MMF diagrams of transformer construction options

[simphys]

Hey, I was hoping for some help here. It's nothing major, I was wondering why the windings of a transformer are represented in such a manner as in the two following top-view cross-sections (I think) of the transformer.

The main question is, normally it is just represented as a normal coil that's wounded around the core, so I'm wondering how these actually look in real life if they have such a layout.

1. Here there are conductors next to each other in the horizontal direction, but I don't understand why instead of just having them next to each other in the vertical direction?

2. for this (I assume this is a multi-layer winding), I don't understand how these are connected to each other when such foil layers are used. I'm thinking, if they're stacked on top of each other they would also have a big capacitance as well? I couldn't really find an example on the internet.

Thanks in advance!

 

[berkeman]

I'm having trouble parsing those drawings. Why don't the coils encircle the core? Can you give a link to where you got those drawings?

 

[simphys]

I'm having trouble parsing those drawings. Why don't the coils encircle the core? Can you give a link to where you got those drawings?

Well, that's what I'm struggling with myself, and these are lecture slides. But it could be that it's from the following book as that's what he based it on:
Fundamentals of Power Electronics 3rd Edition.

 

[simphys]

@berkeman, the same representation has been used here

https://electronics.stackexchange.c...-transformer-why-layers-are-numbered-this-way

 

[berkeman]

@berkeman, the same representation has been used here

https://electronics.stackexchange.c...-transformer-why-layers-are-numbered-this-way

Okay, thanks. This is the missing drawing that finally shows how the coils do wrap around the middle leg of an EE or similar core:

 

[DaveE]

But it could be that it's from the following book as that's what he based it on:
Fundamentals of Power Electronics 3rd Edition.

Yes, it is from Erikson et. al. But it's a common way to sketch currents in this sort of problem. In the first drawing, he's discussing leakage flux, in which case the core can mostly be ignored. The assumption is that flux that couples with the core structure will also couple to the other windings and isn't a significant part of leakage flux. Of course that's not exactly right. But the old rule of thumb, that the leakage inductance in a transformer with a core structure is the same as the winding inductance when the ore is removed is common and good enough for engineers.

In the second drawing, he's discussing the MMF, not the flux, so the core isn't really important and isn't shown. In other words, that drawing is about the H-field, not the B-field.

It is common in these drawings to simplify the sketch to the salient features, for clarity, and ignore things like the assumed symmetries of current return paths, dimensions, and such. That's why textbooks have a bunch of words as well as the pictures. This is an excellent text and worth reading.

 

[DaveE]

I don't understand how these are connected to each other when such foil layers are used.

First, I'm not sure it is foil, he may just have chosen not to draw the individual wires. He just wants to show some simplistic current density. Then these drawings are cross sections, somehow they are connected to make the current flow he wants to show, but he doesn't think is important to his point.

I'm thinking, if they're stacked on top of each other they would also have a big capacitance as well?

Yes, foil windings tend towards more capacitance and less inductance than multiple turns of wire. They tend to have surfaces that are closer on average to the next layer because of less insulation or air gaps. This is just from the geometry of building a capacitor.

 

[Tom.G]

Just something that hasn't been mentioned yet:

In the first figure, those circles with an "X" or a Dot in them usually indicate the polarity of magnetic field lines. The symbols represent an Arrow heading away from you into the page (X, for the feathers), or toward you (Dot for an arrowhead).

I have seen them, rarely, to indicate direction of current flow in a wire.

Cheers,
Tom

 

[DaveE]

Just something that hasn't been mentioned yet:

In the first figure, those circles with an "X" or a Dot in them usually indicate the polarity of magnetic field lines. The symbols represent an Arrow heading away from you into the page (X, for the feathers), or toward you (Dot for an arrowhead).

I have seen them, rarely, to indicate direction of current flow in a wire.

Cheers,
Tom

It's current direction in those diagrams. This isn't uncommon. It is simply the polarity in an 'end on' view of a vector quantity. Any vector will do, so you have to understand the context.

 

[Babadag]

I think SKM presents more details.