Larkinisms

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What, George? Is it supposed to be untrue? Explain how.

Tim

--=20 Deep Friar: a very philosophical monk. Website:

The magnetic flux density, B, is proportional to i for all inductor/transformer configurations.

B = uH, u is approximately constant.

H = N*i/(2*pi*r) for a toroid.

At 0 current the magnetic flux density is 0. Simple as that.

Incorrect. Flux is more fundamental than current, at least for the = purposes of transformers.

So, how much peak flux is applied by a 120V 60Hz sine wave? What is the = resulting flux density in the core? What is the resulting flux when the = secondary is heavily loaded?

At any rate, even when using H, you somehow forgot to count the = amp-turns of primary *and* secondary, which is just silly.

Tim

--=20 Deep Friar: a very philosophical monk. Website:

What "I" are you talking about? A transformer has two windings. And each winding has resistance.

Take a typical power transformer. Connect the primary to the AC line and leave the secondary open. Measure or compute the core flux density. Now short the secondary. At least before it catches fire, shorted-secondary flux density will be about half of the unloaded value.

Loading the secondary of a transformer reduces flux density... just look at the directions of the primary and secondary winding currents. It's surprising how many people get this wrong, and think that a transformer "saturates" if you short its output.

John

Really? How so? I guess most transformers do not need current to work?

You obivously don't understand the difference between magnetic flux and magnetic flux density.

Correct. The current is only a byproduct, and in general it is quite = nonlinear.

Permeability can be made arbitrarily high, so magnetizing current can be = made abritrarily small. A several-kW power transformer made with = stripwound permalloy could be driven beyond full operating voltage, into = saturation, with a transistor radio. Well, assuming transistor radios = had response down to 60Hz.

the=20

the=20

and=20

Evidently you never passed highschool algebra. This is the same = question as "a*x =3D 5, solve for x". Are you dismayed because it's not = a number, but a function?

Tsk tsk... I should know better, trolls don't answer questions...

Tim

--=20 Deep Friar: a very philosophical monk. Website:

Yes. The flux arising from secondary current is in opposition to that arising from primary current. The two currents are related, so the situation is more complicated than George's Theory appreciates.

For the purpose of calculation, it's handy to consider that, for a given frequency, saturation will occur when the effective voltage across the magnetising inductance reaches a particular value. With respect to the primary winding, its resistance is in series with its inductance, so the effective voltage across the inductance reduces as current increases. Because it does so in proportion to the winding resistance, the ratio of inductance to resistance becomes important in mains transformer design.

For audio purposes, the mains transformer should arguably have the lowest possible resistance, to minimise regulation under widely-varying load, and highest inductance, to ensure that the core remains within the most linear region of the BH curve, so minimising higher frequency mains harmonics. Such a transformer would, however, be very unforgiving of abuse, and less safe for general-purpose use.

I agree that George's Theory is common, perhaps because saturation is normally associated with flow rather than force. In this case, it's the difference between two opposing flows that counts.

A perfect transformer with infinite inductance and no winding resistance would be totally transparent to the circuit. The inductance disappears, together with the flux, so it can't saturate no matter how great the current. George's Theory would predict saturation with any current, no matter how small.

On a practical level, it's important to realise that the input impedance of a transformer reduces as current is drawn from the output! George's Theory would get this wrong, too. The ensuing extra primary current must be going somewhere other than through the primary inductance, and so cannot cause saturation.

All of this becomes apparent, George, if you look at the equivalent circuit of a transformer commonly used for purpose of simulation. It should be a proper road-to-Damascus moment. The primary inductance, in series with its resistance, is in parallel with the output. Consequently, transformed current flowing from input to output has no effect on flux, other than to *reduce* it because of the voltage drop across the inductance, caused by the primary winding resistance.

AFAIK most mains transformers are designed so that they are close to saturation with an open secondary. When under rated load Bmax is reduced. Would you expect mechanical transformer noise to reduce, or increase, George, when a secondary load is connected?

Ian

Not fluxes. They share identical fluxes, less leakage inductance.

It's the equal flux that forces equal voltages and *opposite* currents = to flow: EMF =3D -dPhi/dt.

Tim

--=20 Deep Friar: a very philosophical monk. Website:

An easy way to measure the flux density in a transformer core is with a probe coil, namely any old 3rd winding on the same core.

So get a power transformer with one primary and two secondaries. Connect the primary to the AC line. Measure the voltage on secondary 1 and switch a load on/off secondary 2. If flux density increases with load, the voltage on the sec1 winding will go UP as sec2 is loaded.

Of course, it doesn't anything that stupid. It goes down.

At least AlwaysWrong has a new friend.

John

"John Larkin" wrote in = message news: snipped-for-privacy@4ax.com...

Ironically, switching power supplies do this. But they have feedback, = so that's simply inversion of the feedback network.

Tim

--=20 Deep Friar: a very philosophical monk. Website:

Not fluxes. They share identical fluxes, less leakage inductance.

***The two sum to one, obviously, because there is only one core.

It's the equal flux that forces equal voltages and

*opposite* currents to flow: EMF = -dPhi/dt. ***Opposite? How are the currents opposed? Only in the sense that the flux arising from one is in opposition to the flux arising from the other.

Ian

***Careful. Easy to measure the *rate of change* of flux with a coil. If you try to explain to someone who doesn't know, it's important to be rigorous. ***Good, although the anthropomorphism may not be helpful. Also, if you wish to explain to someone who is unsure of the relationship between coil current, voltage, and flux density, an explanation that depends on an assumption of that relationship will appear as a circular argument. ***Learn to read and think before you squabble.

Ian

This is on the money. Jefferson's lack of further response duly noted.

Interesting word. You're assuming that the possession of, or being in, a state of 'stupid' is a purely human trait. I know of a poster here who uses over seventy aliases, with every single one aspiring to be as thick as a brick.

Ergo, if a step upwards, to the level of a brick, advances the quality of the state of stupid, we can not claim the condition to be a purely human one and the 'anthropomorphic' label is being misapplied.

On a side note, does Mister Larkin really deserve this *obviously* misguided MacCarthyistic attack on his beliefs? I think not. Let's not surrender so easily to the darker side of human nature.

mike

that's simply inversion of the feedback network.

A lot of switchers, even simple bucks, have mildly negative output impedances. I've never figured out why.

John

The real question is whether transformers deserved the slanderous description of their behavior.

John

induced voltage on the open winding is not a function of flux density, it is a function of the amplitude of the excitation winding.

This is commonly known as a feedback winding. Commonly used in switchers to control the drive of the transformer. That, in concert with a voltage feedback reference, allows one to regulate the output properly and efficiently, and with great load change response rates.

The real question is why you think that scat-in-the-hat has any brains regarding the subject at all.