----- Original Message -----
From: "Will Matney" <craxd@engineer.com>
To: amps@contesting.com
Subject: Re: [Amps] Magnetic shielding
Date: Sun, 19 Jun 2005 10:04:37 -0500
Borislav,
The form factor "F" below is 1.11 not 4.44. I was thinking of it multiplied by
4 but the 25.8 takes this into account for converting to inches. 4 X 6.45
(conv. factor) is 25.8. Actually this could all be rounded off to 25.8 X 1.11 =
28.638 for a sine wave. Sine wave = 1.11 and square wave = 1.0.
Best,
Will
>
> Borislav,
>
> I dont think coupling to the chassis is what is causing this to
> happen. Microwave oven transformers use pretty cheap steel, and
> they most likely should be ran around 12 kilogauss. Actually, any
> unknown steel should be ran around this. The duty cycle wasn't set
> very high either on those. I think the problem your having is still
> saturation. What you'll probably have to do is go back and refigure
> the turns per volt using 12 kilogauss as the maximum flux density.
> The shielding used was mainly to stop harmonics, etc. You can use a
> one layer winding for this too. Just hook one leg to ground and
> leave the other open. If it's connected, you'll have a short, even
> in using a piece of copper sheet or foil. Let's say if it had one
> turn per volt, that would be 1 volt at a lot of amperage or a
> shorted winding. This could cause one to saturate too. I have
> mounted a many of transformer on a steel chassis and never had a
> problem of having a big hum. If you think that may be the problem,
> mount
> the L brackets on top of some washers raising the core up off
> the chassis a bit. A small air gap means a lot here. Another way is
> to take off the L brackets. Get some extra long screws that hold
> the lams together. Run these down through some aluminum standoffs
> which are long enough that to coil wont hit the chassis. Then mount
> the transformer standing up on these standoffs-spacers.
>
> Formula;
>
> N = V X 10^8 / 25.8 X F X f X a X B X s
>
> B = V X 10^8 / 25.8 X F X f X a X N X s
>
> N = Number of turns in the primary
> V = Primary voltage
> F = Form Factor or 4.44 for a sine wave
> f = Frequency in Hertz
> a = Core area in square inches
> B = Flux Density ( use 12,000 here for Bmax and unknown )
> s = Stacking Factor, probably 0.90 here and determined by the lam
> thickness and steel type. It's range is from 0.85 to 0.95
>
> That should give you the correct number of turns in the primary
> winding. Then just figure the number of turns for the secondary
> from that as Np = Ns. Then multiply Ns by 5% for losses and you'll
> be very close to the output voltage you want. Remember that the
> primary is what determines everything including saturation under no
> load. Saturation under load is caused by more wattage than the core
> can handle. Hope this helps.
>
> Best,
>
> Will
>
>
> ----- Original Message -----
> From: "Borislav Trifonov" <bdt@shaw.ca>
> To: amps@contesting.com
> Subject: [Amps] Magnetic shielding
> Date: Sun, 19 Jun 2005 06:14:13 -0700
>
> >
> > I had a 1.5 kW microwave oven transformer (MOT) that I took apart,
> > removed the magnetic shunts, and when putting it back together, I
> > interleaved the laminations (originally, all the Es were together as one
> > E; likewise the Is). The transformer hums quietly on its own, but when
> > I put it in my project chassis, which is steel, I found out that it had
> > huge leakage as it made the chassis hum very loudly. Knowing that MOTs
> > are made with the bare minimum of materials, I figured that the core was
> > saturating and added 15% more turns to the primary (about as much as I
> > could fit). That helped a bit, but it was still pretty bad. I also
> > added a copper tape around the windings, outside the core, as the 'flux
> > band' sometimes seen in transformers (especially in audio equipment).
> > That made a small difference. The chassis still hums too loudly. I've
> > put in too much work in the current project chassis to replace it with a
> > non-steel one. So I'm wondering about magnetic shielding. Mu-metal has
> > very high permeability but saturates easily and is only useful for weak
> > fields. Often I've seen tube audio amp transformers either completely
> > encased in, or at least wrapped around the edges of the EI, with silicon
> > steel or soft iron, which reduces leakage flux. However, I actually was
> > not able to find any appropriate material. Any suggestions?
> > _______________________________________________
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> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
>
>
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