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Re: [Amps] Magnetic shielding 2

Subject: Re: [Amps] Magnetic shielding 2
From: "Will Matney" <>
Date: Mon, 20 Jun 2005 11:20:08 -0500
List-post: <>

The current drawn by the circuit shouldn't be any more than maybe 5% higher on 
the transformer side than what's on the supply side at the filter capacitor. 
Transformers are figured by power needed (wattage or volt amperes VA). Then the 
primary current is the secondary current plus the losses. One loss is called 
watts per pound loss and are the iron loss in the performance curves. You add 
the I^2 X R losses for the wire to it to determine the primary wattage and 
current. This is also its efficiency which will probably be about 80-85% at a 
worse case. So the power difference will be about 15-20% between the primary 
and secondary. I wouldn't doubt if the maker didn't wind the MOT with mybe 500 
circular mils per ampere which some appliances have. These run hot especially 
when a shunt is used. In your case though the shunt is gone and the core acts 
normally but 500 cir mils per amp is cutting it close. It's according to what 
the original primary and secondary wire size is. Another
  thing that might be screwing you up is just using two inductors for a 
transformer in the simulator. That then wouldn't have any iron in it and throw 
the baby out with the bath water. To roughly determine the core area(A), you 
can use A = 0.16 X sqrt of P (in watts) for 60 Hz. For 50 hz it raises by X 1.2 
or 0.192 X sqrt of P. That would get you in the ballpark of where you need to 



> 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):
> 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
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