Re: Contactor question, Arc Suppression

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Good point Bart.

It doesnt matter what it use to start the "timer" as long as closure 
is at peak voltage (for an inductive load).  Of course, if the load 
was capacitive, closure would need to be at zero volts.  In any case, 
my current system uses a 120V variac that I just plug into the wall 
(no contactor). Even with its output setting at zero volts, it often 
trips the circuit breaker on the variac when plugged in (sometimes, 
it even trips the branch breaker).  I just reset and continue.

Gerry R.

> Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
>
> Hi Sean, Gerry,
>
> From a practical standpoint, "a" detection must first be used (zero 
> crossing is just easy and it doesn't matter what your looking at). 
> The reality is, we can only minimize the arc since contact timing 
> will not be synchronous. There will always be some about of current 
> there, and it will vary. Minimizing that energy during contact 
> closing and opening is the art of longevity to the contactor in a 
> potentially stressful situation, and it will take adjustment to the 
> delay from "whenever" we start the timer to locate the "best 
> overall time" to energize and deenergize the contactor.
>
> Take care,
> Bart
>
> Tesla list 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