Posted: 9/22/2007 10:56:47 AM EDT
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Lately there have been a lot of questions regarding generators in the forum. This article should help the neophytes out a bit. Alternating current or AC electricity is the type of electricity commonly used in homes and businesses throughout the world. While the flow of electrons through a wire in direct current (DC) electricity is continuous in one direction, the current in AC electricity alternates in direction. The back-and-forth motion occurs between 50 and 60 times per second, depending on the electrical system of the country. Virtually all US appliances use 60 cycle devices. AC is created by an AC electric generator, which determines the frequency. Frequency of the AC is a direct function of generator speed. There are a few exceptions, but this will be covered in a minute. The number of “poles” in a generator determines what speed the generator needs to spin to produce 60 cycle (hertz or hz) power. A two pole generator spins 3600RPM, a four pole spins 1800RPM, a six pole spins 1200RPM, an eight pole spins 900RPM, and a 10 pole spins 720RPM. If you are planning to match a generator to an existing engine, this becomes very important. You do not want to have an engine running at 1800RPM when the engine makes peak power and efficiency at 3600RPM. This also means that if a constant 60 cycle source of electricity is desired (it is), the engine speed needs to be held constant. 50 hz power is similar, but the speeds are lower. The exception that was discussed earlier is the newer inverter technology generators. The frequency output of the generator is independent of the engine speed with these models. This is really neat stuff, but it does drive cost and complexity. Voltage will be the next consideration. Most of the high-current devices like a water heater, stove, and dryer in a house operate on 220 volt AC. Either plan to have 220 available or plan to not use the devices. A generator needs to be “sized” to the application. There are several calculators available online. One can cheat the sizing a little bit by staggering the load (unplug refrigerator while running the microwave, etc). Having a generator that is way too large is almost as bad as having one that is way too small. Not only does a larger size generator consume more fuel, the engine will not be happy only running at part load all of the time. The next consideration for a generator should be the fuel source. Gas is the cheapest/easiest to get a hold of and also means a lighter weight generator set. Diesel is a good choice, but the set will be heavier and more expensive. Natural gas/propane can be a good choice, but one can quickly get into trouble if the NG distribution system goes down for a while. If the set will be purchased for SHTF, take into consideration how one will obtain fuel AND how much fuel the set will consume. If you are running a NG/propane set and are planning to run from a 20lb. bottle when the distribution system goes down, practice running the house for a few bottles to understand how long a 20lb. bottle will last. Now that a generator has been selected, one will need to incorporate it into the household electrical system. The quickest/easiest way is to use extension cords for the individual appliances. Should one wish to move up to the next step and run power from the electrical panel, Popular Mechanics did a neat article on installing a small transfer switch a few years ago. Moving up from here in the form of cost and complexity, an automatic transfer switch can be purchased and installed which will transfer the source, well, automatically. If one wishes to wire into one’s breaker box, a transfer switch is a necessity. Duty cycle should be considered into the equation as well. As duty cycle goes up, the durability requirements go up as well. Will the set be used as prime power 24x7 for a month or will it be used to keep the freezer cold by running several times a day. This should be considered a primer and provoke thoughts and questions, not a cover-all, answer everything article. 45cb |
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If anyone wants to " rough out " natural gas / propane consumption on a 7KW. This link has a table, fuel comparisons are under " ENGINE ". Propane / NG - 7KW |