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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