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Old 30th June 2006, 12:34 PM   #1
Nixie is offline Nixie  Canada
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Default Serious transformer problem

I had rewound the secondaries on two 950 W transformers a few months back, both identically. I just tried measuring the inductances of their windings, and to my surprise one of the transformers measures more than twice as high as the other one... WTF
When connected to the mains, both transformers seem to work fine and put out the same voltages on their secondaries, draw the same current from mains with secondaries open, and drop about the same voltage on the secondaries when a heavy load is attached.
Yet they measure so hugely differently, by a factor of at least two, and when I feed the primaries from my scope's 1 kHz square wave source, they load it differently.
I don't get it, how to troubleshoot this. None of the four secondaries could be shorted, because otherwise a winding wouldn't put out the right voltage. The foil electrostatic shield between the windings I'm pretty sure is not shorted, and the two ends are separated so there's no complete turn; it would probably overheat if it was shorted. I don't know what else it could be. I haven't touched the primaries when I was rebuilding the transformers. Both transformers also have a copper flux band outside the core (parallel to the windings), as well as magnetic shielding alloy along the core's perimeter, but all is the same here as well. I'm completely clueless.
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Old 30th June 2006, 01:35 PM   #2
SY is offline SY  United States
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Is there a way you could confirm that the cores are (or aren't) the same? The first thing I'd look for is an unintended gap or two in the one with lower inductance.
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Old 30th June 2006, 01:45 PM   #3
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If they measure the same current when connected to the mains, they must have the same inductance at the mains frequency.


Sy, if there are varying gaps, I think that would affect the unloaded line draw as well. Since he said identical draw, I suspect a frequency dependence.

What freq is your inductance meter?

John
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Old 30th June 2006, 02:00 PM   #4
Nixie is offline Nixie  Canada
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Gap? The core laminations are interleaved, E and I swapped each layer. There is at most a difference of 1 (of about 100) in the number of laminations in each.

Each has four identical windings. I measured current with a shorted secondary, and 150 W bulb as ballast on the primary from mains. Primary and secondary currents were same for both within 5 %.

The multimeter has two ranges and measures with 1 kHz on one and 200 Hz on the other. So for the primaries I get (open secondaries): transformer one: mH range: 146.1 mH, H range: 0.142 H; transformer two: mH range: 284.4 mH, H range: 0.265 H. On the scope, the 1 kHz square wave is loaded as sketched in the attachment, each color is one of the transformers.

[edit] jneutron, the frequency dependence makes sense for 1 kHz, but not the 200 Hz the meter uses for its H range.
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Old 30th June 2006, 02:15 PM   #5
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Default Re: Serious transformer problem

Quote:
Originally posted by Nixie
Gap? The core laminations are interleaved, E and I swapped each layer. There is at most a difference of 1 (of about 100) in the number of laminations in each.

Each has four identical windings. I measured current with a shorted secondary, and 150 W bulb as ballast on the primary from mains. Primary and secondary currents were same for both within 5 %.

The multimeter has two ranges and measures with 1 kHz on one and 200 Hz on the other. So for the primaries I get (open secondaries): transformer one: mH range: 146.1 mH, H range: 0.142 H; transformer two: mH range: 284.4 mH, H range: 0.265 H. On the scope, the 1 kHz square wave is loaded as sketched in the attachment, each color is one of the transformers.
Ah..ok..the meter changes freq to pick up resolution..I believe it is going to 1k for the lower inductance range. So, tranny 1 seems stable with freq, tranny 2 drops inductance as freq goes down.


Your initial statement:

Quote:
Originally posted by Nixie
When connected to the mains, both transformers seem to work fine and put out the same voltages on their secondaries, draw the same current from mains with secondaries open, and drop about the same voltage on the secondaries when a heavy load is attached.
From your test description, you didn't measure the unloaded current of each tranny. Try measuring it unloaded..what is the current draw when the secondaries are open. This will measure the current of the primary due to inductive reactance plus that due to eddy losses in the core.

John
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Old 30th June 2006, 02:18 PM   #6
Nixie is offline Nixie  Canada
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I did: "draw the same current from mains with secondaries open"

I don't know if directionality of the windings can make this frequency dependent difference. I'm not sure all are wound in the same direction (the secondaries are not on top of one another but split-bobbin type arrangement, all on top of the primary and its foil shield). But I guess it shouldn't, as I hadn't connected them to each other.
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Old 30th June 2006, 02:46 PM   #7
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Quote:
Originally posted by Nixie
[edit] jneutron, the frequency dependence makes sense for 1 kHz, but not the 200 Hz the meter uses for its H range.
Agreed. Your whole problem isn't making sense, that's what is so intriguing about it..I love it..


Quote:
Originally posted by Nixie
I did: "draw the same current from mains with secondaries open"
If the inductance at 50(60) hz is different, they hafta draw different currents. Your statement of "within 5%" seems to indicate that difference. I can't believe that the inductance would plummet to identical at line freq..it just hasta be diff. Measure the line current directly, secondaries open, with no series load on the primary.

Quote:
Originally posted by Nixie
I don't know if directionality of the windings can make this frequency dependent difference. I'm not sure all are wound in the same direction (the secondaries are not on top of one another but split-bobbin type arrangement, all on top of the primary and its foil shield). But I guess it shouldn't, as I hadn't connected them to each other.
Winding directionality will have no consequence on the inductance.

Cheers, John..

ps..this problem is fun, thanks.
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Old 30th June 2006, 02:50 PM   #8
poobah is offline poobah  United States
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Nixie,

Your gap occurs between the E's & I's, in each layer, where they meet. The reluctance of your magnetic circuit is primarily a function of this small gap. It may be. 0.004" on tranformer and 0.006" on the other.

Measure the inductance of the secondaries... you will see the same ratio as the primaries.

The question is: does the transformer with the most inductance saturate at full power?



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Old 30th June 2006, 03:23 PM   #9
6h5c is offline 6h5c  Netherlands
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Quote:
Originally posted by Nixie
...
The multimeter has two ranges and measures with 1 kHz on one and 200 Hz on the other. So for the primaries I get (open secondaries): transformer one: mH range: 146.1 mH, H range: 0.142 H; transformer two: mH range: 284.4 mH, H range: 0.265 H. On the scope, the 1 kHz square wave is loaded as sketched in the attachment, each color is one of the transformers.

[edit] jneutron, the frequency dependence makes sense for 1 kHz, but not the 200 Hz the meter uses for its H range.
Hi Nixie,

Just my view: don't you think a primary induction of merely 146.1 / 284.4 mH (!) is a bit low for a fat @ss 950VA tranny? It should be in the range of multiple Henrys. I just measured a 500VA toroidal that I have here and it's about 14H .
Maybe your measurement is wrong?

Regards,

Ray.
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Old 30th June 2006, 03:45 PM   #10
Nixie is offline Nixie  Canada
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Quote:
Originally posted by 6h5c
is a bit low
Well sure, the cores are designed for 60 Hz, not 200 or 1000. An EI choke I wound and I'm certain is at least 5 H measures less than a tenth of that at the meter's frequencies. Yet high frequency cored chokes with known inductance give the expected number at measurement, so I know the meter is working fine. The point is, something's gotta be wrong if I get such a huge difference even if at frequencies that are much above the mains.

Quote:
Originally posted by poobah
Your gap occurs between the E's & I's
Yes, and they are likely all different, since I assembled the thing by hand after rewinding. I hammered a bit on each transformer after assembly to minimize the gap, but there's probably on average up to a tenth of a millimeter variation. Still, due to variation over hundred laminations, it's likely to average out to very similar gaps on each transformer.

Quote:
Measure the inductance of the secondaries... you will see the same ratio as the primaries.
Yes. But again, can I be sure it's the core? It still bugs me the small possibility the electrostatic shield aluminum foil between the primaries and secondaries may have its ends shorting each other, though I just poked it around with a small screwdriver from the side where the ends overlap around a paper insulator, and didn't make any difference. I can't see deeper towards the middle of the bobbin though. Partial shorts like that usually decrease output voltage, but since it's aluminum foil and it's just one turn, who knows.

Quote:
The question is: does the transformer with the most inductance saturate at full power?
Don't you mean with the least inductance?
The sine wave when powered is the same on both transformers, both primaries and secondaries, so I guess neither one does, whether there's a load or not (actually more load should decrease the chance of saturation as it increases resistive losses, no?)
However, the transformer with lower inductance buzzes more. Not the windings, but the magnetic shielding material wrapped around the core (perpendicular to the windings. However, if the core was saturating, it would probably saturate the thin wrap as well, and distort the sine waves on the scope. Might be close however and that could cause the buzzing, or more leakage flux is causing the buzzing. I know it's not the windings since when I squeeze them through the copper flux band I feel no vibration. When I touch the shielding, it's vibrating.
The primary is multitap. Buzzing is loudest with the tap for the highest secondary voltage (least primary turns), as makes sense; I'll probably avoid that one; luckily the middle tap gives me enough voltage, though there is still some buzzing -- which is likely related to the difference in measurement.
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