The thing is, these were initially identical transformers, I just took
them apart and replaced the secondaries. There's no difference between
the cores.
Since they do draw the same current from the mains when secondaries are
open, I guess can't be the shield shorted.
I don't see how gapping could be the issue, since the laminations are
interleaved, Es and Is switching place every other layer; the gapping
varies a bit this way due to me manually reassembling them, but again,
putting a squeeze in the vice makes very little difference, 5% not 100%.
I'm interested in your partially magnetized core suggestion. How can I
check for that, or demagnetize it?
Will Matney wrote:
> Borislav,
>
> I know you've answered some of the following questions below, but I'm adding
> all I can think of to help. Does both transformers have the same amount of
> iron in them? Is the laminations the same size? Are the laminations the same
> thickness? Are you sure both have the same number of turns in the secondary?
> Is the iron type different between the two possibly (this really shouldn't
> make a huge difference in the inductance though unless one has something
> added in the iron which impedes the magnetic flux adding more reluctance).?
> Transformer iron doesn't vary as much as does ferrite, or iron powder where
> you have different AL factors unless it may be something like Permedure, etc.
> Could one core be partially magnetized? If the shield were shorted of course
> you would have a low voltage, high current short for what ever one turn
> equals out to in voltage by using the turns per volt figure. When you applied
> current to the primary, you would sure know it as it would be a big hum and
should blow a fuse, etc. I would definately double check this! Any short
generally acts this way.
>
> What controls the inductance is the number of turns and the amount of iron in
> the core in square centimeters or square inches. A different core material
> could possibly cause it if one type impeded the flux more than another
> (greater reluctance). A gap will effect it some more because of flux
> impedance (reluctance), but really it's used to keep a choke from saturating
> at high current levels, or where you have DC mixed with AC, not for
> transformers. If ones gaps a transformer, it needs to be a very thin gap like
> 0.002" or even thinner as in a C-core type. These are actually ground and
> lapped so they fit really close. Check all this over, and let me know what
> you find. I'll do my best to help.
>
> Best,
>
> Will
>
>
> *********** REPLY SEPARATOR ***********
>
> On 6/30/06 at 9:10 PM Borislav Trifonov wrote:
>
>> I had rewound the secondaries on two 950 W power transformers a few
>> months back, both identically, each with four identical windings side by
>> side. I hadn't touched the primaries. There is a layer of aluminum foil
>> as an electrostatic shield between the primary and secondaries, not
>> making a complete turn, with drain wire. Yesterday I tried measuring the
>> inductances of their windings with my DMM, and to my surprise one of the
>> transformers measures more than twice as high as the other one, on all
>> its winding... WTF!
>> The meter uses 200 Hz in the range I was measuring with, so it gives me
>> a much smaller number than the actual inductance as the laminations are
>> designed for 60 Hz operation. Nonetheless, the huge difference between
>> the two transformers is consistent across all windings, same ratio.
>> Moreover, the transformer with the lower inductance buzzes the outer
>> magnetic shielding more when powered, indicating more leakage (I know
>> the cores do not saturate as I get fine sine waves on the scope for both
>> of them). Measurement of current through shorted secondary when
>> powering the primary through a ballast is the same for both
>> transformers, and both draw the same current from mains when secondaries
>> are open. All secondary windings produce the right voltages, and drop
>> the same under heavy load.
>> Yet, the measurement difference and the buzzing difference clearly
>> indicate something is wrong with one of the transformers. I was
>> thinking partially shorted winding, but then the voltage output would be
>> changed. It's possible the electrostatic shielding foil between the
>> primary and secondaries is shorted, though I'm pretty sure I had the
>> ends of the foil not touching each other (and poking with a needle from
>> the side and shorting the foil on the other transformer didn't seem to
>> create a difference anyway).
>> I considered gapping between the Es and Is of the transformers. However,
>> since I assembled both manually (interleaved, of course), I'd expect the
>> variations in gapping throughout the layers on each transformer to
>> average out to similar values for both transformers, I'd say up to 1/10
>> mm; putting them in the vice to squeeze Is towards Es does increase the
>> meter's measurement about 5%, but the same amount on both transformers,
>> so the ratio remains the same.
>> Well, I'm running out of ideas here. To start to take apart the more
>> buzzing/lower inductance (I guess more leaking) transformer, I'd have to
>> also take the other one apart simultaneously to do comparisons so I know
>> when I've reached the trouble spot. This especially sucks since
>> squeezing in the heavy gauge wire in the amount of space available was
>> very difficult work when I had put these together.
>> Help!
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>
>
>
>
>
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