In this case, the SCR's are used as part of a current limiter that
phase-angle limits the primary side while 3 on/off heater loads are
controlled individually on the secondary (yes, I am aware of the DC
implications of this imbalance). The SCR gate signals can still be present
when all 3 loads are switched off by their respective process controllers.
This is when the supposed current sans power situation exists. It was my
suspicion that the SCR's do not turn off in this condition because this
current is sufficient to keep them latched in conduction, or > the devices
holding current. Since I have PIC control of the firing angle, I can, and
have compensated as (much as I thought was) needed for the inductive phase
shift in the firing angle calculation that spreads the control range across
the current wave form, rather than the voltage. So while phase angle control
is well effected, this current still exists even though the lag is
(apparently) compensated for. Does this mean that it is not perfectly
compensated for? If not, wouldn't this conditiononly exist at only one end
of the control range?

Are you referring to capacitive snubbing to compensate for phase shift? I've
used RC snubbers to limit transients, but not for advancing the phase
angle.....???


>       Even though
>       there is no average power transferred to the inductor,
> the current which does
>       flow must be taken into account for two reasons.
>
>       The first concerns the current capacity of the source,
> and the second involves
>       the switching behavior of SCRs when the voltage and
> current are not in
>       phase. When using SCRs with inductive loads, you must
> use "snubber
>       networks" which allow the SCR to turn off at the current zero.
>
>       John Power

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics