The sensitivity of humans and other mammals, with regard to frequency
happens to peak in the 50-60Hz range. Edison took advantage of this and
Tesla countered with high frequency, high voltage discharges, saying, in
effect, "this is AC, perfectly safe" Both lied (whether they knew it or
not and the not was shown much later) with profit as a motive. Other
hazards such as arcing at switches or poor contacts, worse with DC, were
On Tue, 13 Oct 2009 19:51:24 -0700, Don Kelly wrote:
I recall the breakers at one site I was working at fed compressed air
through the breaker upon opening, just to extinguish any arc that may have
formed (that was a 400V DC setup) - I think that's typical on higher power
DC stuff. The breakers were about the size of a lunchbox.
Hi voltage AC breakers still do use compressed air in some. The 'blast'
is aimed between the arcing contacts to literally blow out the arc.
Lower voltage DC (up to 350VDC) that we used on submarines just used
blow-out coils to create a magnetic field that 'pushed' the
arc-conducting gases up into chutes lined with alternating metal and
insulating plates that would cool and stretch the arc.
But I've seen enough stuff that I know I haven't seen everything :-)
On Wed, 14 Oct 2009 17:30:15 -0400, daestrom wrote:
Interesting - not seen those before. 'ours' were WWII-vintage, and there
was compressed air in the same room as part of the air-start system for
the generators, so I suppose it was no big deal to route it to the
electrical switchboard too.
:-) I'm sure there were all sorts of ways and means of extinguishing
arcs, though - some of which may have worked better than others!
It'd be interesting to know what larger power stations etc. did, too. Had
some friends in NZ with a smaller plant (2,500 kVA) but I've not talked to
them in quite a while, and I don't recall anything obviously resembling
breakers on the site, although I assume they were there somewhere!
Another variant that I worked with was spring to open and the bottom
side of the mechanism pushed a plunger in a cylinder to make a 'gush' of
air that was directed from below the contacts, up between them into the
arc chute. Of course it was just a short burst of air, but the idea was
to blow the hot gases up into the chute where the plates would
separate and cool them.
High voltage air blast breakers for AC have been around since the late 40's.
They were/are modular with series sections and could then be extended to
higher voltage by adding sections. The contact opening in a section was
about 1 inch and 600psi air was driven through the arc, extending it towards
a vent but not actually interrupting the arc until the arc naturally
collapsed at current zero- then the arc products were blown out and the gap
filled with good dielectric (high pressure air). A two gap section was good
for 72KV and 2 of these in series for 161KV. Two gap sections could be
linked together and put on longer columns at higher voltages there was a
loud bang when they operated. The advantage of these breakers from Europe
was that they were smaller, lighter, faster and cheaper than the oil
breakers in use up to that time in North America.
There was a bit of a war of words going on in IEEE PAS regarding the
relative merits of bulk oil breakers and air blast breakers and air
breakers won out. Even the old circuit breakers at, say 15KV up whether
oil or air blast operated on the principle of removing arc products,
replacing them with good dielectric, when the current went through zero.
This principle is used for HV minimum oil and SF6 breakers (blast of oil or
SF6 through the gap).
At lower AC voltages- say 5-15KV such breakers are often used- You could
take one of these and derate it to about 400-500VDC and it would likely
work. That current zero every half cycle makes a big difference.
Have you had any experience with high frequency AC power systems?
I've come across 400hz AC power in some old computer installations
and seen a lot of military surplus aircraft power equipment that
used 400hz AC power. My assumption has always been that higher the
frequency, the smaller the mass of the transformers not only making
equipment smaller but lighter.
In addition to smaller generator/motor and transformer sizes and weights
for a given power, the 400Hz machines can be driven at higher speeds
eliminating some or all gearing in high rpm aircraft usage (up to 24000
rpm at 400 Hz
vs up to 3600 rpm at 60 Hz. ).
For aircraft the distances involved are short so that inductance and
capacitance are not a problem .
In general, for land based applications the advantages are outweighed by the
disadvantages because of the distances involved.
Switching power supplies were not an option in those days.
My statement still stands. I've never seen a switching regulator
that ran at 1Mhz. Just because the chip is capable of 1Mhz does
not mean that that is the switching speed of the regulator. Most
of the design engineers I'm familiar with tend to be conservative.
Your opinion is not that important. Opinions are like tailpipes,
everyone has one, except for those like you who have theirs transposed
with their induction system. FLNF
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