MooseFET said:
MooseFET said:
"Michael A. Terrell" <
[email protected]> wrote:
Till the dragon flies swarm in and start eating the mosquitos. Some
of the dragon flies are bigger than the birds. [.....]
They are amazing things to watch. They can move any which
way change direction in an instant. They seem to work a lot better
than anything we can build.
The most amazing thing about dragonflies is when they land, and line
up at a 45 degree angle, like miniature bi-planes, all in the same
color.
I've seen that same sort of thing too. I think it must be the first
step in the mating or something. They expend everything they have
when they do mate.
I've only seen it a couple times in the last 20+ years. Most of
the time they are in at least a dozen different colors so birds don't
get all of them, in flight while they are still tiny.
The tiny ones are a different type. When the dragonfly emerges from
the water it is nearly or perhaps fully its final size. In the water
they are a complete terror in small scale. They are a jet powered
predator with a special set of jaw parts that can reach out and grab
small fish and tadpoles etc.
[...]
We had a few boards explode in the reflow oven, even after a 24 hour
prebake. We had to return an entire run of blank boards to the vendor,
because over 90% delaminated in the Heller oven, after the proper
prebake.
I've seen delamination for some reason that had nothing to do with
water. I think it must have been a bad batch of epoxy. We had them
tested and the lab results came back with a "darned if I know"
report. The test lab did tell us that the fab house had other metals
in its plating tank but I can't see how that could be the cause.
The vendor insisted that it was doe to improper storage, and
adsorbtion, so we baked the entire batch over the weekend, then sent
them through the Heller oven. You could hear them popping from 20 fet
away.
[.... ESD etc ...]
A lot of plants still have an undersized neutral on their three phase
panels.
I wouldn't put "still" in there. I'd bet that one built today would
have the same problem. Perhaps it would be even worse today because
of the price of copper wire.
The Ocala Microdyne plant was built after the problem was well know,
and the design was done by an EE with a PE, yet he ignored it in his
designs. I spotted the problem right away, but they called in the
engineer's company. They sent a couple redneck wire pullers who looked
inside a couple breakers boxes, and put the covers back on. they told
me, We don't see any problems and left. Then they called Florida Power
who placed a logger in the plant for a few days. A week later they told
the company that the neutrals were undersized. They still didn't believe
it, and neither did the P.E. They pulled additional wire for the
neutrals and the problem went away. They never did acknoledge that i
had found the problem, but the amount of damaged test equipment went
down over 90%, and repeatability on critical tests went up about 500%.
You couldn't convince some of the good ole boys that there was a
problem, and the low bidders wore the worst offenders. A lot of other
20 year old buildings around here still have undersized neutrals.
The harmonic content is hard to get rid of, and if you load the
panel close to its limit, the neutral may burn.
Mismatched power factors (phase angle)[1] also adds to it. New
equipment produces less harmonic content and has a phase closer to
zero so I expect this problem is getting less with time.
The load was fluorescent lighting, and over 1000 switching power
supplies in the test equipment, computers, terminals and our products
goiung though burn in or test. The air conditioning and resistive
heating elements in the burn in chambers were a small part of the total
load.
There is still a bucket of 180Hz out there though. If you measure the
radiation, the 180Hz is typically 3 times as big as the 60Hz. This is
because the 60Hz cancels in 3 phase wiring but the 180Hz doesn't. The
180Hz often doesn't return via the ground wire but instead goes back
through the soil.
[1] I really wish that a different term was used for the harmonic
content part of power factor and that power factor was reserved for
phase.
I think that is the main reason the P.E. and engineers couldn't see
the problems.
The rebar in the concrete was likely doing most of the conducting.
Luckily, the rebar in a slab constructed building is usually all
hooked together electrically. Unlikily some buildings are on multiple
slabs that are not hooked together except via the soil they are
resting on.
There were multiple slabs, but all the rebar was welded before the
concrete was poured. The building was 200' * 200', so a single slab was
out of the question. They were all welded together to prevent sagging
or heaving.
That is a lot less than I would have expected. How was it measured?
Between the two ground systems before they were bonded. Prior to
the update, some of the ESD grounds were tied to the closest outlet, and
casued no end of problems. We still had some old engineers who didn't
belevie in ESD protection. theuir attitude was that any part that could
be damaged by static was already defective. Luckily, they finally
retired.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
