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Re: Building your own HV transformer



Hi Chris,

Depending on your background, this is may be too basic or not basic
enough.

You might have to read a few books.  I don't know what your background
is, but a basic understanding of alternating current circuits is
necessary.  "Basic Electricity"  Dover Publications, $10.95, ISBN
0-486-20973-3, is a US Navy text, and explains it very well, with a
minimum of math.

You don't (technically) have to use Iron for the core.  Iron will have
at least 800 times the permeability of air.  What that means, is you
would need 800 times more primary turns (at 60 HZ) in air, to achieve
the same inductance you will with one turn around a piece of iron.  It
just isn't practical.  Some 'designer' alloys have many more  times the
permeability of ordinary iron.  They also cost a lot.  "Basic
Electricity" shows the permeability of cast iron at around 1,400 at its
peak (permeability rises as flux density increases, up to a point).

I'm trying to keep it real basic here, and put it in terms of someone
with very little electronics experience can understand.  I'm leaving
some things out, for that reason.  

***********

The terms:

VOLTAGE:  Electrical pressure.  (sort of how enthusiasticly electrons
want to get from point A to point B).  The unit of measure is the Volt,
and the symbol is E.  (Electro Motive Force, or EMF)

INDUCTOR:  to keep it basic:  A coil of wire.  The core of the wire coil
can be any material.  (the inductor is the physical thing, the rest of
the terms refer to properties of an inductor)

INDUCTANCE:  If a coil of wire is placed in a changing magnetic field,
it will produce a voltage.  If you put current through a coil of wire,
it produces a magnetic field.  The ability to produce a field (or
voltage) is inductance.  Raise the turns, you raise the inductance.  It
is expressed in Henries, the symbol is L.

REACTANCE (inductive):  If you put Alternating Current (AC) through a
coil of wire, the self-induction (converting its own magnetic field to
an opposing voltage) resists the flow of current (all the while
producing a varying magnetic field).  This opposition is proportional to
the frequency of the applied voltage.

The resistance, of an inductor, is different than an ordinary resistor,
in a circuit.  An inductor returns most of its energy back to the
circuit.  Only the energy given to heat the DC resistance of the wire,
is lost.  A pure reactance would be 100% efficient.  There are no pure
reactances outside of superconductors.  Expressed in OHMS, The symbol is
X (sub) l. (lowercase L)

RESISTANCE:  Opposition to current flow, of any polarity, or frequency. 
(Similar to trying to cram too much water through too small a pipe). 
Expressed in OHMS, the symbol is an upside-down U.

IMPEDANCE:  The combined effects of inductive reactance and resistance,
impeding the flow of current in an AC circuit.  It is expressed in OHMS,
and the symbol is Z.

PERMEABILITY: The ability of a material to conduct (and concentrate)
magnetic lines of force (flux).  Increase permeability, and you increase
inductance. Expressed as mu, symbol u (same symbol as 'micro').

*******

Reactance, Inductance and Impedance, are all expressed in OHMS and can
be used in "ohm's law."

Ohm's law just states the if resistance (ohms) increases, current
decreases (assuming a constant voltage).  If voltage decreases, current
decreases (assuming a constant resistance).

                    E
The formula is R= ----
                    I

***********

The crux of the matter:  If the primary of your transformer doesn't have
enough impedance (for our purposes reactance, since it doesn't consume
power) too much current will flow through the primary, melting the wire
or saturating the iron.

Saturation occurs when you try to put too many lines of magnetic force
in a material that can't conduct any more.  The current in an ordinary
transformer rises rapidly as the ability to conduct magnetic force is
exceeded.  An open core or gap in the core will limit or (for practical
purposes) eliminate the ability to saturate the iron.  A closed core has
a much higher efficiency than an open core. (higher inductance per turn
and greater ability to transmit power - the liability is that if you
short the secondary with a spark or other conductor, a closed core
transformer will self-destruct if you don't add some means to limit the
current,  called "ballasting" the transformer)

A transformer is two coils of wire with mutual magnetic coupling.  If
you provide an AC voltage to one coil, its mate will show a voltage
across its terminals.  The voltage is directly proportional to the turns
ratio.

So if you are building a transformer, you want enough Inductive
Reactance to limit the primary current to reasonable levels, yet to keep
the turns ratio in your favor, you don't want too many primary turns of
wire (when designing a HV transformer).  If you were designing a low
voltage transformer, you would still limit the number of turns to some
compormise between inductance and resistance (resistance dissipates
energy as heat and doesn't return it back to the circuit)  If you are
designing for efficiency, you try to hit along the peak of the
permeability curve with a minimum of resistance (transformers can be
over 90% efficient).

To decrease the primary turns (which directly impacts the number of
secondary turns required to achieve the same output voltage) you must
increase the cross-section of the iron, or increase the inductive
reactance in some other way (like increase the frequency - which has the
effect of exacerbating eddy current losses in the iron).

So on paper, a single turn primary and hundred turn secondary will yield
12,000 volts out with 120 volts in.  In practice (at 60HZ), the
inductance would be too low (unless the core were composed of tons of
iron) and the primary would convert to heat, light, and metal vapor, or
a circuit breaker would open.  A practical 60 HZ transformer will have
at least a few hundred turns of wire on the primary.  A small
transformer would require more turns than a larger one.

The iron, and the way it is made, affect the permeability.  Grain
oriented, annealed, silicon steel is the most power/cost effective
transformer iron.   As iron leave the foundry, the crystals are all
jumbled up.  Rolling or pulling wire through dies orients the grain
(metal crystals) and makes the iron denser.  Heating the iron to a high
enough temperature and allowing it to cool slowly, anneals it (makes it
less springy, and allows it reverse its magnetic polarity with less
reluctance).  Silicon keeps it from losing some of its inductive
properties over time and use.

Tesla coils are resonant air-core transformers.  They work with few
turns in the primary and no iron in the core, because the frequency is
high.

I've attempted to outline the basics.  If you were designing
transformers for a living, there is more to learn.

If I can be of further 'help' (assuming this is help) please E-mail me
directly.

take care
bob