- Location
- Massachusetts
Yes for sure, there is no way I ever wire a transformer without checking the output before putting a load on it.
No load losses - normal vs reverse fed
- Thread starter electrofelon
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Yes for sure, there is no way I ever wire a transformer without checking the output before putting a load on it.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Perhaps they design transformers like this in the uk, but not here in the states? Is that possible?
From the link you posted:
"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."
The very point I've been making!
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.
It may not change what you do and you don't think it affects you. That's fine. Maybe not a big difference, one you can ignore for many practical purposes. But it is there.
Yes. As was I, so I'm not sure what point you are making.
The link iwire posted was SquareD.
Basic transformer calcs don't vary with nationality.............
- Location
- Massachusetts
The point I am making is clear, I work with and measure the voltages from transformers very often
The issue you keep bringing up is not happening in real life on the jobs I have worked on.
End of story, moving on.
The point I am making is clear, I work with and measure the voltages from transformers very often[/quote]
As did I. From 40V to 13,800V. Industrial power electronics if you'll recall.
Then why this from SquareD on the link you brought up?
"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."
Don't you think SquareD just might, just might have a sound technical basis for that categoric statement ?
As did I. From 40V to 13,800V. Industrial power electronics if you'll recall.
Then why this from SquareD on the link you brought up?
"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."
Don't you think SquareD just might, just might have a sound technical basis for that categoric statement ?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Respectfully, if you read iwire's entire statement, he was NOT referring to control power transformers, neither is that the subject of the thread. Although you are correct WRT CPTs, let's please stay on topic and cease the back and forth over details not germane to the OP.
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
- Occupation
- EE (Field - as little design as possible)
Here is a GE paper on backfed inrush. The example they use is a 75kva, 480D,208Y
max inrush forward fed - 11X
max inrush reverse fed - 37X
as mentioned by iwire and templdl:
The taps are on the wrong side to correct for overvoltage. If the primary voltage is high, setting the taps down will lower the output voltage but leaves the transformer prone to over-fluxing (careful with the pronunciation, please), increasing magnetizing current - it will run hot.
And from Bes:
The paper discusses compensated windings, "Some manufacturers also build larger transformers with compensated windings." But they don't identify any culprits. I've never seen one (larger than 3kva compensated), but apparently they are out there.
Interestingly the paper does not discuss ground detectors. The only method discussed is corner grounding.
ice
max inrush forward fed - 11X
max inrush reverse fed - 37X
as mentioned by iwire and templdl:
The taps are on the wrong side to correct for overvoltage. If the primary voltage is high, setting the taps down will lower the output voltage but leaves the transformer prone to over-fluxing (careful with the pronunciation, please), increasing magnetizing current - it will run hot.
And from Bes:
The paper discusses compensated windings, "Some manufacturers also build larger transformers with compensated windings." But they don't identify any culprits. I've never seen one (larger than 3kva compensated), but apparently they are out there.
Interestingly the paper does not discuss ground detectors. The only method discussed is corner grounding.
ice
Nor was I referring to just those.
My point is valid for all transformers.
How would you interpret compensated windings?
Physically, what exactly would you do to compensate?
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
- Occupation
- EE (Field - as little design as possible)
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
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
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
AFAIK, if the connection is made at just the right time (voltage peak) the inrush may be just the normal magnetizing current, with at most a factor of two that could be attributed to hysteresis.
If the initial current in the winding (zero) is the same as the steady state current would be at the same point in the cycle, then there should not be a major transient.
It is not like a motor which has to come up to speed.
Sent from my XT1585 using Tapatalk
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
- Occupation
- EE (Field - as little design as possible)
Out of my area - I'm definitely not a transformer designer. Maybe just add the turns required to bump the voltage to where the impedance pulls the voltage down to spec when it is full loaded. Then again, maybe there is magic excrement in the flux path.
Control transformer applications can be tough. For example, AC coil contactors have a high inrush. Maybe control xfm mfg are working to compensate for that.
ice
- Location
- Massachusetts
No, as has been pointed out by a number of folks by now, no.
Regarding windings compensation it has to do with CPTs. For example a 480-120 CPT, if you apply 480v on the primary you will get more than 120v on the secondary. The ratio is 1:less than 4 to compensate for control coil inrushes. If they had a 1:4 ration as a power transformer has the voltage drop and the instant the coil(s) are first energized may be significant enough to impedc the coil from pulling in. By compensatiing the secondary winding it allows for a stiffer secondary voltage when the coil(s) are first energized.
A good teaching lesson is a buddy of mine wanted to go on the cheep when he attempted to use a 120v voltmeters called to 480v and as tempted to use a 480-120 CPT instead of a more expensive 480-120 PT and wondered why his VM read higher than 120v.
Another eye opener is in you use a 480-120CPT to step up from 120 and expect to get 480 and actually get something significantly less than 480v.
Got it in one.
:thumbsup:
Quite so. And that point seems to be being missed here. Intentionally or otherwise.
- Location
- Massachusetts
No not missed.
Just because others have different views from your own does not mean they missed something.
Here is the quote you responded too
CPT = control power transformer.
Most of us that you feel have 'have missed something' have been talking about power transformers.
- Location
- Massachusetts
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.
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