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Lundahl Power Transformers best

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Posted on September 30, 2009 at 18:46:06
Leo Mmmm
 Kevin I have read material posted on Lundahl power transformers claiming that DC on the mains is potentially saturating transformers or causing many to hum. I personally have measured between 60 to 90 mv of dc on the mains at my home. The gap used by Lundahl is supposed to prevent this problem. Kevin what is your experience with power transformers?

I also wonder how effective the Lundahl is at dealing with noise which is not common mode given its C core design? I wonder if a sloppy EI core many have a lower bandwith.

RE: Lundahl Power Transformers best, posted on October 1, 2009 at 18:53:29
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 Here are Per Lundahl's comments regarding your questions:

Mains transformer air gap and core saturation.
Theoretically there is a risk that the core in high mu core mains transformers saturate as a result of DC from the mains. Biggest risk is probably in toroid transformers. In C-core transformers and lamination core transformers there are always airgaps. To be sure that the airgap is big enough, and consistent between individual transformers, we gap all our mains transformers. I must admit however, that it is a decision based on theoretical discussions, and not on experimenting. Thus we (Lundahl
Transformers) have no real experience from power supplies with saturating mains transformer cores.

What you can expect when the core saturates is current spikes (as transformer inductance drops suddenly with each period). The current spikes can create ground noise and magnetic field noise which can be picked up by other devices. It can also create a noise in the mains transformer, mainly from the coils due to magnetic forces between adjacent wires.

Differential mode noise
A transformer with bad coupling will have an increased leakage inductance and thus a reduced bandwidth. It will act as a low pass filter.
But the leakage inductance is actually magnetic energy stored in the air around the transformer. Thus you can understand that such a transformer will also have a bigger magnetic stray field. I do not think this is the right way to go. I think it is better to try to stop HF noise in the chokes or capacitors of the power supply.

Kevin Carter
K&K Audio
www.kandkaudio.com
RE: Lundahl Power Transformers best, posted on October 2, 2009 at 16:33:11
Leo Mmm
 Thanks Kevin
My own experience has shown that EI power transformers create large magnetic fields and negatively affect sorrounding circuitry. I will be giving the Lundahl power transformers a try.

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