Re: inductor/transformer concepts

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "D.C. Cox by way of Terry Fritz <twftesla-at-qwest-dot-net>" <resonance-at-jvlnet-dot-com>




Dave:

Obtain a copy of the book "Practical Transformer Design Handbook", 2nd
edition by Eric Lowdon, 1989, Tab Books,   ISBN 0-8306-3212-3.

It is comprehensive and will answer all your questions regarding flux,
cores, etc.

TAB Books, Inc., is located at Blue Ridge Summit, PA  17294  (no st. adr
necessary)

Best regards,

Dr. Resonance




----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, August 04, 2002 11:12 AM
Subject: Re: inductor/transformer concepts


> Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<davep-at-quik-dot-com>
>
> > I'm interested in building a transformer for my coil.
>
> > I've realized I don't have an EE degree and will need
>
> > to learn several key concepts before I design one.
>
> There are texts available, the usual intro is a
> one term course.  Also the web sites referenced.
>
> > heres a few questions I have regarding magnetic flux.  I'm just
> > trying to understand key relationships and concepts.
>
>
> > primary turns intended to induce flux in a core:
>
> > too many turns
>
> ???
> More turns will limit the current.
> Limiting the current reduces the flux.
> Reducing the flux avoids saturation.
>
> > or too small core can cause core saturation.
>
> true.
>
> > too little turns or too large of a core could not create
>
> > enough flux to induce current in sec.
>
> Almost never a problem.
>
> > as a wire can be compared to a pipe and current is volume
>
> > that flows within that pipe, so as to magnetic flux that
>
> > flows within the core.  (this seems incomplete)
> Close enough.  There is a parameter, which i forget
>
> name of, which is analogous to 'resistance'.  (I took
> the course a Long time ago....)
>
>
> > core materials: soft iron is used for lower frequencies
>
> > and ferrite cores are used for higher frequencies.
>
> Each time the current switches direction, it induces
> _electrical_ currents in an iron core.
> This current, thru the _electrical_ resistance of the
> core causes _loss_.
> The _loss_ rises with frequency.
> Ferrites have acceptable magnetic performance AND
> very high _electrical_ resistance, so minimize
> _electrical_ losses in core.
>
> > why?
>
> cf above.
>
> > I figure that it is the material's capability to retain...
>
> > ok, let me rephrase that. I look at it like this:
> > the flux moves through the material.  but instead of looking at
>
> > it like a wave; i'm looking at like a 'particle' or charge.
>
> > so the core material 'molecule' holds a specific spin or flux
>
> > for a specific amount of time.
>
> Overly complex.  cf above.
>
> > the material's characteristics state that it can hold this flux
>
> > for x time.  this determines how the flux propagates through
>
> > the material.
>
> There IS a time dependency on the propagation of flux
> thru the core, however it is much slower than involved
> here.  cf above.
>
> > for soft iron, more flux is needed to raise material's flux, but
>
> > it retains it for a longer time.  but ferrites respond to a
>
> > smaller amount of flux, but retain it for a shorter period.
>
> cf above.
>
> > soft iron used in a ferrite application would then not receive
>
> > enough flux to be 'efficient'(too little flux), and a ferrite
>
> > used in a soft iron  application would then become saturated.
> cf above.
>
>
> > how does an air core relate to the above scheme?
>
> No losses due to core electrical currents (sometimes
> called eddy currents): good.
> 'saturates' at low flux density, requiring larger
> coils and 'core' space: bad.
>
> > or how does the field of a coil change when it does not have a
>
> > core (as in it has an air core)?
> Same primary current will produce less flux, less
>
> secondary output.
>
> For fixed voltage, same coil will draw more current
> (possible overheat) due to lower inductance, for lack
> of a core.
> ('power transformers' for 50/80 Hz work are ALWAYS
> cored....)
>
>
> > Also is it true that eddy currents resist the pri coils flux?
>
> A bit.
>
> > (and resist the secondaries induction of current).
>
>
> > why use laminations?
>
> To reduce eddy currents.
> The voltages driving the currents are small, so
> the resistance of stacked laminations is sufficient
> to be helpful.  Also :laminations commonly have
> coatings, or an oxide layer, which helps block
> eddy currents.
> Also: the laminations can be oriented 'preferentially'
> for working magnetic flux flow and crosswise
> (blocking) to eddy (electrical) current flow.
> (one might think of ferrites as 'molecular scale'
> laminations.
>
> > why does this help?
>
> cf above.
>
> > now there are many fields in the core.
>
> Laminations exist (to put it another way) to ease
> magnetic fields (tho a solid core WOULD be (a little)
> better WHILE blocking/limiting eddy current
> --
> best
> dwp
>
> ...the net of a million lies...
> Vernor Vinge
> There are Many Web Sites which Say Many Things.
> -me
>
>