Thanks for the feedback Trev. OK - Let's just cover that again in a little more detail (Some of this is plagiarized, some isn't, but it's all factual):
Don't power Thyristors fire only at the mains period and also have a needed, adequate, low bandwidth [relative to 1 - 40MHz]? Here's a Wiki link: http://en.wikipedia.org/wiki/Thyristor Harmonics are usually defined as frequencies that are integer multiples of the fundamental frequency. When these harmonic frequencies are summed together with the fundamental power frequency (50 or 60Hz normally), the result is a distorted voltage and/or current waveform, no longer the pure sine wave that originated back at the generator. But numerous people have asked why are just the harmonic frequencies present, such as 180Hz, 240Hz, 300Hz, and so on? How does the electronic equipment know to generate only those frequencies? What’s wrong with 178Hz or 316Hz? Waveforms that contain those frequencies can exist in electrical distribution systems, though they aren’t that common. They are called “interharmonics,” or frequencies between the harmonic frequencies. A special category of those interharmonic frequencies are the subharmonics, which are frequencies below the fundamental frequency. These are more common, and are often the source of the voltage fluctuations that result in light flicker. Subharmonics can also be produced as the result of the interaction between harmonic and interharmonic frequencies. For example, 180Hz and 185Hz signals together would result in a 5Hz signal that falls in the subharmonic region. Since there is a pair of rectifiers (one for positive half cycle of sine wave, one for negative half cycle) for each of the three phases, this is referred to as a six-pulse or six-pole converter. When the control circuitry of the converter turns off one SCR (or thyristor or whatever type of rectifier is used) and turns on the other, there is an overlap period where both devices are turned on. This is because such devices don’t really stop the current flow until the current waveform goes to zero. Having two devices turned on at once is effectively a short circuit between the phases, which results in a very large current flow for a very short time, until the first device goes off completely. This is the commutation period, and is a synchronous process to the power frequency. As you can see in Figure 1, these notches occur six times in each power frequency cycle. The “rule” on the resulting harmonic currents is H = n x p +/-1, where “n” is integer 1, 2, 3, etc., and “p” is the number of poles (six, in this case). Hence, the dominate harmonics would be 5, 7, 11, 13, 17, 19, and so on. Another common source of harmonics are single-phase rectified loads, such as found in the front end of most electronic equipment from PCs to TVs to DVD players. The power supply circuitry in these units only conducts current during a period near the peak of the voltage waveform. The width of the pulse is dependent on how much current that the load needs. The more current, the wider the pulse; the less current, the narrower the pulse. This pulsed waveform happens over and over again each cycle, though over time its width may vary. The mathematical theories regarding such waveforms state that they can be created from the summation of the fundamental frequency waveform along with a series of waveforms that have a weighting or scaling factor applied to their amplitude and have frequencies that are two times the fundamental frequency, three times the fundamental, and so on. This is the same definition as a harmonic. If you take the fundamental (60Hz) at a factor of 1, the third harmonic (180Hz) at 0.94, fifth harmonic at 0.78, seventh at 0.58, ninth at 0.36, and so on, and combine all of those signals, you would end up with a single waveform. What about the even-numbered harmonics? So far, all of them that we have considered have been the odd numbers. Even numbered harmonics are not normally found in electrical systems, unless there is current drawn on only one half of the sine wave. This can be from half wave rectifiers that only work on half the cycle, or from full wave rectifiers with something broken so that only half of the rectifiers are working. Waveforms with even harmonics are usually visually detectable because they will lose the symmetry in the waveform, where the one half doesn’t look like a mirror of the other half. The way that electrical power is used in most equipment today is much more likely to result in odd harmonic currents, which in turn result in harmonic voltages. The subharmonic waveforms themselves are a physical phenomena i.e. they are real. They last, on average for about 11 microseconds, not long enough for "normal" hearing to detect, so you don't need a particularly high detection sample rate to pick them up on instrumentation. Right now I'm picking up 45.45, 92.36 and 314.89Hz subharmonics on a setting of 11.025KHz/8-bits/mono/Ref. freq. 440Hz (the ultrastable note of A4). Odd and even harmonics, so there's quite a mix in the Hum. Did you see this? http://www.sengpielaudio.com/calculator-dba-spl.htm - Halfway down the page it shows the flat response curve graph of dBC receptor response. Local - Yes. Top-of-pole mounted AC-DC rectifiers. That's why Hum is local (although it's everywhere) and why I went to Greenbank, West Virginia (USA Jodrell Bank - No transmitters allowed) to prove to myself that Hum is a localized thing. Hope that answered your and everyone else's questions. On Dec 10, 11:26 am, Trev <[email protected]> wrote: > Don't power Thyristors fire only at the mains period and also have a > needed, adequate, low bandwidth [relative to 1 - 40MHz]? Also > transformed supplies at the local level don't use power regulation to > alter phase loading , at least not here,afaik. I can see load > controllers having some nearby effect, but they don't seem natural > candidates for Hum, in themselves, in the wider picture. Easy to pick > up LF power leakages and these have caused some identified Hum over > the years, along with loose transformer windings/laminations. > Switching power supplies fall into the local category, and they also > could contribute to hum via load imbalances as they cause reactive > loads at night. These technologies are not new and been on my suspect > list for years, but they are variable in their loading- and hum is > very constant in my area. > > On Dec 9, 3:00 pm, Vic <[email protected]> wrote: > > > > > Trev - It all comes down to a piece of electronics called a Thyristor > > - In the case of the Hum, a Power Thyristor. As more electronics are > > mounted on the poles drawing their power from the lines, more and more > > thyristors are in operation as electronic equipment requires DC power. > > Hence, the Hum phenomena is becoming more and more widespread. > > > It really comes down to incompetence on the part of the electricity > > utilities and mega-salesmanship on the part of vendors of this stuff. > > > There are safe alternatives, there are cheaper alternatives. There are > > also alternatives that are far more technologically stable and that > > are upgradeable as technology advances without the need for additional > > equipment. The equipment that has been installed and is being > > installed right now results in communications speeds over the power > > lines that are 8 years out-of-date and cannot give any additional > > speed witohut being ripped out and replaced. -- You received this message because you are subscribed to the Google Groups "Hum Sufferers" group. 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