Re: Interesting inductor observation

["Matt Behrend" <electronxlc-at-earthlink-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Gavin, All,

I have had some experience here.  I tried winding a large 
transformer core for my ballast.  When I was taking measurements 
of its inductance the inductance decreased as I increased the 
current (not good for a current limitter!).  After only a few amps, the 
current waveform began looking more like (sin t)^3 than sin t.  This 
effect rapidly grew worse as current was increased.  After I cut a 
gap in the core I could run more current while maintaining a stable 
inductance.  The inductance of anything with a core is not constant 
because permeability is a function of flux density.  Power inductors 
are often rated with a minimum inductance at the rated current.

Here is a useful site
http://www.ee.surrey.ac.uk/Workshop/advice/coils/index.html

http://www.ee.surrey.ac.uk/Workshop/advice/coils/terms.html
(check the section on magnetic flux density to see a BH curve)

Matt Behrend


> Something that might be of interest...
> 
> I was talking to a transformer specialist today. 
> Although not an EE, he has been designing and 
> manufacturing transformers and inductors for the 
> last few years. He says that when designing an 
> inductor for a current limiting application, it 
> is not enough to simply specify the inductance 
> and the line voltage for the inductor.
> 
> Apparently, different inductors with the same 
> inductance (but different physical designs) will 
> often behave completely differently when placed 
> into a live circuit.
> 
> He said that an example is when you tune a TC's 
> input current with an variac-inductor, measure 
> the inductance of the variac, and build a fixed 
> inductor with the same inductance. It seems that 
> you will invariably get different performance 
> results from the coil when you switch between the 
> inductors.
> 
> This effect does not apear to be attributable to 
> core saturation in the inductors. Also, the 
> impedance of the two inductors is measured as 
> being (almost) the same.
> 
> Does anyone out there know why this effect 
> occurs? Presumably it is a magnetic effect - 
> possibly to do with the flux density or 
> magnetising currents? Or maybe it is just a 
> tuning phenomenon that is peculiar to the TC.
> 
> I would be interested to know what others think 
> about this, or if anyone else has experiences 
> this.
> 
> Safe coiling,
> 
> Gavin Hubbard
>