I have a PIC based AC Phase-angle power controller design that I am trying
to fit to a customer's application.
I mention it here because it's PIC controlled, but the part of it I'm
working on is basically an AC Power EE problem:

The controller fires gates on SCR/Diode pairs on each leg of a three phase
load. The zero-crossing timing is derived optically from the AC line, and
the command input to the PIC sets the gate delay, or phase angle to fire at.
It was intended for resistive loads, where the relationship between voltage
and current is quite simple.

I need to use it to gate the SCR's into a transformer primary, which is a
simple 3-wire delta. The secondary is through a three-phase bridge rectifier
to a wye connected resistive load. You might envision it feeding the
resistive equivalent of a 3-phase AC motor stator, but ignore the concept of
motion.

I am trying to assess the net effect on the primary side of the coupling
transformer, ie: as long as the rectifier is fairly well balanced, there
should be no DC reflected back, right?
Will the current at the transformer primary lag, and will that effect my
zero-cross detect timing, since it is done by LED's? If so, the controller
will fire past the zero-crossings and make some spectacular fireworks, since
it deals with hundreds of Volts at hundreds of Amps.

Please let me know if anyone has experience with this sort of thing that
would like to discuss it either on or off-list.

Chris

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