Chris Jones wrote:


Although he is rude, Phil is usually right, and this time is no exception.

I suggest you get a hall-effect current transducer connected to a DSO so
that you can measure inrush current, and try out switching the
transformer on at both the peak of the mains voltage, and also at the
zero-crossing, and look at the current waveforms. When you run out of
working triacs you could also google it.

The key to understanding this is to realise that the magnetic flux is
proportional to the time-integral of the applied voltage, and that in
continuous operation the flux is normally close to zero when the voltage
is close to maximum, and the flux is close to maximum when the voltage


http://sound.westhost.com/articles/inrush.htm


** FYI:

I had a large input to the writing of that article - Rod is a friend and colleague, we talk and email regularly.

The most surprising thing is how the in-rush surge of an unloaded transformer consists of brief pulses all with the same polarity - it's DC current.

Also, if you power a 240V tranny from 120V, surges are eliminated.



..... Phil