On Sun, 01 Aug 2010 11:43:25 -0400, hanslg@aol.com wrote:
>When you run 120 volts supplies on outlets on different phases but common
>return, be aware that if they are of the "old" type, diodes feeding directly
>into capacitors, so called "top-" or "pulse" rectification, due to the
>harmonics in the currents through the supplies, the current will add up, not
>subtract in the return line. You can end up with 40 amps in the return line
>with both lines loaded to 20 amps. I have seen this happened in computer
>rooms in schools where they had many computers loading 2x120V outlets.
Your observations are partly right and partly wrong.
The classroom example is correct, because it is almost certainly a 3-phase
system. The home example is not, because virtually all homes are single-phase
(120-0-120). Here's why.
Virtually EVERY electronic power supply, linear or switcher, has a capacitor
input filter, and the CURRENT to recharge that capacitor flows primarily in
short pulses at the top (and bottom) of the AC waveform. As a result, that
current is rich in harmonics. In a single phase system, all of that current
cancels in the neutral (assuming equal and identical loading of both sides of
the power line).
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 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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