Sourcing shielded power transformers

[coppercone2]

https://www.farnell.com/datasheets/2007294.pdf

yes the measurement is not worth while because the span is almost 70% with the data sheet, but it does still show you that things like bolts are flux concentrators, and that there is more energy near the corners of a transformer (you can still do relative tests compared to a R core or doughnut).

a zero Gauss chamber to find a null and then using the earths magnetic field might be the easiest way to determine sensitivity.

still useful to see if there was an effect before and after with something like a shield or component being installed.

[Sengcid]

Hello,

The main problem I have experienced is stray flux – mainly third and higher odd harmonics (150/180Hz is easily audible) due to high flux density (approaching saturation - not enough iron and/or turns). Toroidal transformers are definitely better than rectangular (E + I) types.

Stray flux can be mapped with a simple search coil – ten turns of thin wire about 2-3cm diameter, input to an amplifier/oscilloscope/spectrum analyser/headphones. I have one held together with cotton thread (used a darning needle to thread through and around) then three layers of nail varnish and glued into the end of a plastic tube (ex. ballpoint).

One can reduce MMF and flux levels in the core by taking load current from a secondary. If I remember correctly the load must be inductive for the current to be in the correct phase. The penalty is more heat.

With machine wound toroidal transformers look out for axial flux (50 or 60Hz) due to net turns (windings applied in the same direction). Inductive voltage dividers and ratio transformers employ “No-Net Loop” and, even better, “Balanced No-Net-Loop” winding schemes to avoid this. A simple way to reduce this effect is to add the requisite number of turns around the outer circumference and connect in series with the primary.

Another common source of “hum” is even harmonics – radiated by the pulses of current when the bridge rectifier diodes conduct. The solution is to use tightly twisted pairs from the transformer to bridge rectifier and from bridge rectifier to the smoothing capacitor.

N.B. Ideally all pairs, whether current carrying or voltage sensing, should be twisted or coaxial.

The current pulses also have an impact on the heat dissipation due to the I-squared R rule. As a rule of thumb the pulses are ten times higher than the average (PSU DC output current) for one tenth of the time, resulting in ten-fold higher I2R losses compared to what one might expect.