[TowerTalk] Fwd: shack wiring
jim.thom at telus.net
Mon Aug 2 04:16:50 PDT 2010
Date: Sun, 01 Aug 2010 09:56:23 -0700
From: "Jim Brown" <jim at audiosystemsgroup.com>
Subject: Re: [TowerTalk] Fwd: shack wiring
In a 3-phase system, any harmonic whose number is divisible by three will ADD
in the neutral rather than cancel. That's because the three phases are
displaced by 120 degrees at the fundamental, and their third harmonic is
displaced by 3x120 degrees (360 degrees). Likewise, the sixth is displaced by
720 degrees, the 9th by 1080 degrees, etc. Those harmonics add in the neutral
(AND in leakage currents on ground conductors). And it is those harmonics that
we hear as "ground buzz."
Now, almost none of us has 3-phase at home, but the mains power wiring in the
alley or underground that feeds us IS 3-phase, and a form of 3-phase power
distribution called High Leg Delta is widely used in cities, towns, and even
some rural areas to feed both residences and businesses from the same lines. A
center-tapped transformer on one of the phases feeds residences, while
businesses that need 3-phase power get all three phases (but no neutral). The
catch is that residences DO get a neutral, and all that harmonic noise from
the businesses goes to ground on our neutral. If you hear "ground buzz",
that's what you're listening to!
## If the business's don't get a neutral... then how do the business's
obtain 120 vac ?? The usual deal with 208/120 3 phase is...
Phase to neutral = 120 vac IE: 208/sqrt 3 Perhaps the
business's are all stuck with 208 vac single + 3 phase??
## I read a while back abt hi tech business's.. that huge hundreds of
120 vac PC's... were really cooking the 600v primary /208-120 sec
distribution xfmr..located on each floor of a 4 story building. The 600 vac
was fed to the riser. The problem with all the 120 vac loads was the harmonics.
The neutral current was double normal.. and sometimes more than double normal.
## each floor had a 50 kva xfmr... and the average load was 37 kva. The 50 kva
xfmr was ready to burn up... and the hvac in eacg xfmr room on each floor...was
running flat out. The 'fix' was to use these new type xfmrs... amphorous
with a 3rd winding em. They killed the harmonics.. and the new style 50 kva
xfmr doesn't heat up. The original solution was to replace the xfmr with
a 75/100 kva conventional unit.
## the power up here is always on abt harmonics. Those silly CF lights
have real lousy power factor .48 The kicker is.. in zl/vk land... they have
perfect pf. The ones from both zl/vk and NA..come out of the same factory...
in china ! The ones soild here are missing the small choke..used to cancel
the XC from the leading PF of the CF lights...go figure.
## some power co's in the usa were abt to penalize folks for lousy pf...
but of course that got squashed. CF's are all the rage...even with .48 pf
All they have to do is add the choke back in. You can see where it goes on the
pc board inside em. There is a jumper across the 2 x holes in na models.
## bottom line is... big city sucks 480 kw worth of CF lights... but the power
co has to provide 1 million VA to do it.
## In EU.. any power supply > 75 watts has to have PF correction built into
it plus harmonics have to be suppressed. I stopped by the telco where I just
retired from.... and noticed they had changed out all the old style Ferro resonant
400A rectifiers with these new tiny switching types. the new ones have almost
unity PF. [ >.99] To pull that off, they have improved the pf.. plus killed all the
harmonics. New ones are tiny things.... 6 will easily fit into a 24" wide rack
6" tall x 15" deep]. heat sinks never get hotter than 102 deg F. Old style units
would cook a room.
Later... Jim VE7RF
If you can look at the buzz on an audio spectrum analyzer, you'll see those
harmonics. I've got screen shots an FFT analyzer and an extended discussion of
all of this in a couple of tutorials that are on my website. See either the
Ham Interfacing tutorial or the White Paper on Power and Grounding for Audio
and Video Systems. http://audiosystemsgroup.com/publish.htm
73, Jim Brown K9YC
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