Re: Contactor question, Arc Suppression

["Tesla list" <tesla@xxxxxxxxxx>]

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.

> Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
>
> Hi Terry,
>
> Yep, that would work with a simple zero crossing detector, but for 
> it to work, the electromechanical energize time from rest to contact 
> must be "monkey'd" with. It would take a zero crossing detection and 
> delay to the next half cycle to time out the energize vs. current 
> crossing scenario. Obviously, a pot on the delay would be worthwhile 
> to fine tune the delay time required, and hopefully, a consistent 
> energize time on the coil (questionable). However, if you can simply 
> suppress the arc, this timed situation becomes null and void (but a 
> cool approach nevertheless)!
>
> Take care,
> Bart
>
> Tesla list wrote:
>
> > Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>
> >
> > Hi,
> >
> > There is a "trick".  I am not sure it is worth all the trouble...
> >
> > You make a circuit the monitors the current and voltage along with 
> > some delay electronics.  The idea is to only open the contactor 
> > when the current passes through zero.  Close it as the voltage 
> > passes through zero...
> >
> > Then, the contacts only have to take the nominal current rather 
> > than peak or break situations...  But the circuit and all the 
> > trouble it takes to make it has to be balanced against just getting 
> > a really big contactor in the first place...
> >
> > It can be done with some simple stuff in very controlled 
> > situations, but it is sort of an "art form"...
> >
> > An alternate is to get contactors with fail safe switches that 
> > detect a failed contact.  If it welds closed, a warning goes out...
> >
> > Contactors weld closed all the time...  Probably their #1 failure mode....
> >
> > Maybe best to use really big hydraulic circuit breakers...  They 
> > know how to hand the big currents ;-))
> >
> > Cheers,
> >
> >     Terry