"Bert Hickman" wrote in message
news:(E-Mail Removed) ...
> The noise problem appears to be that the cracked epoxy is permitting
> slight physical movement between the primary and secondary
> windings. Under short-circuit, repulsion forces between the two
> windings is sufficient to allow relative movement or flexing the
> secondary windings during current peaks. The magnetic forces scale
> as the square of secondary current, so doubling the output current
> quadruples these forces.
> Minimizing noise involves preventing any differential movement
> between windings or flexing of the secondary winding. Since the
> secondary winding protrudes significantly beyond the primary
> windings in your transformer, you are introducing a significant
> amount of leakage flux. The free (outermost) portions of the
> secondary are attracted to neighboring turns while the inner
> portions (that overlap the primary) are repelled by the closer-
> proximity primary windings. Flexural forces, particularly at the
> top- and bottom-most turns, may have contributed to the destruction of the
> original epoxy. And, if you are using an E-I core, winding separation
> forces may be increasing the air gap
> in the core magnetic path. If so, this may also contribute to
> transformer noise and unexpectedly higher primary current
> under heavy loading.
> If you do decide to rewind, you may wish to consider changing
> the primary windings so that their outer diameter extends to
> the outermost edges of the secondary winding to reduce secondary
> flexural forces. The entire primary and secondary assembly needs
> to be epoxied together. Also, the winding assembly needs to be
> clamped/braced to place it under heavy compression to handle
> axial repulsion forces. Interleaving of E-core lamination should be
> used instead of E-I laminations to better handle axial spreading
> forces from the braced winding stack.
> Since you're dealing with similar short-circuit forces, you may
> also wish to locate a book on power transformer design to see
> how these forces are dealt with in large transformers. Some
> suggested books include "Transformer Engineering Design and
> Practice" by Kulkarni and Khaparde, "Power Transformers,
> Principles and Applications" by Winders, or Waters, "The
> Short-Circuit Strength of Power Transformers".
I appreciate the ideas. This transformer is used in a test set that dates
back to 1971, and AFAIK it is one of the first commercially available high
current primary injection test sets. By 1975, when I first became involved
with them, the output transformers were redesigned with a more efficient
configuration, with the high current output winding closely wrapped around
the core, and the additional turn wound next to the higher current loop for
twice the voltage and half the current. EIL's version, the BTS-50, which had
an output rated at 7500 amperes at 7.5 volts. It was fairly quiet up to
about 10 kA, and not too bad at 50-75 kA which was its maximum capability.
In 1981 Multi-Amp introduced the CB8160, which had a 5-stage output
transformer with dual windings rated at 10,000 amps at 11.5 volts. EIL had a
similar design called the BTS-1000. Both test sets were rated at 100 kVA and
supposedly could produce 100,000 amps into a short circuit. But the
Multi-Amp design was quieter and more efficient, with a lower internal
impedance due to its shorter and wider winding. At ETI, we designed a
PI-4000 which was based on an old EIL PS-600, which unfortunately was not as
well designed as the Multi-Amp, and it was noisy and had a much higher
internal impedance. Finally we emulated the CB8160 and used C-cores instead
of EI and had much better results.
Meanwhile, I had become convinced that a high current transformer could be
ideally constructed with toroidal primary cores, and around 1995 I designed
and built a prototype from four 1.4 kVA toroids and a four-turn secondary of
1" x 1/4" bus, with outputs of 2.8, 5.6, and 11.2 VAC at true continuous
current of 2000, 1000, and 500 amps. It proved to be very quiet and
efficient, and could produce up to about 12,000 amps into a short.
http://www.pauleschoen.com/pix/PI1000X-1.JPG
http://www.pauleschoen.com/pix/PI1000X-02.JPG
Finally I was able to design and build a toroid-based larger test set, the
PI-800 and the PI-1600, which has an output of 6 VAC at 1600 amps, and can
produce up to 20,000 amps quietly and efficiently:
http://www.pauleschoen.com/pix/PI800%20003.jpg
http://www.pauleschoen.com/pix/PI800%20007.jpg
http://www.pauleschoen.com/pix/PI1000+MCB-4.JPG
http://www.pauleschoen.com/pix/PI1000+MCB.JPG
http://www.pauleschoen.com/pix/PI2auxXfmr1.JPG
http://www.pauleschoen.com/pix/PI2aux-Rinside.JPG
http://www.pauleschoen.com/pix/PI2000-1-small.JPG
http://www.pauleschoen.com/pix/PI2000-2-small.JPG
We tried to design a much larger toroid test set but ran into some problems
and chose to use our tried and true PI4000B as a two-piece unit. This now
ETI's most popular design but we have had some problems with reliability. It
can be seen in one of the videos.
I'll have a look at some of the books you have recommended. I think I have a
good understanding of high current transformer design and I would really
like to build a toroidal replacement for this customer's old test set. But
it would cost at least $4000 additional and I think this transformer can be
repaired, even though it is not a good design. I think it will be much more
apparent what is causing the noise once I remove it from the enclosure. It
has to be a mechanical problem such as a loose lamination or foreign
material that I can't see. Once it's out I may be able to pour epoxy into
the gaps and solidly brace the output bus. It is also possible that the
magnetic flux is leaking into nearby steel panels and causing them to
vibrate. I'll know more next week.
Stay tunes, and thanks for the information and discussion.
Paul
Similar Threads
Linear Mode
