RE: Soft transformer turn on without a variac

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> 

I don't believe that zero crossing switching will solve the problem.  I 
think the problem is that the variac core retains a magnetic bias, so if 
the initial turn-on phase is going the "wrong" way, the current will be 
high regardless of turn on timing and variac loading.

A Negative Temperature Coefficient (NTC) thermistor is the simplest 
solution and is what I use.  The one I use was pulled from some power 
supplies at work, so I can't advertise a ready source, and the vendor 
(KETEMA) seems to have vanished.  An alternative to using a thermistor is 
to have a current limiting resistor in series with the variac, and have a 
relay short it out 1-2 AC cycles after turn-on.

Regards, Gary Lau
MA, USA


Original poster: Thomas <tom-at-pwrcom-dot-com.au>

Another option may be a solid state or mechanical zero crossing switch so
that power is only applied to the variac when the mains voltage is 0V.

http://www.phenixcontrols-dot-com/Services/Engineering/Zero_Crossing_Mechanical/
Zero_Crossing_Mechanical.htm

http://www.celduc-relais-dot-com/uk/relais.asp

  > Original poster: Bill Vanyo <vanyo-at-echoes-dot-net>
  >
  > Hi All,
  >
  > 	Anyone remember this discussion?  Basically, the
  > problem I want to
  > solve, and that was discussed previously, was how to turn on
  > power to a
  > variac without tripping breakers.  For my own setup, I have a variac
  > rated 50 amps 240 volts fed from a 30 amp 240 household line (electric
  > clothes dryer).  I set the variac at half way to step down to
  > 120.  I'm
  > using 3 (soon 4) unmodified 15/60 Franceformers.
  >
  > 	What I'm looking for, if anyone can help me with this,
  > is a cut and
  > dried plan using readily available parts (part numbers & sources would
  > be ideal), a la Terry's NST Protection filter - just
  > something so I can
  > turn on power to the variac (turned down to the low end) without
  > tripping the house breakers.  Right now I'm using a big old knife
  > switch, which more often than not trips the breaker.
  >
  > 	I suspect many other novice coilers could benefit from
  > a solution to
  > this too.
  >
  > 	Thanks,
  > 	Bill V.
  >
  > Tesla list wrote:
  >  >
  >  > Original poster: "Rick W by way of Terry Fritz
  > <teslalist-at-qwest-dot-net>"
  > <potluck-at-xmission-dot-com>
  >  >
  >  > Hi Peter,
  >  >
  >  > I had that problem also with a stack of 2 variacs for 120
  > volts and a stack
  >  > of 6 variacs for 240 volts. The scheme I'm currently using
  > is shown in the
  >  > link. For 240 volts I'm using an 18 ohm resistor. I
  > believe the wattages can
  >  > be less but that's what I had in the resistance range I
  > wanted. This works
  >  > really well. I don't add load (ramp up the variacs) until
  > both contactors
  >  > are pulled in.
  >  >
  >  > http://xmission-dot-com/pub/users/p/potluck/pics/rampup.jpg
  >  >
  >  > I see Scot D. is using an inductor. I tried that and found
  > the same problem.
  >  > The current draw feeding six 240 volt paralleled variacs
  > is rather low after
  >  > the intitial small spike. Much lower than I had expected
  > so these worked
  >  > very well.
  >  >
  >  > Also, the post about Negative Temperature Coefficient
  > Thermistors in power
  >  > supplies is very interesting. Very interesting indeed. I
  > too will be looking
  >  > into that.
  >  >
  >  > Rick W.
  >  > Salt Lake
  >  > ----- Original Message -----
  >  > From: Tesla list <tesla-at-pupman-dot-com>
  >  > To: <tesla-at-pupman-dot-com>
  >  > Sent: Wednesday, July 23, 2003 6:53 PM
  >  > Subject: Soft transformer turn on without a variac
  >  >
  >  >  > Original poster: "Peter Terren by way of Terry Fritz
  >  > <teslalist-at-qwest-dot-net>" <pterren-at-iinet-dot-net.au>
  >  >  >
  >  >  > Hi all.
  >  >  > For my 5 kW museum
  >  >  > coil,
  >  >  >
  >  >
  > <http://members.iinet-dot-net.au/~pterren/Scitech.htm>http://membe
rs.iinet-dot-net.a
   > u/~pterren/Scitech.htm
   >  > ,   I need to have a single switch for turn on preferably without a
variac
   >  > to keep things simple for the staff.  The present system has solid
state
   >  > relays but is complicated and unreliable. I need to turn on the motor
   > first
   >  > then a couple of seconds later soft turn on the power to the
transformer.
   >  > Even switching  on power to a normal variac gives a spike that will
trip
   > my
   >  > lower current breakers. The whole system will have to be electrically
   >  > quiet.  Does anyone have experience with inrush limiters or have other
   >  > suggestions?
   >  >
   >  > Cheers
   >  > Peter (Tesla Downunder)
   >  >
   >  >
   >  >
   >  >
   >  >
   >  >
   >  >