Primer on Transformers needed

[Y2KEDDIE]

I am curious about the differences in transformer and choke design. I note core material, air gap size, laminations vs. Toroid are major differences. I just would like to understand in laymen’s terms how these things affect operation.
Microwave high voltage transformers are of unique design; the cores have special spacers. How do they effect operation.
Audio transformers have different cores vs. power supply, what are the differences. Does air gap play a role? Core material?
What is the difference in transformers used in high current capacity such as battery chargers vs. DC welder supplies?
In a linear supply often there is a choke input filter to help with regulation. I’ve seen chokes used in pipeline cathodic protection supplies where there’s low voltage high current output. What is the difference in these chokes?
Sorry for the discontinuity In my questions. I realize there are many things considered. I just would like some simple explanations.

 

[Gryd3]

I am curious about the differences in transformer and choke design. I note core material, air gap size, laminations vs. Toroid are major differences. I just would like to understand in laymen’s terms how these things affect operation.
Microwave high voltage transformers are of unique design; the cores have special spacers. How do they effect operation.
Audio transformers have different cores vs. power supply, what are the differences. Does air gap play a role? Core material?
What is the difference in transformers used in high current capacity such as battery chargers vs. DC welder supplies?
In a linear supply often there is a choke input filter to help with regulation. I’ve seen chokes used in pipeline cathodic protection supplies where there’s low voltage high current output. What is the difference in these chokes?
Sorry for the discontinuity In my questions. I realize there are many things considered. I just would like some simple explanations.

I can lightly help. Not too far in depth though.

Do you understand the mechanics of how a transformer works?
Do you understand how the magnetic field is created, and are you familiar with magnetic saturation?
Have you come across mention of eddy currents within a transformer?

At the very least, looking these up will answer your question regarding toroidal vs conventional transformers and the difference in laminated cores. This should also cover your question regarding high power application.
I am not aware of special spacing being used though, so I will need to dig around myself to complete my own picture

 

[Y2KEDDIE]

First, thanks for responding.

Yes, I have some theory (insigts) as to how they work but, I was hoping for some practical explanations. Most transformers shown on a schematic use the same symbols but obviously there's a lot of differences. I seen others posting questrions about these differences and the same answers: (theroy) I was hoping someone who had actual experiences mnufactuering transformers could shed some light. Text books are great for general theory but I am just a simple country boy.

 

[Arouse1973]

Core material effects many factors but mostly the permeability and hysteresis of the core. This effectively is how easy the core will transport magnetic flux. A cores resistance to magnetic flux is called reluctance so a core with a high permeability will have a low reluctance. But due the tolerance in manufacturing and the temperature effect on permeability an air core is usually used to help with this.

This helps to maintain a more reliable inductance value. It is like putting a resistor inside the core which will reduce magnetic flux which may also be needed in some applications. The down side of this is fringing flux which concentrates around the gap, this cause unwanted radiation and interference. So this has to managed correctly in the design.

Air gaps in chokes prevent saturation of the core which reduces the inductance and can turn the core into an air core.

Laminations reduce eddy current which cause heating of the core. This heat is wasted energy which effects the efficiency of the transformer. Heat will also change the inductance of the coil which in some cases is not desirable.

Toroid cores are used to reduce the size of the transformer generally and they also radiate less have a higher permeability which requires less copper windings to produce the required inductance. Silicon is added to the steel to help reduce the electrical conductivity and hence reduce eddy currents.

High speed cores again use material suited for high frequency operation like NiZn. I don't know what material is used for microwave core, probably some exotic material with a very narrow hysteresis loop.

That's all I know

Thanks
Adam

 

[davenn]

This effectively is how easy the core will transport magnetic flux.

Not really in favour of that comment, Adam

Since the Primary and secondary windings are, in 99% of the time, wound on top of each other. ALL the magnetic field from the primary
cuts through the secondary windings and nothing needs to be transported anywhere.
The primary purpose of the core is concentrate the flux density to within the transformer, rather than having the field radiating far and wide.
And concentrating that flux density improves the efficiency over an air cored transformer.

Dave

 

[Colin Mitchell]

"This effectively is how easy the core will transport magnetic flux."
"Not really in favour of that comment, Adam"

The core does have an enormous effect when the windings are on different legs.
The core also has an enormous effect when the secondary is not drawing current.
The core is called a magnetic circuit and the most important part of this item is the cross-sectional area.
It must be large enough to absorb the magnetic flux generated by the primary winding.
To explain this, we can remove the secondary winding.
We now connect the "transformer" to the supply (which must be AC) and as the incoming voltage increases, it produces "magnetic lines of force" from the primary winding. This is commonly called ELECTROMAGNETIC LIKES OF FORCE or ELECTROMAGNETISM and the "magnetic lines of force pass into the core and also cut all the other turns of the primary winding.
The lines of force that enter the core produce microscopic currents in the material and these current produce magnetic flux that emerges from the core to cut the turns of the primary winding.
The overall effect is this:
As the incoming voltage rises from 0v to a few volts, the current increases and a considerable amount of magnetism (magnetic lines) is produced and the core responds with a considerable re-emission of magnetic lines that produces a voltage in the primary that is opposite to the incoming voltage.
As the incoming voltage increases, the "back voltage" also increases and the result can be as high as 99% back voltage.
This means only a very small forward voltage results and this small forward voltage can only allow a very small current to flow in the primary winding.
That is why the transformer only allows a very small current to flow when the secondary is not loaded.

 

[davenn]

The core does have an enormous effect when the windings are on different legs.

yes that would be true, but as I stated, that's very rare for power transformers

 

[Colin Mitchell]

"yes that would be true, but as I stated, that's very rare for power transformers"

I am sorry to say you are totally wrong.

Most transformers are wound with the primary and secondary side-by-side so you can provide 5,000v isolation.

 

[davenn]

I rarely see side by side windings 95% are secondary on top of primary

 

[Colin Mitchell]

"I rarely see side by side windings 95% are secondary on top of primary"
Everything in Australia is on split bobbins to achieve the 5,000v isolation.

 

[Arouse1973]

Not really in favour of that comment, Adam

Since the Primary and secondary windings are, in 99% of the time, wound on top of each other. ALL the magnetic field from the primary
cuts through the secondary windings and nothing needs to be transported anywhere.
The primary purpose of the core is concentrate the flux density to within the transformer, rather than having the field radiating far and wide.
And concentrating that flux density improves the efficiency over an air cored transformer.

Dave

Hi Dave
That's fair enough, I wasn't talking about any particular type of transformer or inductor. I was merely adding something about core materials and trying to keep it basic. Regardless of transformer construction, permeability is the ratio of B(flux density) to H (magnetizing force) and as I understand it is the ability of a material to conduct magnetic flux.
Thanks
Adam

 

[BobK]

Hmmm. Try taking the primary off a power transformer core and plug it into your AC outlet. What will happen?

Bob

 

[davenn]

I wouldn't recommend it haha
unless you are the Mythbusters

 

[Y2KEDDIE]

Ok, from your comments I understand:

An air gap is needed to prevent saturation, but too much air gap will reduce inductance and efficiency.

Laminatitions are needed to prevent eddy currents.

High permability core is required to lower reluctance (increase efficancy.

Torroids offer higher prmability cores and better sheilding.

A few questions keeping me awake at night:

A t higher frequencies are air coils used without cores because the effiency is greater , not requiring a iron core? Is i possible to saturate an air coil?

Why do baluns (like those used to match 300 ohm twin lead to 75 Ohm unbalanced coax use ferrite cores instead of air cores.

How do swinging chokes work in a power supply? Does it have something to do with the air gap?

Is the difference between power supply transformers and audio transformers the type of core material? Permability and reluctance dependent on variable frequencies?