Re: Building your own HV transformer

Keep in mind I have little or no idea what I am doing. I know how a
transformer works, but you lost me with all the terms and stuff I
don't know about. I've seen all the pictures in books and the like
about what one looks like, but, I need just a little more detail. Like:

Do you have to use Iron for the core?

What are the core requirements, like how does it relate to voltage and
how are they setup (spacing, different configurations, details)?

What do more turns on the primary do, like what is the difference
between 2 turns and 20?

Tanks in advance to anyone who answers this.

---Tesla List <tesla-at-pupman-dot-com> wrote:
> Original Poster: Bob Misiura <misiura-at-nccoast-dot-net> 
> Great to see someone interested in building HV transformers.
> The Iron rods will surely cause eddy current losses.  Maybe he intends
> to run the thing at high loss for short periods.  Like he says "the
> design is untested."  I figure large number of typos in that article
. .
> . 500 feet of wire in the primary and 800 in the secondary, not too
> likely!  I might believe 8,000 but just don't buy only 100 turns in
> primary because I did build a open core transformer.
> I decided I needed a better transformer than the oil burner ignition
> transformer I was running and didn't really have a good idea where to
> find a suitable core.  Finding cores wasn't the problem, finding one
> with enough space to accommodate the insulation required was a
> I have a book "The Tesla High Frequency Coil," by Haller & Cunningham
> (originally 1910, but reprinted by Lindsay Publications in 1995). 
> Misters Hallar and Cunningham outline building a Tesla coil from
> scratch.  And I do mean scratch!  They give instructions on cutting
> laminations for the induction motor used to turn the rotary gap!
> The HV transformer they outline is a sort of big induction coil. 
> instructions call for a lot of stuff that isn't readily available
> anymore, but are easily substituted.  
> The core I used is a bundle of steel wire (about 11 awg) used for
> hanging ceiling support.  It is one and a half inches in diameter and
> twenty inches long.  The primary is wound in four layers of 350 turns
> each (I brought out the start and finish ends of the wire and can
> configure the primary for different turns ratio and primary
> With 120 volts, 60 cycles, I operate with two windings in parallel and
> in series with the remaining two, for a total of three windings or
> turns.  With 140 volts in, I use all four windings, and with a
> transistor chopped 100 volts DC, 200 cycles, I run all in parallel.  
> The primary (excitation) current with no load, and three windings in
> series, is on the order of one and a quarter amps.  Secondary output
> over 8,000 volts at a design nominal of 900 watts (easily pushed to
> KW, due to the size, and thermal inertia of the transformer).
> The secondary consists of four homemade bobbins wound with a total of
> 67,800 turns of 32 AWG magnet wire.  The inside bobbins have 55 layers
> each and the outside 45 layers each.  I used one section of 2" PVC
> for the primary to secondary insulation, and the bobbins were made
> 2-3/8" ID tubing (Plexiglas end plates) placed over the primary
> insulator.
> I rigged a vacuum impregnation system using a pressure cooker and
> pump and impregnated each coil with liquid paraffin wax, assembled the
> transformer in a oak box and back-filled the box with molten wax.  
> If I use the transformer as an induction coil, the output voltage is
> to 80 KV chopped at a 80 HZ rate, with a 12 mfd capacitor across the
> 'points.'  The tesla coil is ecstatic  with the increased voltage and
> rep rate, but it is hell on primary capacitors.  The 12 mfd has to
> endure a spike of ~600 volts on it.
> To characterize the iron (they just don't include hysteresis curves
> ceiling wire) I wound a smaller test coil and scaled it up for the
> coil.  I arbitrarily set a limit of 1 amp, figuring that %10 was a
> reasonable amount of power to waste to excite the iron.  The
> is directly proportional to the cross section of the iron in the core
> (allow about one diameter to stick out either end past the primary). 
> The inductance is proportional to the square of the turns (increase
> turns by a factor of two, increase the inductance by a factor of
> An open core will not saturate at any reasonable input voltage (the
> primary would melt first).  An open core does tend to self limit
> to an extent.  I run un-balasted at all times. If I want to lower the
> current I lower the input voltage, increase the primary turns, or
> increase the speed of the rotary spark gap (unsynchronized, 6000 RPM,
> eight breaks per revolution, with two fixed gaps in series)
> My primary winding and iron can be slid out of the box because the PVC
> tube protrudes from either end of the box and isn't encased in wax. 
> I've experimented with finer and coarser wire with very little
> difference in the performance at 60 cycles (at least in the 12-18 AWG
> range).  The gauge of the core wire makes a profound difference in the
> operation as an induction coil.  (I even tried running steel wool
> through my garden shredder to make powdered iron - the permeability
> faster than the eddy currents at 60 cycles - but allowed operation to
> over 1 KHZ - and colored everything within about twenty feet of the
> shredder a nice rust tint in a few days)
> It only took seven days to wind the secondary, using a primitive motor
> driven winding lathe.  There is about 13 miles of wire in the
> secondary.  It took about six months to complete the entire project,
> from planning to playing.  It wasn't particularly hard, and was very
> rewarding.  I was sorry to finish it; building it was fun.
> The three phase transformer sounds like it might be a winner if you
> enough room for the insulation.  Do you plan to cut part of the core
> away, or wind the primary/secondary in pairs on the outer legs and
> them in series?  
> Don't be too tempted by high a secondary voltage, the insulation
> requirements in the transformer get hairy over ~5,000 volts/winding,
> the TC's capacitor design gets bigger and more expensive.  
> Consider the weight of the finished transformer, mine weighs in at
> fifty pounds and I neglected to allow for handles on the case. 
> If I do another one, I will increase the size of the core to allow a
> more winder-friendly turns ratio.
> To characterize your three phase core, you only need to wind a test
> with a few hundred turns of wire and use a variac to bring up the
> voltage on it.  If and when you reach saturation, the current really
> takes off with a small increase in input voltage.  Bear in mind the
> current will drop a little as the primary wire heats because of
> resistance increases at higher temperatures.  The core must be
closed to
> see a sharp rise in current, even a small air gap will cause a sloppy
> 'knee' in the curve.
> The existing primary turns and voltage rating, if you have them, may
> give some information.
> My copy of the ARRL Handbook, 1990, devotes about three pages to 60HZ
> transformer design, but they are chock full of the basics.  If you
> to go with the transformer/induction coil route (I recommend it) get
> Lindsay's reprint, make the cross section of the core larger than
> 1-1/2," and put handles on the box (or wheels).
> Post your experiences! 
> good luck.

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