[Amps] Magnetic shielding 2

[Borislav Trifonov]

I just want to let you guys know you've helped out a lot with the 
various suggestions, even though I'm not in radio, so I'm very grateful.

OK, say I was to start from my requirements and fitting a transformer to 
the application, instead of the other way around.  Now, my load is DC 
plasma modulated at audio frequencies (above 500 Hz).  I think I 
mentioned this on this list under another topic.

Here's a simulation of my current power supply with the expected load 
(represented by the current sink): http://www.cs.ubc.ca/~trifonov/hv.png

The transformer is just the two coupled inductors, as simulation is too 
slow with more complex transformer models.  I've simulated the lowest 
frequency I expect to draw, 500 Hz.  Total (for both channels) is 380 mA 
sine peak (not peak to peak as LTSpice uses peak), over a 400 mA DC 
offset.  I need at least 2 kV after the filter, with a bit of leeway for 
inserting a regulator.  The rectifiers I have drop about 15 V each.  The 
choke in the diagram is 4 H (estimated by calculation, as I wound it by 
hand on a large core).

So my first question is about something I don't understand at all.  When 
I ask the simulator for the RMS through the current source, it says 
478.43 mA.  However, when I ask for the RMS through the secondary 
(represented by the second inductor), I get 783.31 mA.  Sorry if this is 
a stupid question, but why is that?  Should I assume that the second 
number is the actual rating I should be looking for in a transformer, or 
is LTSPice giving me erroneous numbers here?

I'm waiting for a 2 kW industrial control transformer to come in the 
mail.  If the secondary 120 V windings are not on top of the 600 V 
primary, I could rip off the 600 V winding, rewind for 2 kV and drive it 
backwards.  But before I do this, I'm wondering if the smaller MOT I'm 
using right now is big enough to be rewound in the above application. 
When I multiplied the secondary current waveform by the voltage across 
it, LTSpice says an average of 939.97 W, so I figure the MOT core should 
handle it.  But again, I don't know if I can trust those numbers to be 
even approximately close.

The MOT is 16 lbs.  Core (center beam) crossection is 41x54 mm^2, or 
3.43 in^2.  Using 12000 G in the formula, I'm calculating 189 primary 
turns.  Currently primary is 14 AWG and secondary is 18 AWG, with space 
left from the removed shunts.  Assuming the RMS current LTSpice is 
giving me on the primary, 13 A, is correct, then I should be using AWG 9 
which would fill up the whole MOT core space...what the...  I hope I'm 
mistaken in these calculations, or the 13 A is wrong (though it probably 
isn't much smaller given MOTs have bad power factors).


Will Matney wrote:
> Borislav,
> 
> 19 kilogauss is too high for M-6 or Hipersil material for about 28 or 29 gauge. It's max is about 17 kg., and it's recommended to be ran at 15 kg. because after 15 kg., the magnetizing current raises sharply and doesn't gain that much for it. After about 18-19 kg, it goes into saturation where the permeability drops off sharply, you get a distorted waveform, and it starts to act like a short. Then it will finally burn out. Also, did you see the correction for the formula from using 4.44 and use 1.11? I messed up there when I was in a hurry and didn't proof read it. If your using the 1.11, it's correct.
> 
> I'm afraid there's no other fix but to rewind it to make it work correctly. Even potting it will still leave it saturating, plus running hot. Eventually under load, it will finally burn out. If this will be ran ICAS as most amateur amps really are, you can get by with using about 700 to 800 circular mils per ampere for sizing the wire. If you want CCS, then it needs to be ran over 1000. The problem with 1000 is you may not be able to fit all the wire on the core. If you have the availability of extra wire, I'd cut the whole coil off and start from scratch making a new bobbin too. For layer insulation use 7-10 mil paper, and between the different windings use 15-20 mil. Use about 40 mil for the bobbin. There's several places on ebay carrying fish and kraft paper pretty cheap. That's the only solution and the correct cure I know. Maybe others on here may know of something else to do with it.
> 
> Best,
> 
> Will