[TCML] Ballast -- Before or After Variac?

[Jeremy Scott <supertux1@xxxxxxxxx>]

Hi all,

I was just wondering where the optimal place in the circuit is for the ballast?

Should it go in series with the variac input, or series with the variac output? 

Modeling it in SPICE, it appears to function either way. 

The ballast is an arc welder with a range of 6mH to 18mH (measured), 
output terminals shorted.

The variac is a 30A Powerstat with an input of 125mH to an output range 
of 0-180mH (measured - 120VAC in, 0-140VAC out.)

I haven't hooked it up and measured the output for real yet, but SPICE modeling shows some interesting results:

When placed AFTER the variac on the variable voltage line, it limits current to 35A on the low end to 50A on the high end. 

When placed BEFORE the variac on the 120VAC voltage line, it limits 
current to 28A on the low end to 45A on the high end.

So, both results look okay -- this is a 50A circuit, so which is best?



      
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