On Mon, 13 Aug 2012 11:58:44 +1000, "Phil Allison"
wrote:


"Peter"
"Phil Allison"

Magnetic saturation is an instantaneous phenomenon.

Well yes, it is a quite rapid action, but does not occur till the
current in the primary is sufficiently high. Saturation does not
occur over the whole input wave, and drops and reverses as the sine
wave input reverses. So during the non saturated period, the input
and output voltages are proportional to turns ratio.


** Strangely enough, the peak saturation currents occurs near to each AC
supply voltage zero crossing.

Yes phil, that is interesting, and why I referred to current, as the
current amp-turns causes the magnetising force to result in core
saturation. When the secondary load is small, the primary current is
nearly at 90 degrees to supply voltage as the transformer is
essentially an inductor, so the high current part of the wave will be
near 90 degrees. Of course, when the core saturates, the inductance
drops to very low value, and the main limiting factor is the primary
circuit resistance, which in the case you quoted gave 15 times rated
input current.
In the case you quoted, the failure in a matter of seconds means you
have little time to turn things off before a catastrophic transformer
failure.

peter





With a small transformer and double the rated AC input voltage, the (off
load) current wave is very peaky in shape and rises to about 15 times the
usual RMS value.

Shorted turns in the primary results in a few seconds, the resistance drops
suddenly and blows a fuse even 10 times the correct size.

The PSU in question contained only electros, resistors and zeners - all
operating well within their ratings in normal circumstances, so well able to
take a brief increase in voltage.


... Phil