[R-390] Question about VARIAC

[2002tii]

Barry wrote:

>I guess what I was getting at was impedences.  If I were to tap the input at
>one turn (or two or three), then I assume this would appear as a very low
>impedence and wouldn't work very well (most likely incurring a very heavy
>current draw and hopefully trip the circuit breaker before burning out the
>transformer.  If there were double, triple, etc., the amount of turns, this,
>too, would affect the input impedence, would it not?
>
>I guess what I was trying to say is that I figure there's a point where the
>number of turns matters but just wasn't sure where that point is.  Is that
>incorrect?

Just like a transformer, an autoformer is a constant-power device 
(neglecting losses).  So, whatever current is drawn by the load at 
the voltage selected is transformed to the input according to the 
constant-power law (v1 x i1 = v2 x i2).  As long as what you are 
doing is hooking the input up to most or all of the winding, the 
voltage step-up ratio will be small and the load won't draw much more 
current than it would draw at the autoformer input voltage.

If you were to hook 120 V to just the "bottom" 43 turns (between turn 
0 and turn 43), the voltage between turn 0 and the top of the winding 
(turn 292) would be 815 V.  This could make a load hooked between 
turn 0 and the variable tap try to draw much more current than it 
would draw at 120 V (assuming the load is more-or-less 
resistive).  The insulation resistance may not be up to this, so it 
is not a good idea in any case.  Additionally, any transformer works 
like a transformer only at frequencies where the primary has 
significantly more inductance than the reflected load impedance.  As 
the input frequency decreasess, the primary acts more and more like a 
resistor, and draws current even with no load.  At the limit -- DC -- 
a transformer winding is just a wire-wound resistor.  If you added a 
tap at the second or third turn and applied the input there, I think 
it is safe to say there would not be sufficient inductance to keep 
the no-load current at negligible levels at 60 Hz.  I do not know if 
the inductance of the first 43 turns has sufficient inductance to 
keep the no-load current at negligible levels at 60 Hz.

Best regards,

Don