I have a few basic electrical engineering questions. My purpose is a home wind/ hydro electric project to sell back to the grid. My questions are these: 1. Which generates greater power: running at a faster RPM, or building a larger generator? 2. Is there a limit on RPM usefulness in building a generator? 3. If I can make a generator run at 20,000 RPM (yes, I believe I could achieve that rotational speed), will that produce a useful current gain over the same generator running at, say, 3,600 RPM? I am also not sure if I should construct an AC generator, or a DC generator hooked to a DC to AC inverter. Any input would be greatly appreciated, thanks!
What should the priorities be in designing a home generator
currentgenerator
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Realistically, not a lot. An electrical generator is any electro-mechanical device that converts mechanical energy (typically a spinning shaft) into electrical energy (a current). This is the exact opposite of the operation of an electric motor which converts a current (electrical energy) into mechanical energy. As such, some motors can also function as generators if the shaft is externally driven.
Generator is an umbrella term; there are various types of generators, but the only ones pertinent to your question are a dynamo and an alternator. A dynamo is a common generator used on bicycles to power lights, but it has been used for many other purposes as well. It incorporates a "commutator" which periodically switches the direction of the current flow from the rotor to the external circuitry to generate DC power.
If the commutator is removed, a dynamo is essentially an alternator producing AC power. The frequency of the resulting signal is determined by the windings of the generator and the rotational speed of the internal rotor. Technically, any generator that produces AC power is an alternator, but usually only the smaller AC generators driven by internal combustion engines (such as the one in a car) are known as such.
The actual alternator in the generator should be harder to turn when there is more current being drawn from it. The current flowing though the coil windings will generate a force trying to turn it.
You can do an experiment with a DC motor. If you short the two leads to the motoro together, you'll see it takes more force to rotate the shaft than if they are not connected.
You can also try turning on the headlights or another large electric load on your car when the engine is idling. You should be able to notice a that the engine is working harder by listening carefully to the sound.
So, your alternator almost certainly takes more power to turn when you have a larger load. The question really is why that does not show up in higher fuel usage.
It could just be that the the generator is so inefficient at low power output that the difference in fuel usage due to the increased load is not noticeable. If this is the case, the lost energy is likely leaving the generator as incompletely burned fuel or excess heat in the engine exhaust, rather than anything to do with the electrical part of the generator.
If you are going to consistently run at low loads, then try to tune the engine to run more efficiently at these small loads. This could be as easy as adjusting the carb depending on what type of engine it is.
Alternately you could consider getting a smaller generator that runs more efficiently at lower loads. If you use it alot, this could pay for itself in the money you save on fuel in the long run. Honda makes some very nice, very small generators that are impressively efficient and quiet.
Another option could be to only run your existing generator at full load, but only for short periods of time. You could, for example, turn on the generator for 5 minutes and use all the power output to quickly charge a bank of batteries, then shut off the generator and run your small load from the batteries until they start to go dead, then repeat. This would lower your fuel usage significantly.
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The factors that you are describing are just components of what matters here which is power (Instantaneous ability to do work or deliver power) and energy (work done or power x time acting).
You need a better understanding of power and energy and there is so much on the internet on this that repeating it here is not a good idea.
T address your questions, plus a few notes:
To do this the equipment must be certified as suitable. Among other things it MUST be certified as "anti islanding" - ie it cannot send energy into the grid when the grid is unpowered. There is zero chance of getting approval for connection of DIY equipment to the gid without a certified anti-islanding arrangement from an approved supplier. The most usual and probably easiest method is to drive a certified ant-islanding "grid tie" inverter. These ost about $US1 per Watt on ebay for top quality brands (eg SMA) and rather less for brands of lesser repute. As a rule you don;t get what you don't pay for in this area. You can but non-certified grid tie inverters at low prices but connection is prohibited and probably also illegal. FWIW - the reasons fro requireing anti-isalnding are not overly good technically but rules are rules.
Essentially "No", although too small a generator for available power will limit output. .
Speed is not directly tied to power. It is often possible to build a more compact generator if it is DESIGNED to operate at higher speed but this is not "a given" and is complex. The power output is set by the available power to drive the generator as long as the generator is able to handle it. If eg a wind turbine receives 100 Watt of power at the shaft, you could design a slow or fast generator (or alternator) that worked about as well BUT you could only approach and not exceed 100 Watts no matter what speed you used.
Yes,sort of - but aspects such as how is the power produced, are as great or greater influences in design speed. Most wind turbines rortate such that the blade tips exceed wind speed by a factor K. A k of 1 to 2 is slow, 2 to 6 say is normal, 6 to 9 is rather fast and 9+ is getting frenetic. Wind Turbines with TSR (Tip speed ratios wrt wind) of 12:1 exist but are noisy and prone to erosion by rain and dust. High speed devices fly apart more easily. A TSR = 12 machine is a work of art & insanity and I'd be wary of standing too near to one.
As above, it's essentially irrelevant, and 20,000 RPM is hard to design for well in that sort of environment - a rotor that rotates 300+ times every second is trying to kill itself and you every moment it is running.
AC is almost always used for reasons which become clear once you look at real world designs.