On 2/14/2015 11:43 PM, Phil Allison wrote:
mike wrote:


If you don't turn it on/off at zero crossings, the saturation
state of the core can make a great difference in the next weld.


** Zero crossing switching is the worst possible for creating large in-rush surges and hence fully magnetising the core of an AC supply transformer.

Switching on at a voltage peak, while under load, is the best option.

A triac switch will always go off near zero current, which will be nowhere near zero volts.


Well, I don't recall mentioning volts at all.

** You did, cos "zero crossing" means at zero voltage crossings.


Yes, triacs switch off at the zero crossing of the current.
Not much you can do about that.


** Then why did you say it was needed ??


I question not turning it on at zero volts???


** Really ? Did you try Googling the topic?


Please explain the large in-rush current when there's zero volts
on the transformer primary?


** The surge current peak comes a little after the first voltage peak.

Consider that it takes half the time for the applied voltage wave to first average zero if you switch at a peak - half a cycle later instead of a whole cycle - minimising the low frequency component of the wave.


... Phil



if you say so.
As long as you stay out of saturation, the main component of the
primary current is due to the shorted secondary.
I think I'll give my SCR the benefit of switching on when there's zero
voltage.