Re: inductor/transformer concepts

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

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