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Re: Powerstat Surge



Tesla List wrote:
> 
> > Subject: Re: Powerstat Surge
> 
> >From hullr-at-whitlock-dot-comTue May 28 22:06:46 1996
> Date: Tue, 28 May 1996 09:25:33 -0700
> From: Richard Hull <hullr-at-whitlock-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Powerstat Surge
> 
> Tesla List wrote:
> >
> > >From Esondrmn-at-aol-dot-comSat May 25 11:50:30 1996
> > Date: Fri, 24 May 1996 16:49:56 -0400
> > From: Esondrmn-at-aol-dot-com
> > To: tesla-at-pupman-dot-com
> > Subject: Re: Powerstat Surge
> >
> > In a message dated 96-05-24 00:10:29 EDT, you write:
> >
> > >
> > >[ I wonder if it matters what order things are in.  I have the variacs
> > first,
> > >and the arc welder between them and the pole pig. -- Chip ]
> > >
> > >
> > >
> > Chip,
> 
> In a series circuit, such as the primary power circuit, the order in
> which a component is in in a circuit is of no consequence.  I use a 5 ohm
> 200 watt resistor in seires with my variacs and place a 120 VAC relay
> across the variac input terminals as well.  The normally open contacts
> (should be rated the full load of the variac) are placed across the
> resistor.  After the inrush current is over, and the voltage across the
> variac rises, the relay pulls in and shunts the resistor out of the
> circuit.
> 
> Richard Hull, TCBOR
> 
> [ Just a quick question appended 'cause it's easier for me...
> 
>   The primary power circuit that I use is not quite a pure series circuit
>   in my eyes.  The power comes in through the plug and energizes the variac.
>   The power from the variac then goes to the pole pig off one side, and to
>   the welder from the other.  From the welder, the power goes to the pig.
>   If we could take the pig and welder out of the circuit, we would still
>   measure a small amount of current that is used to energize the variac.
>   Now, if the variac had no impedance for some reason, I believe that the
>   breaker would trip.  From what I have been hearing, and what I believe about
>   inductors and AC, there is a period of time where the core of a
>   transformer has to magnetize.  I believe that this time period (1/120th
>   sec. or so) is when a large surge of current can flow.
> 
>   A______
>          3
>          3
>          3<------------------ Pole Pig -|
>   B______3_________ Welder _____________|
>                |            |
>                |/\/\/\/\/\/\|
>                  oven element
> 
> A and B are the inputs to the variac.
> 
> I believe that the best way to limit the current at startup would be to have
> a resistor at A or B that was cut out after energizing the variac as
> mentioned.
> So, the question is "Is my understanding about the primary power circuit
> correct?"
> 
>  ---- Chip]


Chip,

Assuming that you have the variac turned all the way down when power is 
applied, (the normal case) you still have a series circuit.  With the 
variac turned down at power up, all the AC line sees is the variac core 
only.  The other stuff is invisible.  The resistor should go is series 
with the core of the variac at point A or B, it doesn't matter.  For the 
sake of the drawing let us assume A.  A set of normally open contacts 
from a relay are placed across the resistor.  The relay's coil is placed 
directly across the variac's windings A to B.

By the way, you should never just throw a switch and have the system just 
blast on!  This is a big no-no! With little neon circuits under 500 
Watts, this might be OK, but not with pole pigs.  I am assuming you never 
do this and use the variac to slowly bring up the power.  In this case 
the line sees only the variac core at powerup.

Richard Hull, TCBOR