In a thread in SEB there was a discussion on transformer failure modes that
also mentioned gaps in the magnetic path. I never fully understood the
function of gaps in the core, but I observed that they are generally
present in iron core inductors, but not in most transformers.
I found some information at
http://www.micrometals.com/appnotes/appnotedownloads/id4hf.pdf, where it is
explained that the gap size can maximize energy storage in an inductor by
balancing the point of magnetic saturation (and core heating) with winding
losses. It seems that a wider (or longer) core gap extends the point of
magnetic saturation by allowing more current to flow through the windings,
so the effect is to lower the inductance. A smaller gap will have higher
inductance, but will saturate the core much more quickly, resulting in less
energy storage.
As an inductor is used more for energy storage, a gap (whether actually cut
in the magnetic material or distributed as with powdered iron), allows more
energy storage by allowing more current flow, and energy is proportional to
the square of the current. For a transformer, as I understand it, the
energy is transferred from the primary to secondary by mutual inductance,
so the absense of a gap results in higher inductance and a higher volts per
turn.
More information can be found at
http://ece-www.colorado.edu/~ecen4517/course_material/Exp6/Inductor.pdf,
which describes filter inductor design.
I would like to get a better understanding of the characteristics of
transformers and inductors to know how best to design high current 50/60 Hz
transformers as well as switch mode boost converters using inductors.
The transformers I have made use toroidal primary cores with 120/240 VAC
windings, and secondaries consisting of several turns of bus bar or welding
cable to produce up to 10s of thousands of amps. They will usually produce
15 to 30 times their nominal output currents for short pulses.
The switch mode boost converter I have designed uses a 10 uH inductor at
100 kHz to boost 12 VDC to 25 or 45 VDC at about 800 mA. However, I
recently found that a small pot core inductor rated at 6.7 amps seemed to
work better than a larger toroidal inductor rated at 10.8 amps. I think
this might be because the smaller inductor starts to saturate sooner,
lowering its inductance but allowing more current to flow, resulting in
higher energy storage. The larger inductor is probably allowing much less
current and hence less energy, so it cannot produce the power for the
higher voltage load. I can probably drop the frequency to 75 kHz or 60 KHz
and maybe get the output I need.
Thanks for any thoughts and discussion.
Paul
