>  ...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

The SCR shuts off when the current goes through zero. If the load is
resistive, this will coincide with the voltage going through zero, and
all will be simple. With an inductive load, the current will cross zero
after the voltage does. This means that immediately after shut-off,
the voltage across the SCR will rise very rapidly to the instantaneous
value of the line voltage (which will not be zero at this point). The polarity
of this voltage will be opposite to that which existed during conduction,
since the voltage has already passed through zero. At this point, either
of two things can happen.

If the SCR is operating as a diode, it cannot turn on again until the end
of this half-cycle since the voltage is the wrong polarity.
If the SCR is operated inside of a bridge or if you are really using a triac,
there is the possibility that it will immediately turn back on again when
the voltage across it shoots steeply up. This is due to the dV/dT effect.
If the slope of the voltage is too high, capacitive current will be injected
into the SCR junction, and if this current exceeds a certain minimum,
the SCR can refire. Once fired, the SCR will remain on for the entire
half-cycle. To prevent this, a series RC circuit is placed across the
SCR to blunt the edge of the voltage step. Reducing the slope of the
voltage reduces the capacitive current ( I = C dV/dt ). The symptom
of this happening is that the SCR appears never to turn off.

This can only happen if the voltage and current are not in phase (i.e.
with an inductive load). With a heavy resistive load across the
inductor (a heavy resistive load on the secondary will do), the phase
shift between voltage and current reduces to a small amount, which
means that the voltage across the SCR at shut-off will be small, and
the SCR may not refire. With no resistive load, the phase shift
becomes 90 degrees, which puts peak voltage across the SCR right
at the wrong time (when the current goes to zero). The result is that
the refiring behavior becomes a property of the load.

Some energy will be dissipated in the R of the snubber network, so
care must be taken in choosing R and C. Refer to application notes
for SCRs and triacs; this subject is discussed there.

John Power

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body