Jim,
I knew that Dr. Middleton's class would come in handy someday.
Very good explanation.
73
Don
N8DE
ex-W8QHW ... Go Bearcats!
Quoting Jim Brown <jim@audiosystemsgroup.com>:
> 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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