Re: Serious transformer problem


"Prune"

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!


** Forget using the DMM as a primary inductance meter for large, ungapped
transformers.

Such transformer primaries have very large and NON LINEAR impedances when
tested with low and medium frequencies. Non linear in the sense that the
impedance is a function of the actual level of the test signal used.


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.


** Nonsense.

You have confused "inductance" with "impedance".

Only the latter varies proportionally with frequency.




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.


** That last one proves there is nothing basically wrong.

Off load primary current at rated voltage ( aka I mag) is a good indicator
of transformer's integrity.

If one *really* did have a lower primary inductance - Imag would be much
higher.



All secoindary 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.


** Not really.

I suspect one tranny's laminations have not been as neatly re-stacked as the
other.




........ Phil






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