Re: ? improve K&W charger power factor ?

[George Tyler <gtyler@mweb.co.za>]

----- Original Message -----
From: Lee Hart <leeahart@earthlink.net>
To: <ev@listproc.sjsu.edu>
Sent: Thursday, May 31, 2001 6:59 AM
Subject: Re: ? improve K&W charger power factor ?


> George Tyler wrote:
> > An Inductor is not the same as a transformer...

> They are the same physical device. "Inductor" is what it is;

Except for the gap

> "transformer" is what it does. A transformer is an application for an
> inductor with 2 or more coupled windings on the same core.

 > > An inductor with no gap will saturate at low currents.
>
> Low *DC* currents. In the context we are discussing here (inductors in
> AC circuits), there is no need for a gap in the core.

If there is no other windings it will saturate at the same instantaneous
current ,ac or dc

>
> The inductance of a transformer's windings are not particularly
> important when it is used as a transformer. It works as long as it has
> "enough" inductance to limit the current to a reasonable value when
> operated with no load on the secondary, and "enough" core so it doesn't
> saturate in a single AC half cycle.

True.

Lee, I am going to talk in terms that I know, they might be a bit different
to what you guys are used to as we use different units in South africa to
you in the States,
Flux density  is measured in Tesla
we measure wire in mm,
area in metres, etc


Consider this, Take a mains transformer, say a 100watt rating, strip the
secondary just to avoid confusion, and measure the magnetising current when
connected to the mains. Lets say it is 50ma.
That winding is designed to be just below saturation at the highest mains
voltage.(230v here). I.e. it saturates at +- 60ma. (this is not the whole
story but let's keep it simple, in fact saturation is not dependant on
current but on the volt-second product, all else being equal.)
        The Peak flux density is now around 1.4 Teslas, if I now insert a
gap in the magnetic circuit the flux density does not change!.  I can now
vary the gap to get any current I desire and the core will not saturate.
This winding is designed to take 0.5amps so I adjust the gap for this.

Now, we have two "inductors" on the same cores with the same windings.

                1) the first passes 50 ma and will saturate at anything
more.
                2) passes 500ma.

You cannot decrease the turns on the first one to decrease the inductance as
you need that number to support the volt-second product. This now shows us
the easiest way to design an inductor for ac.

                1) work out the voltage dropped across it and calculate the
turns needed
                    for the core we want to use.
                2)calculate the wire size
                3) gap for the required current
You should actually start by calculating the window area- core area product
and selecting a core from this. All of this means that a closed magnetic
circuit (like a tape wound core or toriod is not really sutable for an
inductor. The toriodial inductor cores, (micrometals, etc) have gaps built
in all the way round because they are make from powder mixed with a binder.
(I work as a consulant for a transformer company, but this does not mean I
know all the answers, just some, and I am also willing to discuss and learn)
> -- George Tyler