Re: Saturation in transformers.



Ian Iveson wrote:

> Patrick Turner wrote
>
> >> > Your explanation wouldn't be easy for anyone without serious
> >> > knowledge
> >> > to follow.
>
> >> Why not? I followed it OK. I think.
>
> >> Generally there are as many ways of explaining as there are
> >> variables.
>
> >> > Fs = 22.6 x V x 10,000
> >> > ------------------
> >> > B x Np x Afe
> >> >
> >> > Where
> >> > Fs = frequency of saturation,
> >> > 22.6 is a constant for all equations,
> >> > V = voltage in rms across the primary,
> >> > Np = primary turns,
> >> > B = maximum allowable magnetic field strength in Tesla in
> >> > the
> >> > core,
> >> > Afe = cross sectional area of the central core leg, in
> >> > square
> >> > mm.
> >> >
> >> > The saturation is a voltage caused phenomena.
> >>
> >> No.
> >>
> >> Fundamentally, magnetic field depends on current. A voltage
> >> without
> >> a current will not cause a magnetic field. Saturation arises from
> >> a
> >> magnetic field, therefore it is directly related to current, not
> >> voltage.
> >>
> >> By your own formula, you should be able to see that saturation
> >> depends not just on voltage, but also on frequency. For an
> >> inductor,
> >> the upshot of voltage and frequency is current.
> >
> > If you read RDH4, you'd see where they say saturation is a voltage
> > related phenomena,
>
> The laws of physics won't change if I read RDH. Of course current is
> related to voltage...look up Ohm's Law.

If you are not into reading RDH, then i suggest many other books may
provide a
better source of info than I have tendered to the group so far.

But we are dealing with the non linear behaviour of transformer iron....



>
>
> > and sure there is a magnetizing current in a tranny, whether
> > loaded or
> > not,
> > and that current is due to voltage applied across an inductance.
> > but once that voltage exceeds a threshold the steel saturates,
>
> Rubbish. Only true at a particular frequency. You must know better
> than this, so I take it you are squirming, as usual.

Well look at the formula I quoted above from my website.
Its the well known *transfromer equation* that relates the
applied voltage, frequency, Bmax, number of turns and the core section
area.

I am not squirming after having desinged and wound dozens of very fine
power and output transformers.

>
>
> > and the coil becomes a short circuit when the steels field cannot
> > continue to
> > oppose the current flow of the current from the applied voltage.
>
> No. Some time ago I pointed out an error in RDH. Perhaps you ignored
> me and failed to amend the diagram? The inductance does not plummet
> as depicted, but actually trails off more gradually as current
> increases.

If you examined the current wave form as V across a winding is
increased,
you will find there is a sudden increase in the 3H after what is called
the saturation
phenomena.
The iron appears to have magnetic store of energy to oppose the AC flow
for *parts* of the cycle, ie, the top and bottom part of the wave, so we
see
large current *spikes* where the coil acts as if it has become a short
circuit
for part of the cycle.
But for other parts of the cycle around the zero crossing region the
iron energy is providing
some oposition to the flow of current in the wire.

If you looked, you would know.

>
>
> The situation is exacerbated by what you, with your crazy notion of
> feedback, would call positive current feedback: as current
> increases, reactance falls, so current increases further, etcetera.

There is no crazy notion of FB.

Electro magnetics isn't easy to understand.

But why does not a large current flow when you apply
a voltage across a coil?
The magnetic field the applied voltage sets up opposes the flow.




>
> But, so considered, the feedback is less than unity so the result is
> finite...whereas with a short it would be infinite. In fact an
> equilibrium is reached for any given current, and reactance does not
> disappear as quickly as you think, or as RDH shows.

I didn't say the L dissapears once a threshold has been reached; but it
is as if
there is a sudden large reduction fall in inductance at saturation.



>
>
> Of course there is also primary resistance, which remains
> unaffected, so that is two reasons why the coil does *not* become a
> short.

Well when the inductance ceases to oppose the flow during the wave
cycles due to
saturation, the applied voltage to the primary tends to be a short
circuit current, ie,
the mains applied voltage sees only the DCR of the primary.
This situation *is* regarded as a short circuit.


>
> Further, you may notice that a saturated transformer gets hot. A
> short dissipates no power (only current, no voltage drop), so where
> do you think the heat is coming from?

Two things, core losses, and copper losses.

>
>
> > The very field in an inductor opposes the current flow...
> >>
> >> If you think of it as a load resistance in parallel with an
> >> iron-cored inductor with a given core and number of windings, it
> >> is
> >> the current through the inductor that produces the magnetic
> >> field,
> >> hence too much current leads to saturation.
> >
> > The too-much-current occurs after a voltage threshold hold has
> > been
> > reached.
>
> Only at a particular frequency (sigh...). Do you remember any school
> maths?

See my formula.

Seems to me my maths are better than yours.

> Remember the common questions that ask you to reduce an
> equation to its simplest form? Why was that an important skill to
> learn? Because it allows you to see the essential relationship
> without the complications of mathematical clutter and particular
> circumstance.

The transformer equation is the simplest way to relate the design
parameters of a transformer.

>
>
> In this case you would substitute voltage and frequency for current,
> and end up with a very simple formula. That would improve your basic
> grasp of the relationship, which would be true for *all cases*.
>
> Armed with this fundamental understanding, you can quite easily
> derive whatever practical formula you wish, depending on the
> particular problem you are addressing. If you are interested in
> frequency and voltage, then you can derive the formula you gave from
> RDH.

I did derive my formula from what is in RDH4 and a few other old books
that explain everything.
Often their explanations are totally incomprehensible.
Electro Magnetics was designed by the schitzoprenic brother of the
God Of Triodes, and this dude made the use of iron, wire, and volts
to be as confusing as possible.

>
>
> Fundamental understanding is, in other words, transferable. Without
> it, particular knowledge is just empty fact. That's why you have to
> keep reading the book...you lack the conceptual framework to
> construct your own thoughts.
>
> If you know the core, and how many turns on the windings, and how
> much current through each, you are home and dry. The only reason you
> need to drag voltage and frequency in is in order to calculate the
> current.
>
> You won't see what I mean, and neither will Phil. Oh well.

I do wind much better transformers than I can buy from anyone.

My conceptual understanding levels are good enough.

Phil knows more about it than you do, and he can explain it when he
isn't
acting like a complete fukken idiot.

Patrick Turner.

>
>
> cheers, Ian

.