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