Re: Current Limiting and Impedence

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Gerald  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Syd,

You can analyse the ballast being driven by a current source or by a 
voltage source. and should get the same answer.  Most analyse the effective 
inductance of NST's by measuring the short circuit current and open circuit 
voltage to determine the transformer's impedance and assume all of the 
impedance is inductive reactance.  On a ballasted pig, for example, when 
the pigs output is shorted by the sparkgap, this short reflects onto the 
pigs input and the ballast see's the full line voltage.  So I believe volts 
per turn is very appropriate way to think of a ballast.  Once you know the 
volts per turn, you know the dphi/dt in the core and a gap isn't going to 
change that.  Given a fixed line frequency and the core material,  dphi/dt 
=>phi and core area will determine the B and H in the core (with or without 
a gap).  A gap, however, will create magnetic poles at the gap and the H 
field in the gap will increase by   u/uo since the flux normal to the gap 
is continous across the boundary.  The magnetic path is effectively 
increased cuz integral H.dl is a larger value. The amp turns (NI) 
=  integral{H.dl}. So as the magnetic path is increased, the current the 
ballast will allow will also increase (with no change in the core) and the 
effective inductance will go down.

Note, I am not taking into account the effects a capacitive load and the 
SRSG's inductive kick have on increasing the current beyond short circuit 
levels. Certainly,  these effects need to be considered when designing in 
the margins for saturation.


> Original poster: syd <tesla@xxxxxxxxxxxxxxx>
>
> Hello Gerry,
>
> The introduction of an air gap reduces the effective permeability of the 
> core, which in turn decreases the magnetic flux density (B). B needs to 
> remain below the saturation point for the core material you are using, 
> typically 1.6 tesla for grain oriented silicon steel (common in 
> transformers). In a current driven winding, such as in a ballast inductor, 
> the volts per turn isn't so important; what IS important is the current, 
> the number of turns, the magnetic path length, and the permeability of the 
> core.

This is all true for a current driven ballast.  However, the design of the 
ballast is suppose to control the maximum current and the depth of the gap 
is one of the variables in doing this.  I think of current as a controlled 
variable not an independent variable.

The bottom line, I believe, is that voltage driven and current driven 
analysis are equally valid approaches.

Gerry R.