No load losses - normal vs reverse fed

[Besoeker]

Most of us that you feel have 'have missed something' have been talking about power transformers.

So have I.

 

[iwire]

So have I.

I am done, find someone new to bicker with

 

[Besoeker]

Here's a test for you.

Find one post of yours anywhere on this forum were someone on this forum was able to change your opinion.

In your view you are always correct, that cannot be true.

Might I gently suggest that you stick to the topic rather than personal judgements?

 

[iceworm]

... The comment about lower than expected voltage doesn't just apply to control transformers. It is just basic transformer calcs. The no-load voltage is higher than the on-load voltage. Thus, the turns ratio cannot be the same as the designed operational voltage ratio. It really is that simple ...

Nor was I referring to just those.
My point is valid for all transformers.

Bes -
I can't tell what your point is. I read all the posts and you are jumping around a lot.

Compensated windings has nothing to do with the No-Load voltage is higher than the Full-Load voltage. That difference is due to transformer impedance. This voltage drop has nothing to do with compensated windings, forward or reverse fed, or even the turns ratio.

For non-compensated, (non-CPT, in the US we would call this a power transformer) the turns ratio is the same as the voltage ratio. And that would be true forward fed or reverse fed. Unless you have a valid reference - I'm sticking with my story.

And yes, I am discounting, "Some manufacturers also build larger transformers with compensated windings." as referenced by the GE paper, cause those are sufficiently weird that I've never seen one. And I can't tell you about IEC equipment, so they doesn't count either. However, even if those were included, that doesn't change:

The voltage drop from No-Load to Full-Load is from the transformer impedance.

And I think you are wrong with the compensated windings just being a change in the turns ratio. I'm thinking magic excrement in the flux loop has at least as much to do with it as turns ratio.

ice

 

[Besoeker]

Bes -
I can't tell what your point is. I read all the posts and you are jumping around a lot.

Compensated windings has nothing to do with the No-Load voltage is higher than the Full-Load voltage. That difference is due to transformer impedance. This voltage drop has nothing to do with compensated windings, forward or reverse fed, or even the turns ratio.

For non-compensated, (non-CPT, in the US we would call this a power transformer) the turns ratio is the same as the voltage ratio. And that would be true forward fed or reverse fed. Unless you have a valid reference - I'm sticking with my story.

And yes, I am discounting, "Some manufacturers also build larger transformers with compensated windings." as referenced by the GE paper, cause those are sufficiently weird that I've never seen one. And I can't tell you about IEC equipment, so they doesn't count either. However, even if those were included, that doesn't change:

The voltage drop from No-Load to Full-Load is from the transformer impedance.

And I think you are wrong with the compensated windings just being a change in the turns ratio. I'm thinking magic excrement in the flux loop has at least as much to do with it as turns ratio.

ice

So what mechanism is used to compensate?

 

[Besoeker]

And I think you are wrong with the compensated windings just being a change in the turns ratio.

Not sure I suggested that.

 

[GoldDigger]

The way I see this B is stating that some or maybe all European power transformers have compensated windings.
US electricians and EEs who are familiar with US power transformers state that few if any have compensated windings.
Neither side has produced any evidence to refute the other position, only to support their own.
The two positions are *not contradictory*, so I do not see the point in just repeating the same assertions!

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[iwire]

The way I see this B is stating that some or maybe all European power transformers have compensated windings.
US electricians and EEs who are familiar with US power transformers state that few if any have compensated windings.
Neither side has produced any evidence to refute the other position, only to support their own.
The two positions are *not contradictory*, so I do not see the point in just repeating the same assertions!

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I agree with this entirely and both myself and electrofelon suggested that is the case but Bes would have none of that, he told us that was not possible.

 

[iwire]

Perhaps they design transformers like this in the uk, but not here in the states? Is that possible?

The link iwire posted was SquareD.

Basic transformer calcs don't vary with nationality.............

And in almost everyone of my posts I made clear I was talking about transformers I have worked with which I think we all know I work in the US.

 

[Besoeker]

I agree with this entirely and both myself and electrofelon suggested that is the case but Bes would have none of that, he told us that was not possible.

Do you agree/accept that no-load and on-load voltages differ?

 

[iwire]

Do you agree/accept that no-load and on-load voltages differ?

Changing the perimeters of the question again.

 

[Besoeker]

Changing the perimeters of the question again.

Can you just answer the simple question.
Do you agree/accept that no-load and on-load voltages differ?

 

[iceworm]

Do you agree/accept that no-load and on-load voltages differ?

There you go again. Stop it.

The difference between No-Load voltage and Full-Load voltage has nothing to do with compensated windings. It is caused by transformer impedance.

ice

 

[Besoeker]

There you go again. Stop it.

The difference between No-Load voltage and Full-Load voltage has nothing to do with compensated windings. It is caused by transformer impedance.

ice

But it has to do with turns ratio. Isn't that why, when used in reverse, the expected voltage may not be achieved as SquareD have stated?

 

[GoldDigger]

But it has to do with turns ratio. Isn't that why, when used in reverse, the expected voltage may not be achieved as SquareD have stated?

Ding! Everyone to their corners.

Transformer impedance causes full load voltage to be lower than no load voltage.

The only question is whether a transformer is designed to produce its nominal voltage at full load or at zero load.
That is a question of normal practice rather than a mandatory design feature.

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[iwire]

Ding! Everyone to their corners.

Transformer impedance causes full load voltage to be lower than no load voltage.

The only question is whether a transformer is designed to produce its nominal voltage at full load or at zero load.
That is a question of normal practice rather than a mandatory design feature.

Again I agree, I think you have summarized this nicely. At least that has been my understanding of this thread.

 

[iwire]

But it has to do with turns ratio. Isn't that why, when used in reverse, the expected voltage may not be achieved as SquareD have stated?

Again Sq D is talking about Control Transformers when talking about compensated windings. It does not say all transformers have compensated windings

The practice of backfeeding a general purpose transformer is NOT recommended, especially in transformers smaller than 3Kva. Backfeeding is not allowed for any Industrial Control Transformers of any size, because windings are compensated and backfeeding will result in lower than expected output voltage.

 

[big john]

It has to be to get the right on-load voltage. It isn't a fudge.

It's called a compensated winding. As far as I know it does not exist outside of control transformers.

The turns ratios on power transformers are often accurate within several decimals of nomimal nameplate voltages.

As far as OPs question, reverse feeding transformers can absolutely result in higher excitation current, making it harder to select primary protection due to nuisance tripping, but I don't know what actually causes that.

 

[GoldDigger]

As far as OPs question, reverse feeding transformers can absolutely result in higher excitation current, making it harder to select primary protection due to nuisance tripping, but I don't know what actually causes that.

I won't say this is the right answer, but it seems to me that the inner windings couple very tightly to the iron core and are very subject to transient effects of magnetization, while the outer windings are less tightly coupled (some of the inductance is air core) and so the surge current is more controlled.
That is in line with the primary of a one-way transformer being on the outside so that the taps are easier to run to terminals.
Do you have a reference for us Phil?

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[electrofelon]

Interesting aside:
Has anyone ever seen anything on minimum inrush (normal install forward feed)?

Had an occasion to advise on a temp power install. 225kva, 408D/208Y, fed with a 100A CB. Normal loading (primary side) was 50A.

I figured it would be more likely than not to trip. IEEE says 1 in 6 times the inrush will be max. All we had for instrumentation was a peak reading fluke clamp-on. That thing just oozed on line. As I recall, the inrush was less than 100A.

Considering 1 in 6 is max inrush, is 1 in 6 min inrush and the rest are scattered in between? Anybody seen any actual test results, or papers on tests?

Plenty of papers on max inrush. Haven't ever seen one on min inrush.

ice

I have seen plenty of motors/transformers/Ac's that have been wired by people who are not electricians. This means that usually they did not utilize the larger OCPD sizing allowed by the NEC as compared to "normal" loads. I cant recall ever having trouble getting any of these to start.

At my house, I have a 2400 to 240 15 KVA used in reverse, which then feeds another 2400 to 240. Its on a 90 amp breaker. I have been turning it on and off very frequently for various reasons, and it may have tripped out a few times upon energizing, but maybe not - its a homeline 2 pole so sometimes that handle tie makes the operation funky..... so hard to tell.

P.S. good paper, nice summary.