Re: [CAD_CAM_EDM_DRO] Re: Power supply softstarts

dgoadby wrote:

>One of the biggest problems is toroidal transformers. Due to the fact
>that they maintain the polarity at the point when they were switched
>off there is chance of a big inrush current depending on where you
>are in the cycle at switch on. A basic soft start could be created
>using a zero-crossing switch. I would also put a small resistor in
>series with each of the primaries to take the surge.
>
>

This is a common misconception. Actually, remanent magnetic field is
insignificant
compared to the phase of the power line at turn-on. The transformer
integrates
magnetic field during each half-cycle of the line, and then
de-integrates that
during the next half cycle. The transformer iron comes close to saturation
just as the line voltage approaches zero, then the field comes back down
as the
voltage rises in the opposite direction. If, however, the transformer
is started
when the line voltage is at zero crossing, it has to integrate flux for
the complete
half cycle, with no flux in the opposite direction to de-integrate
first. So, it will
attempt to develop TWICE the normal peak flux on that first half cycle.

The remanent magnetic field is usually just a couple of percent of peak
flux.
But, here we have a condition of 200% normal! No wonder the transformer
saturates!

There are only a couple of ways to fix this, without redesigning the
transformer
to mitigate the problem. Series inductors or resistors, or a voltage
ramp up
are the most obvious techniques.

Jon