| |
 |
|
|
Science Forum Index » Electronics - Design Forum » Ultimate high current transformer...
Page 1 of 1
|
| Author |
Message |
| Paul E. Schoen... |
 Posted: Tue Jul 15, 2008 11:04 pm |
|
|
|
Guest
|
For the testing of circuit breakers, we need to generate high current
pulses of several cycles at up to 10x or 12x the continuous rating of the
breaker, some of which are as large as 6,000 amperes. So we need at least
60,000 amperes into the breaker, which has a measured impedance of
something like 100-150 uOhms, which means an applied voltage of at least
6-9 volts. We have built transformers that produce about 10-12 VAC and
rated at 4000 to 6000 amps continuous, and their internal impedance has
varied from about 90 to 150 uOhms, depending on design. So at best, a
transformer that can produce 12 VAC, with 90 uOhms impedance, could push
63,000 amps into a 100 uOhm breaker, but only 57,000 into the 150 uOhm
breaker. And most of the transformers are not that good. But recently I
measured the internal impedance of a 3000 amp transformer, with about 10.5
VAC output, and it was about 70 uOhms.
This transformer consists of three separate E-I cores, 9" high and 12.5"
wide, two of which are 4.75" thick, and one is 2.5" thick. The entire
transformer is 19.5" long, with two side-by-side 3/8" x 4" bus bars bent in
a loop 4.62" apart. The smaller section is connected to a continuously
variable source of 0-520 VAC, and the second and third section primaries
are either shorted or connected to 480 VAC in such a way that 5 discrete
overlapping ranges of output are produced. We connected a solid busbar
across the output, and produced 2500 amps at about 20% of the adjustable
source. The voltage across the output of the transformer was 0.207 VAC into
this load, and 0.380 VAC at the same setting with the load removed, so I
determined the internal impedance as (0.38-0.207)/2500 or 69 uOhms. The
load impedance would be 0.207/2500 or 83 uOhms. So the maximum possible
output current would be 10.5/(83+69) = 69 kA, and into a 150 uOhm breaker
it would be about 50 kA.
There are larger transformers that are made with heavier busbar and more
sections. But the best I have seen had about 90 uOhm impedance and maybe 12
VAC output, so it would not do much better, although the larger size is
necessary for long-time overload testing at 3x rated current.
We were going to try a toroidal design that looked promising, but the
project was abandoned because of management issues. This design used two
pieces of 1/2" x 2" bus through the holes of two sets of toroidal cores
placed in horizontal stacks. But the toroidal geometry meant that the bus
bars had to be separated by about 7", and it seems that the wider
separation increases inductance and hence impedance. The ideal transformer
appears to be wide and low, and not excessively long.
We have now received some transformers made with C-cores, with
approximately the same shape as the one described above, but the bus
separation is 5.12", and the overall length is about the same, yet this one
has only one small section and one large section. There is a considerably
longer extension of the bus bars out of the end of the transformer to the
connection to the load. So we get an internal impedance of about 70 uOhms,
but only a 3.75 VAC maximum output. We actually use it as a 2000 ampere
transformer by using a series connection which provides 7.5 VAC but at 280
uOhms, so we would expect about 30,000 amps into a short. We plan to add a
second section which will add three more large cores and be used in
parallel for 4000 amps continuous output.
These C-cores are constructed in the usual way, with a single winding on a
spool around the double wide portion in the middle leg, and then the bus
bar is wrapped around it, which gives a minimum spacing between the bus
bars of about 5". I wanted to try using two smaller primary windings, one
on the top C-core, and one on the bottom, and then wrap the secondary bus
as close as possible around the middle section, which would allow a spacing
of about 3.75". I think this would make an improvement, with an impedance
about 75% of that with the present design. The overall size would be
larger, but there is room, and this design might also provide more space
for air cooling of the bus, which gets quite hot in the present design. I
think I have seen some transformers wound this way, but none with a single
turn of bus for high current. Transformers are not my strong point, so I
would like some opinions by those who are better at magnetics theory and
applications.
Thanks,
Paul |
|
|
| Back to top |
|
| BobW... |
| Posted: Tue Jul 15, 2008 11:19 pm |
|
|
|
Guest
|
"Paul E. Schoen" <pstech at (no spam) smart.net> wrote:
[snip]
Quote: So we need at least 60,000 amperes into the breaker...
[snip]
Thanks,
Paul
WOW!
Consider that my contribution to your project.
Bob
--
== All google group posts are automatically deleted due to spam == |
|
|
| Back to top |
|
| |
|
Page 1 of 1
All times are GMT - 5 Hours
The time now is Sat Nov 22, 2008 3:59 pm
|
|