Original poster: "Sean Taylor" <sstaylor@xxxxxxxx>
Hi Gerry,
I think we're thinking 2 different things - it's true that if you energize
an inductor at a voltage peak, you'll end up with a negative current and
end up farther from saturation, leading to a lower peak current. However,
There is a larger peak current closer to the switching time. For finite
switching time devices (in 60 Hz land, namely relays), in the interest of
keeping the current low near the switching time, I would switch at a zero
voltage crossing. This way, the current peak comes later.
Sean Taylor
Urbana, IL
On Thu, 18 Aug 2005 00:22:47 -0500, Tesla list <tesla@xxxxxxxxxx> wrote:
Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
Hi Sean,
What you say is true, but at steady state conditions with the variac
set to zero out, the zero current crossing occurs at peak voltage. The
load a variac presents at these conditions is inductive and switching
needs to occur at a time that would normally be zero current. If you
switch power on at zero volts to a variac, the inrush current will be
very high cause the initial current at turnon will be zero instead of a
minus current (for a positive going zero voltage crossing). Keep in
mind, that current lags the voltage for an inductive load.
Gerry R.
Original poster: "Sean Taylor" <sstaylor@xxxxxxxx>
On Wed, 17 Aug 2005 08:39:27 -0500, Tesla list <tesla@xxxxxxxxxx> wrote:
Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
Hi Terry and Bart,
For closing a contactor driving a variac, zero volt crossing is not the
correct time to close. You need to close the contactor at peak voltage
(which would correspond to zero current with the variac set to zero
volts out) otherwise the intial conditions on the variac will cause the
inrush current to be very high.
Opening the contactor should also be done at zero current crossing.
Gerry R.
The current through the variac any time before you close the contactor
is
0 (assuming it's been sitting "a very long time"). It is actually best
to
close it at 0 volts because at that point, the dI/dt is 0, so you have
the
slowest ramp-up of current, minimizing the current while the contacts
are
bouncing slightly. If the contactor were to be closed at peak voltage,
the dI/dt would be at its peak, and you'd end up with a "high" current
relatively quickly - same reason why a larger variac can blow a breaker
when it's plugged in. While the idle current may be low, the inrush
current in one half cycle (in the transient) can be much greater!
Sean Taylor
Urbana, IL