {Ref Pole Pigs Sec 112}
The
> use of a variac will affect the secondary's inductive reactance, because
a
> variac is actually an autotransformer. With some of the larger Tesla
coils
> it may be adviseable to measure the actual inductive reactance of the
> composite secondary rather than trying to determine it with a formula,
> since some of the factors entering into play are not readily known.
>
> Neon transformers sometimes have Power Correction, Magnetic Shunts, and
> other features that may affect the actual inductive reactance. {Ref
> Transformers}
>
> If a transformer has been modified by removing magnetic shunts, then the
> current rating will be greater than that indicated by its original
> nameplate. {Ref Modifying Transformers}
>
> *** end of article
>
> 1) OK, what do you think about this as a possible entry in the Guide?
> If you think an equation is necessary, useful, available, just say
> so. If you know the equation, please supply it. Identify all
> variables. I will attempt to normalize equations so they are
> somewhat consistent.
> What is wrong? If you can, write what you would propose to put in
> its place.
> What is missing? Be as explicit as you can.
>
>
>
> 2) I need the actual formula for computing transformer impedance.
>
> Someone on the list said Ztrans=Etrans/Itrans. Is that right?
> Does that mean that a Neon with a tag that says 12KV -at- 30ma
> has Ztrans=12000/.030 ? That seems AWFULLY High!
> That implies that a 12KV Neon produces 30ma when you short
> out the secondary and its voltage drops to ZERO.
>
> That further implies that the Neon WASTES 360 Watts
> when the current is 30ma. Can't be, can it?
> Isn't the 30ma the rated continuous current.. and isn't
> 12000 the OPEN circuit voltage with NO LOAD?
> (Or have I misunderstood the tag rating?)
>
> I am more familiar with regular transformers, where
> the power rating is the product of the voltage at
> rated current, and the rated current.
>
> Shouldn't the circuit be loaded down
> until it produces the 30ma and then measure the voltage...
> Then the difference in voltage from 12000 would be the
> voltage being dropped ACROSS the impedance at a draw of 30ma
> and the formula would now be:
> Ztrans=(E1-E2)/I where E1=open circuit voltage (12KV)
> E2=voltage at terminals when load draws rated current (Say 11KV)
> I=rated current (.030 Amps)
>
> ****
>
> Thank you for reading this. I would appreciate any constructive
criticism
> you might have regarding this particular article, or the Guide itself.
>
> Fr. Tom McGahee
> >>
>
>
> Fr. McGahee,
>
> I have two thoughts on the subject. One is maybe we should mention the
use
> of Power Factor Correction caps to be used with neon sign transformers to
> reduce the input current requirements and thus the size of the variac
needed.
> I would imagine most Tesla coils do use neon transformer power due to
their
> cost and availability.
>
Ed,
(I got your coil revision and have updated the original file)
Yes, Power Correction Caps are important, and should be included. I found a
little on this by Richard Quick. What I'll need to know from the list
members is what values to use for the most popular neons (9K 12K 15K at 30
60 90 ma). I don't have any data on the values yet. I hope someone reads
this and lends a hand! I don't know whether there is a formula people are
following, or just word of mouth.
> The second thought is that I think this idea of matching the tank
capacitor
> to the transformer confuses many folks and sends them off down the wrong
> track. If they get the transformer first, then calculate the matching
> capacitor, they end up trying to design the whole Tesla coil project
around
> the capacitor.
I agree totally, and I am re-working the text to indicate this fact right
up front, like in the second paragraph or so.
> I like to look at the problem differently. First I decide
> how much power I am going to be working with and how much space is
available
> in the basement, garage, etc. and decide how large I want to make the
> secondary. For me, the design of the secondary is always first.
This is my belief also, as it is this component more than any other that
really governs the whole design process. This is where the resonant
frequency is really determined.
> Then I
> calculate it's resonant frequency - unloaded and loaded. Then I design
and
> calculate the inductance of a suitable primary. This is easy for me now
as I
> will always use a flat spiral primary with 14 turns. The only decisions
are
> the wire or copper tubing size and the spacing between turns. Now I
> calculate a capacitor that will resonate with the loaded secondary using
> about 80% of the primary to allow for tuning and larger toroids. At this
> point you now need to make sure your transformer has enough current
capacity
> to fully charge the cap.
>
> Does this make sense?
>
Sure, it is the same way that I think most of us REALLY work it up. And if
you don't have enough current capacity to charge the capacitor to the
desired voltage in the desired time, then you throw more neons at it until
you do. The thing is to use equations wherever they are available: not as
gods in their own right, but as guides so that we don't get too far off.
By the way, it seems to me that the firing voltage of the gap is sort of
very important here, because it enters into the charge time part of the
equation. A smaller spark gap means a quicker charge time (all other
factors being equal, of course!) with resultant lower energy per discharge.
Seems to me that the matching would be somewhat affected by the firing
voltage and repetition rate desired. Or do we not speak of desired
repetition rate and instead just take what we get? (In which case it seems
to me that the product of the repetition rate and the energy per discharge
would then yield a constant value for a given transformer). Yet I haven't
seen mention of these factors in the few postings that I have found on the
subject. Anybody out there know if my conjectures here are anywhere near
the mark? Are there any formulae out there that seem to work? If so, please
let me know.
> Ed Sonderman
Ed, thanks again for your taking the time to respond, and for the
thoughtfulness of your response. Thanks also for your posting to Project
Coiler. At present I only have responses from about six individuals.
Hopefully more will trickle in soon. We really need some good standardized
coil data so we can all improve out coiling.
Fr. Tom McGahee