[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Current Limiting and Impedence



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.