Transformer operation question

• posted

I have another question for the group. Take any transformer, in this case, I'll use a torus shaped core; one with the primary spiraling around the core all the way (360 deg.). As I understand, the torus design is efficient and there is low flux leakage from the core. If this is the case, how is the magnetic flux cutting through the secondary (and the primary) to produce current? It seems that flux only has to pass through the inner area of the coiled wire to generate current without actually cutting across the wire itself yet transformers are obviously efficient devices. Does this have to do with flux actually collapsing and expanding out of and into the core due to the AC? The problem I see with the toroid design, the flux is always kept in the core because the windings are looped back on itself due to the torus shape.

Thanks, John

• posted

Ah yes, you have detected one of the classical "over-simplifications" used to teach electro-magnetic theory.

Strictly speaking, it is not necessary for conductors to "cut" lines of flux to generate a voltage, although that is the conventional explanation. What IS necessary is a change the number of lines of flux enclosed by the electrical circuit. It is NOT necessary for the conductors to even be IN the magnetic field, only that they enclose it.

Sounds wierd, I know, but that's the way it is!

• posted

in article ceZnb.20093\$ snipped-for-privacy@bgtnsc05-news.ops.worldnet.att.net, jriegle at snipped-for-privacy@att.net wrote on 10/29/03 4:49 PM:

In this question, there a number of implicit assumptions made that are not justified.

First of all, the leakage reactance of a transformer is the same as the reactance would be without a core present. The core only ends up increasing the magnetizing inductance. That is a necessary requirement to approach the properties of an ideal transformer.

Most EEs and physcicists as well do not truly understand faraday's law of induction. I may be one of those. To get around that, they are indoctrinated with two approximations, the flux linkage law and the flux cutting law. Neither is correct but are good in many situations.

For example, In a dc generator, where the exciting field is rather constant, the flux cutting law is used. E = v x B neglecting constants and signs. Similarly, for transformers a low frequencies with not displacement current

curl E = dB/dt.

The real problem arises when there is both motion (flux cutting) and varying flux linkages as in transformers whose shape is changing at the same time a the flux is changing. In such situations, understanding is what counts, not the half ass rules. Fortunately, that is not the case for typical transformers. Maxwell's equations rule.

Bill

The basic correct formulation is that of Ma

• posted

in article snipped-for-privacy@giganews.com, BFoelsch at snipped-for-privacy@snet.ditch.this.net wrote on 10/29/03 5:08 PM:

*Lines of flux* is not a truly meaningful term. *Flux* by itself will be the correct term. It is really an integral of B over an area. Until you go into quantum mechanic, a totally unnecessary approach. the field is not quantized into lines.

Probably, because Faraday was not mathematical, he used the concept of lines to help his thinking. Maybe he also used iron filings to visualize magnetic fields getting a presentation that looked like lines.

Bill

• posted

indoctrinated

---------- Actually, this is not a problem -just a bit messy. In general for rotating machines, as an example, there will be both transformer and speed voltage terms. Think in terms of d(Li)/dt rather than Ldi/dt (or go deeper, through Maxwell) That is the effects of varying flux and changing position can readily be handled. There are many energy conversion texts, and all those at graduate levels, which do this. . Maxwell's equations rule and can be used to lead to the same results assuming a quasi-static situation which is valid in most cases, particularly considering the approximations needed to allow solution within a lifetime.

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

• posted

Thanks for the comments. It sounds intriguing that a current can be produced in a coil of wire that contains a magnetic flux even though the coil does not have to be in the flux, just surrounding it! John

• posted

in article xvkob.200682\$ snipped-for-privacy@bgtnsc04-news.ops.worldnet.att.net, jriegle at snipped-for-privacy@att.net wrote on 10/30/03 7:17 PM:

Just go back to the integral form of the Maxwell induced emf equation. Because it is difficult for me to put it into normal mathematical notation merely using characters, I will describ what it says.

The line integral of EMF around with respect to the line element is

-the time rate of change of the surface integral of B with respect to the areal elements overa surface bounded by the line integral path.

In other words, the emf induced around a closed loop equals the rate of change of magnetic flux through the loop.

There is nothing about curring or lines of force required. There are no mental crutches such as flux cutting to be used.

Bill

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