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