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Re: AutoTransformer vs. Variac

In a message dated 99-04-10 08:04:33 EDT, you write:

<< 
> First being most important. What's the difference between an
> AutoTransformer and a Variac? My pop being an electrical engineer says
> that the two are totally different things, yet I see many coiler's sites
> reffering to either.

Tom,

There's one important difference; the variac is an *variable*
autotransformer....you can adjust the voltage output as needed by
turning a dial knob.  Some variacs are true isolated variable transformers
however.
 
> Second. I have a dual NST TC and I want to install a Variac (or
> AutoTransformer) to control the input voltage.
> the first NST is a 12kV / 60mA  /  720VA and the second NST is 12kV /
> 60mA / 360VA. What type of Variac (or AutoTransformer) do I need. What
> rating?

Variacs can take a lot of abuse, but I would use a 10 amp and 7.5 amp
variac respectively,  to cover a possible poor power factor, or resonant
charging conditions, or to cover the extra current if you turn up the input
above 120 volts.  It's not all that critical, especially for short TC runs.
 
> I have been told in on my my earlier messages that the reason the power
> rating differs is that the 360VA NST has a power factor capacitor built
> in. Now I understand that typically NST's work -at- 50% the power rating.
> Is that mean that if I add a PFC to the other NST will my power in fact
> decrease or increase? 

Any NST can have a pfc built in or not.  The pfc will not affect the power
draw, only the apparent power.  An ammeter will show a difference, a
wattmeter won't since a wattmeter reads only true power.  If you add
a PFC to an NST that doesn't have one, it will reduce the apparent
power input.  Output will stay the same in any case.

> How would I wire the cap's if I have the two
> NST's in parallel. The approx cuircit looks like this :
> [Image]
> I assume the PFC is on the primary. What cap am I looking for?

I didn't look at the drawing, but the caps go across the pri. input.  Try
something in the range of 30uF to 150uF whatever gives the lowest
current input is best, it's usually not that critical.
  
> Correct me if I wrong but according to the specs of the NST's I
> currently draw about 13 A from the circuit. Is that mean that I can't
> add another NST because the house wiring is rated at 15 A and so is the
> fuse?

That's probably true.  Do you have a 240 volt air conditioner or
stove outlet?  THen you can wire the two NST primaries in series,
and use a 240 volt variac, etc. 

Happy, and safe coiling, 
John Freau
 
 --
                  -=[Tom Heiber]=- >>