[Phil Allison]

"Phil Allison"

4. Details of the magnetic surge behaviour of commercial transformers are
not to be found on any web site I know of - it is a shame that makers
are not made to specify it.


** This PDF has some test data on zero crossing switching and surge
behaviours of AC supply transformers.

The very first paragraph confirms my findings.

http://relays.tycoelectronics.com/ap...fs/13c3206.pdf


Bewa

Figures 3 and 4 have been printed inverted and the labelling of A and B is
reversed.




........ Phil

[Eeyore]

Phil Allison wrote:

4. Details of the magnetic surge behaviour of commercial transformers are
not to be found on any web site I know of - it is a shame that makers are
not made to specify it.

Plitron has some stuff on toroids. It would make your hair stand on end.

Graham

[Patrick Turner]

Phil Allison wrote:

"Phil Allison"

4. Details of the magnetic surge behaviour of commercial transformers are
not to be found on any web site I know of - it is a shame that makers
are not made to specify it.

** This PDF has some test data on zero crossing switching and surge
behaviours of AC supply transformers.

The very first paragraph confirms my findings.

http://relays.tycoelectronics.com/ap...fs/13c3206.pdf

Bewa

Figures 3 and 4 have been printed inverted and the labelling of A and B is
reversed.

....... Phil

Thanks Phil, the pdf shows what happens with wave forms of inrush currents
due to magnetic saturations.

Toroidals sometimes make a hum at turn on which takes a number of cycles or
maybe a second to "fade down" to being silent, and sometimes they either do or
don't
so probably it is due to the actual initial switched instant and whether it is
at the peak
of a wave or not.

Obviously, protection against such inrush currents is given with some series
resistance added and it doesn't take much to have a large effect, ie,
a resistance of 10 times the primary R would reduce the inrush I by plenty,
so if RwP = 2 ohms, and the Rseries = 20 ohms, then the inrush current is very
much reduced.
The resistance in the domestic mains house wiring is also in series with the
primary
and could help to limit inrush.
If I plug a 2kW heater into my shed outlet, maybe there is a 10V mains drop for
an 8 amp draw, so the mains resistance is 10/8 ohms.
This isn't much R though, only 1.25ohms, so there can be no reliance on
house wiring and power lead for significant extra series resistance.

I tend to use more shuntable Rseries than 10 x RwP because the nuisance fuse
blowing is mainly
due to having to charge large caps which takes longer than the magnetic traumas.

The pdf also says that solid state switching relays can sustain huge saturation
currents
because the saturation is worst if the turn on occurs at the zero crossing
instant, and these relays
tend to make sure that switching occurs at zero crossing.

So what sort of sort of solid state mains switching can be used to
minimise inrush current problems by only allowing switching at peak voltage wave
crests?

There would be a big advantage to use such switching fitted to all amps
since it would reduce the need for such high value mains fuses and this would
give more
sensitive protection.

Patrick Turner.