whit3rd wrote:

Remember Phil's
comment that a 240V transformer on 120V excitation wouldn't saturate-
because a 240V transformer has an oversize core for 120V excitation.

** Switching at a zero crossing creates a short term, low frequency component in the AC wave - a component approximately equal to the same AC voltage at half frequency.

Most AC supply transformers operate right on their low frequency limit - but if you only apply half voltage then that limit is now at half the original frequency - so the switch on condition is tolerated.

IOW, a given saturation condition is proportional to V / f.

I often see this in action when testing US model amplifiers meant for a 120V at 60Hz supply. A step-down device only adjusts the voltage leaving the frequency at 50Hz. The resulting magnetising current is the SAME as if one had applied 144VC to the transformer instead of 120V so Imag is way higher.

Some transformers are OK with this and others run very hot PLUS it has a significant effect on the VA rating of a suitable step down transformer.



.... Phil