[TCML] a compact 5.6 kVRMS ferroresonant transformer

[Chris <spamsandwich@xxxxxxxxxx>]

I'd like to report a successful attempt to wind a small high voltage
transformer on four steel C-cores. 

Each has a 20x40.5 mm cross-section and will support 1.9T. The assembly
supports one Volt per turn in saturation at 50 Hz. 

The cores themselves have the thinnest commercially available lamination
(0.23mm) and are potentially suitable for 400 Hz applications too.
 
Interestingly, these cores achieve the same energy density per unit
volume as a sensibly dimensioned Tesla coil capacitor bank built from
twelve of the much loved 942CD12P22K-F (1200 VDC 0.22uF) parts in
series.

The center-pole hosts a stack of eight tape wrapped, former-less square
windings each 1000 turns of 0.315mm wire. These are very easy to produce
on a winding machine between two saucer-sized plates on a mandrel. 

A square spacer with rounder corners and slightly beveled edges defines
the winding and makes it easy to release the wire from the mandrel.
Access holes in the saucers allow the first polyester tape loops to be
inserted to secure the wire bunch before completing wrapping off the
mandrel. 

Square windings are wonderful because the wire is bent and remains
square and orderly even when one thousand turns are held loose in the
hand. Each winding is about 80mm x 80 mm when finished.

Eight such windings are stacked into a 100 mm high stack and then
insulated from the cores with two layers of fiber-glass PCB material.
Most of the core window is assigned to high voltage creepage distance
with 10 mm being achievable.

Final assembly is done with stainless steel screw/band hose clamps with
an electrical connection being made from core to the mid-point of the
secondary and optionally, primary.

The core is driven from one or both of the outer legs. 

Self-resonance between the magnetizing inductor with the capacitor band
falls at 40 Hz. Cautiously energizing the core through resistors gives a
peak reactive power of about 60 VA (leading) since the capacitors are
driven above resonance without magnetic shunts and with very little
leakage inductance. This is unlike a NST design. 

After reaching a critical voltage, the core gives a loud buzz and enters
resonance with a sudden rise to 5.6 kV RMS (8000 Vpeak) at the
secondary. Input current drops back to about half the previous maximum
value.

Voltage regulation is remarkable. An external Ohmic or inductive shunt
is required unless a steel magnetic shunt can be bolted on externally. 

It looks possible to drive just one leg of the core so that there is
minimal input current with a short on the secondary. In this case, the
flux path becomes a figure of eight.

Something between a hard, regulated voltage source a weak current source
should be possible by simply mismatching the number of turns on the two
outer legs.

Before connecting anything sparky, we should protect the windings from
RF energy with some series long (10 Watt) damper resistors of about one
to two hundred Ohms and perhaps some VDRs too. 

LC charging from the external inductive shunt may tend to double the
steady-state voltages at 50Hz to +/- 8 kV peak. Safety gaps of 7 mm
(needle) or 2.5 mm (sphere) would hold-off this doubled voltage. 

Of most concern is the strength of the secondary wire which is highly
stressed. Are eight winding segments enough?

Has someone tried this before?
      

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