On 08/08/17 08:41 AM, James Knott wrote:
That ignores a real factor in power distribution, line loss. As you get further from the source,the line voltage will decrease. This can be compensated for, to a degree, by using taps on the transformer. But even then, changing loads will cause changing voltage. This is why the voltage drops at peak demand times. So, everything is designed for nominal voltages, with the expectation there will be variations.
That depends on a LOT of things. Power _distribution_ is usually at a higher voltage since the Joule–Lenz law means that resistive line losses are based on current. A local 'substation' has the transformer that drops the voltage to that required for the building. Domestic and industrial may not be the same. It's more efficient to push that final stage as close to the 'load' as possible. But there's another factor that comes into play. We're talking about A/C -- that's so we can play with the voltage via transformers, thank you very much Tesla and Westinghouse. But few A/C loads are purely resistive. Motors, which dominate, present and inductive load. There are also devices that present a capacitive load, but motors predominate. This generates a phase shift so that the voltage and current vectors are no longer at 90 degrees. This causes a power loss. It is also pushed back towards the generator and can damage plant and equipment.
From the consumers POV it means that more power is being consumed to get the same amount of work done. When the voltage and current are completely out of phase no useful work is being done even though the power is being transmitted.
There are ways to ameliorate this. One is to put a honking great capacitor across the motor. You see this done with the so-called 'energy efficient' domestic refrigerators at Sears. There are also firms that sell capacitor banks to retrofit older equipment such as refrigerated display cabinets in stores and the walk-in fridges in some restaurants and warehouses. Yes, there are refrigerator mechanism that don't use electric motors; Einstein invented one. But they aren't very popular. If you look at your (even domestic) electricity bill it probably has a footnote about the assumed phase shift. Some industrial plants may have a phase shift monitor. Some of those may be tied to an alarm. The use of three-phase makes phase shift easier to handle under many conditions. Three phase motors offer more possibilities for load balancing. Some domestic settings might have a 3-phase feed for a street with alternating taps supplying what amount to single phase for individual houses. Back in my university-vacation days I worked as an 'apprentice' to a firm of electricians upgrading older houses in England, houses built between the wars or just after the 'last war'. I learnt a lot about the 'Code' and what was the minimum standard and what was a sensible way of re-wiring and bring those houses up to the modern standard. Honestly, even by the UK 'Code' of the 1970s I find the electrical code here in Canada falls short. IIR the later upgrade to the German-driven EU standard for electrical installation and wiring components was even more vigorous. But back then I was shown a fair deal about the domestic/household end of the power distribution scheme worked. The gap between 2-phase and 3-phase was an important aspect of that. The threshold of what a single house could pull of a phase before it had to be converted to 3-phase was another. Wiring a house that had 3-phase coming in but had room-level 2-phase because the appliances were all 2-phase added complications. In a industrial/factory setting that wasn't an issue for the same reason its not an issue on a normal street or in a condominium hi-rise. The 'peeling off' of individual phases, local transformers & taps, is more manageable in those settings. In a single house, an old Victorian monolith that had been converted to individual flats where each one had to have its own meters or a between-the-wars cottage that had been extended and now had a a hi-tech 'entertainment centre' that was sucking power, figuring out a 'balance' is another matter. But for many settings, phases losses of power WITHOUT any voltage drop are the issue. https://en.wikipedia.org/wiki/AC_power#Active.2C_reactive.2C_and_apparent_po... https://en.wikipedia.org/wiki/Power_factor https://en.wikipedia.org/wiki/Power_factor#Importance_of_power_factor_in_dis... And of relevance to us: <quote src=:https://en.wikipedia.org/wiki/Power_factor#Switched-mode_power_supplies"> A particularly important class of non-linear loads is the millions of personal computers that typically incorporate switched-mode power supplies (SMPS) with rated output power ranging from a few watts to more than 1 kW. Historically, these very-low-cost power supplies incorporated a simple full-wave rectifier that conducted only when the mains instantaneous voltage exceeded the voltage on the input capacitors. This leads to very high ratios of peak-to-average input current, which also lead to a low distortion power factor and potentially serious phase and neutral loading concerns. A typical switched-mode power supply first converts the AC mains to a DC bus by means of a bridge rectifier or a similar circuit. The output voltage is then derived from this DC bus. The problem with this is that the rectifier is a non-linear device, so the input current is highly non-linear. That means that the input current has energy at harmonics of the frequency of the voltage. This presents a particular problem for the power companies, because they cannot compensate for the harmonic current by adding simple capacitors or inductors, as they could for the reactive power drawn by a linear load. Many jurisdictions are beginning to legally require power factor correction for all power supplies above a certain power level. Regulatory agencies such as the EU have set harmonic limits as a method of improving power factor. Declining component cost has hastened implementation of two different methods. To comply with current EU standard EN61000-3-2, all switched-mode power supplies with output power more than 75 W must include passive power factor correction, at least. 80 Plus power supply certification requires a power factor of 0.9 or more.[ </quote> -- A: Yes. > Q: Are you sure? >> A: Because it reverses the logical flow of conversation. >>> Q: Why is top posting frowned upon? -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse+owner@opensuse.org