Topband: measuring transformer core inductance

[Tom Rauch]

I wouldn't want people to think this is a big problem with all cores, or
even most cores we use. In my experience, this is a non-issue in the soft
iron cores we normally use at higher frequencies.

I would expect it to be more common if we use the wrong materials. Some
cores have been specified in articles and books with VERY high initial
permeability. The result is transformers wound on the cores are on a steep
downward impedance slope above lower LF. This impedance rolloff, caused by
selecting very low frequency materials (some ui's are in the range of 10000)
requires use of excessive number of turns. This in turn makes the dc problem
more severe, because the core has more ampere-turns. Compounding the problem
caused by having more ampere-turns, the excessively high permeability core
types also often have a "harder" magnetic material that may be damaged by
heating or extreme flux levels.

I've seen some Beverage transformers requiring as many as 20 turns of wire
or more, a sure sign something is wrong in the design!

> >If you damage a core with DC, you can heat it to the Curie temperature
> >to demagnetize it.

> You can also essentially demagnetize the core by cycling it around its
> hysteresis loop several times using gradually diminishing values of d.c.
in
> a coil wound on the core.

You can also just select the correct core, and not have a problem to cure.

73 Tom