Consider two step-up ferrite transformers:
a) 12V/230V/10W
b) 12V/380V/10W
Both have the same core material and size.
Both are loaded with 10W.
A transformer b) will probably have a little lower efficiency (higher losses) because of higher turn ratio. Why? Where are the additional losses?

Is it just secondary wire resistance losses or something else?

 

Consider two step-up ferrite transformers:
a) 12V/230V/10W
b) 12V/380V/10W
Both have the same core material and size.
Both are loaded with 10W.
A transformer b) will probably have a little lower efficiency (higher losses) because of higher turn ratio. Why? Where are the additional losses?

Is it just secondary wire resistance losses or something else?

Hi,

This question might be a little too general. Without knowing the construction it's hard to say which one would be more efficient. The coupling constant could be an issue though but it's hard to nail down without knowing the actual construction of each, which means wire size, etc.

 

Keep in mind that High-Frequency-Transformers operate partially on Magic-Spells, and Incantations,
which make take years of trial-and-error to decipher.

They don't have any "Simple-Rules", and every detail is dependent upon every-other-detail to work,
especially when optimizing for the smallest footprint that still has acceptable performance.
.
.
.

 

If both transformers use the same wire size, then more turns imply more losses from DC resistance. This will be a factor, but not the only one or even the most important one.

 

Really, there are simple rules for transformers that usually give useful results. Unfortunately they are far from universally optimum.
A very interesting article in QST a while back describing the development of a power transformer for a mains powered high voltage power supply came up with one important point, which was that the standard core dimension formula was based on optimizing cost, rather than efficiency. They found that increasing the core stack to reduce leakage flux also reduced the spikes that caused poor regulation.
My point being that when looking at any "design rule" it is very important to understand what it is intended to optimize. Rules for efficiency were obviously ignored in the design of those RF noise producing doorbell transformers, as one example of design for maximum profit.

 

If both transformers use the same wire size, then more turns imply more losses from DC resistance. This will be a factor, but not the only one or even the most important one.

Can you tell what are the other factors?

All I know adding more layers increases resistance by some number (the diameter of more outer turns increases), so R losses increases.

The next is decreased coupling factor since middle distance between primary and secondary increases.

 

The next thing to consider is flux density and core saturation. As you approach the nonlinear part of the B-H curve (flux density vs. field strength) you put more power in for no additional return. This will certainly compromise efficiency.

 

If both transformers use the same wire size, then more turns imply more losses from DC resistance. This will be a factor, but not the only one or even the most important one.

Both transformers are putting out 10W, so the secondary with most turns will be carrying less current, which will result in less resistive loss per turn.

 

Both transformers are putting out 10W, so the secondary with most turns will be carrying less current, which will result in less resistive loss per turn.

Yes, I was thinking more turns, more current, more losses. Transformers can trick the mind like that.

 

The next thing to consider is the coupling coefficient or leakage inductance. This part of the primary impedance that is not coupled to the secondary is also a potential source of loss of efficiency.

 

so the secondary with most turns will be carrying less current, which will result in less resistive loss per turn.

Less current but smaller wire and more turns (more wire length) so the resistance is higher. The resistance is up by squared. (more or less) the winding needs to fit in the same area so the wire diameter must be smaller, and the turns must be more (longer wire)

 

Unspecified details matter tremendously.

 

Consider two step-up ferrite transformers:
a) 12V/230V/10W
b) 12V/380V/10W
Both have the same core material and size.
Both are loaded with 10W.
A transformer b) will probably have a little lower efficiency (higher losses) because of higher turn ratio. Why? Where are the additional losses?

Is it just secondary wire resistance losses or something else?

You're right that transformer (b) with the higher turn ratio will likely have a bit lower efficiency. This is mainly due to increased resistance in the secondary winding because more turns of wire mean higher resistance.

But it’s not just about the wire resistance – the higher voltage can also lead to increased core losses (like eddy currents and hysteresis losses) because the magnetic flux changes more rapidly. So, it's a combo of wire resistance and additional core losses.

 

because the magnetic flux changes more rapidly.

Looks to me like the primary wave form will be the same for both supplies. I think the core loss is the same.

 

If both are pulling 10 Watts that means the 380V is only pulling ~26mA while the 230 is pulling ~43mA.
In terms of efficiency, transformer "b" seems to be more efficient since you need less current to achieve 10 Watts

This is a similar reason why power lines use such high voltage ... the losses you encounter due to IR (current over resistance) loss are far less when you increase the transmission voltage ... in the case with the transformers, transformer "a's" load resistance would be about 5k while transformer "b's" load resistance would be about 14k

 

Increasing the amount of core material will usually reduce the leakage flux, improving efficiency. The standar formulasdo not explain that part.

 

If both transformers use the same wire size, then more turns imply more losses from DC resistance. This will be a factor, but not the only one or even the most important one.

Yes sure, but the way transformers are usually designed is that the goal is to keep the efficiency somewhat high while still meeting the specs. That means that a transformer with higher output voltage would have thicker wire if there was a resistance issue. Maybe that would induce more skin effect though but that's the way it goes. So the skin effect losses would be the game changer in a real transformer design, although probably minimal for a small output voltage ratio one transformer to the other.

We might also see an issue regarding the distribution of the field over a larger physical area with more turns. That probably means a slight increase in leakage inductance.

 

"Skin Effect" at typical mains power frequencies is rather limited. At higher frequencies it is much greater. This is fortunate.

 

"Skin Effect" at typical mains power frequencies is rather limited. At higher frequencies it is much greater. This is fortunate.

Hi there,

I hate to disagree, but I am only going to disagree a little because you are right for the most part. It depends a little on how you look at the problem and what wire size is being used.

Skin effect could be a problem even at 60Hz because thick wire still has a problem with this. I think the max is around 1/4 inch diameter, but this is going from memory now. That covers a lot though that's why I only disagree a little. When I worked in the power industry we did have to design with fairly thick wire sometimes because the converters were super high power.

The other point is that skin effect is still skin effect, and what I was referring to was even the smallest effect that would tip the balance. I'll try to illustrate.
Say we have transformer A with efficiency 90 percent, and transformer B with more turns but the efficiency is 89.999 percent. That's what I meant, even that small difference, because if it is a difference then it's a difference and it's mainly to help understand the issue not really to be practical about it unless we are building a scientific instrument. Most of us here, for what we do most of the time, this would not matter, but academically it's something to be noted.

 

OK, I see the point. But for transformers that I can lift with two hands it is usually not a big deal.