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## Re: [Amps] Magnetic shielding 2 Correction

 To: amps@contesting.com Re: [Amps] Magnetic shielding 2 Correction "Will Matney" Mon, 20 Jun 2005 13:47:22 -0500
 ```Borislav, I done it again, I will have to start proof reading better. I get in a hurry and hit the send button way to fast. OK, I said; "Then the primary current is the secondary current plus the losses". That should have said power and not current or; Then the primary power is the secondary power plus the losses. Sorry about that. In other words, wattage in equals wattage out plus the losses. Best, Will > > Borislav, > > 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 be. > > Best, > > Will > > > > > > > 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 > > _______________________________________________ > > Amps mailing list > > Amps@contesting.com > > http://lists.contesting.com/mailman/listinfo/amps > > > -- > ___________________________________________________________ > Sign-up for Ads Free at Mail.com > http://promo.mail.com/adsfreejump.htm > > _______________________________________________ > Amps mailing list > Amps@contesting.com > http://lists.contesting.com/mailman/listinfo/amps -- ___________________________________________________________ Sign-up for Ads Free at Mail.com http://promo.mail.com/adsfreejump.htm _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps ```
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