"Mike Monett"

OK, it's clear due to the phase lag of an inductor, switching an
inductive load at a zero-cross will give high inrush current.


** Absolute nonsense.

The inrush surge is caused by *core saturation* and consequent temporary
LOSS of primary inductance.

The core saturates because of the DC component ( really a low frequency
transient) generated by switching a sine wave.


It
should be switched at the peak, or some other method should be used
to limit the inrush current.


** A rapid "fade up" using a triac with phase control is one way.


But I was still having problems understanding the inrush current due
to remanence. Modern transformers use low-remanence steel, so why
should there be a problem?



** Where it exists, it just makes the worst case inrush surge larger.

The peak surge current approaches the zero inductance situation in many
cases.

So, I pk = V pk / R primary

Eg.

A 1 kVA transformer ( R primary =1 ohm) can be expected to exhibit 300
amp peak surges on a 230 volt supply.





.......... Phil