Paul E. Schoen wrote in message
...

"Phil Allison" wrote in message
...

"Paul E. Schoen"


Toroidal trannies do seem to have higher inrush currents due to
remanent
magnetism.

** Nonsense.

The Wiki says otherwise. And I have had a lot of experience with high
power
toroids and have seen the effects of high inrush current.
http://en.wikipedia.org/wiki/Inrush_current


If there are an unequal number of positive and negative half-cycles
applied, the effect is a net DC component that magnetizes the core.

** Utter ********.

I have personally observed this in our circuit breaker test sets. We take
care to apply voltage at about 70 degrees phase angle to obtain an initial
current peak equal to those that follow, and adjust the timing so that
there are equal numbers of positive and negative half-cycles. Inrush
current on successive pulses is minimal. But if there is an unequal
number,
and a net DC component, there is always a much higher peak inrush current.
This is easily seen with short duration pulses of several cycles. In
breaker testing, of course, the breaker may trip at any time, sometimes
causing net DC, and the next operation usually exhibits high inrush.

Some other references:
http://relays.tycoelectronics.com/ap...fs/13c3206.pdf
http://powerelectronics.com/mag/powe...tions_solving/
http://www.melcontransformers.info/i...ransformer.pdf

Paul




Can a toroidal former become permanently magnetised and cause excessive
current draw. ? One, in front of me from an amp, drawing excess current ,
does not attract a small piece of iron. I don't know what the main
secondaries are supposed to provide, but can give 300 watts into 4 ohms.
I've wound off the primaries (yes external to the secondaries) to try and
understand what is going on with this transfornmer. Applying 70 volt ac ,
from a variac, across series secondaries is ok but increase to 75 volt and
the current draw goes up to 1/4 amp. Should be able to take twice that
voltage. Absolutely no visible problem , to the wiring or the coating of the
toroid and both secondaries measure DMM the same DC. Tomorrow will apply
something a bit over 70 volt with temperature sensitive labels fitted
around. I will even try a TV degaussing coil.