# Electronic – When/where to open and close a transformers output

inrush-currentsolid-state-relaytransformer

I came across Beware of Zero-Crossover Switching of Transformers (TE Connectivity App Note) where they discuss about how you shouldn't switch a transformer at the zero crossing because it would have the highest inrush.

However, if I am looking to switch on secondary, would this still apply ? In this case, would the zero crossing be a good place to switch (open and close) or it doesn't matter at all ?

The arguments against zero crossing switching transformers applies to primary of secondary. The claim (largely borne out in practice) is that the core saturates when subject to the rising voltage while unmagnetised. This is generally not an intuitive outcome. It makes sense when you consider that in an inductor under steady state conditions voltage and current are 90 degrees out of phase. By switching the voltage on at the peak of the voltage waveform you have maximum voltage and zero current so the 90 degree relationship is automatically established as the initial condition. You still need to build the "magnetising current field" but you are at the best starting point.

Nicely summed up in the final reference below:

Instead, it would be better to close the switch at the peak of the input AC line voltage. Since the inductor's current is initially zero as before, switching this way puts the applied voltage and the inductor's current immediately in quadrature with each other (or really close to it) and there is no transient event or current settling phenomenon.

The current starts out already settled.

At a minimum what happens is that the current waveform is offset relative to zero so that a current sinusoid DC offset by Ipeak/2 initially occurs. This leads to about double expected peak currents if the core does not saturate - and some sources suggest even higher initial surges, due to saturation effects.

The subject is covered reasonably well in Rod Elliot's inrush current -
and especially section 4. "Inductive & Transformer Inrush".

Useful paper here - less severe claims
Effect of Switching angle on Magnetizing flux and Inrush current of a Transformer
(ie saturation effects not dealt with)

Open electrical wiki - transformer inrush

Some practical experiences reported here To Zero Cross or Not To Zero Cross.