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" > 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? > > _______________________________________________ > > 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