wow. . . now that's a lot of questions.
with the dog cart, it would be helpful to know what sort of loads and terrain you're expecting to travel over. . . .for light loads (you and a week's groceries, for instance), paved roads and/or well-maintained gravel, bike tires at high inflation pressured are probably your best bet. If there's going to be significant sand, turf or mud, then ATV tires are probably a better choice, but the extra weight and rolling resistance are going to limit the speed, mass and distance the dogs can pull.
structurally, you can make the thing as big or small as you want. . . mass scales with size. if you're just looking for a way to bop down to the 7-11, and you have a newfoundland or st. bernard, then a two-wheel 'surrey' (think dog rickshaw) has a lot to recommend it. the key thing to keep in mind is that you want the loaded surrey to very nearly balance across its axle. . . something like 5 percent of the load should be on the dog, at the most. . . but you don't want to lift the dog UP, either. have a look at the sort of thing the Amish build for inspiration.
(end of digression)
I'm not sure that studying the earth's magnetic field is going to help with understanding wind turbines. (generally speaking, that's the phrase for the whole machine) that's because most scientists don't completely understand the earth's field, either.
the 'rotor' can be (interchangeably) the bladed portion of the wind turbine, or the portion that holds the magnets and, well, rotates. just to confuse things, the bladed portion is more correctly the 'turbine' of a wind turbine. (things can get really arcane if you start talking about multi-disc turbine rotors and variable incidence stators, but I won't go there; that's getting pretty deep into mechanical engineering for the overview you asked for. plus it really isn't germaine to the energy levels Fieldlines tends to discuss.)
the assembly of magnet rotor and stator/s can be referred to as an alternator (or as slang, as a 'generator' or 'gen'). This nomenclature is clouded by the fact that a permanent magnet DC motor becomes a generator (in the real sense of the word) when its shaft is rotated by an external force (such as a turbine rotor)
now to the AC/DC debate: a brush-type permanent magnet DC motor or a un-modified car alternator pressed into service as a the load for the turbine rotor, will produce DC power. A brushless motor, and induction motor conversion, or a home-built alternator, will produce 'wild AC' which will vary in voltage and frequency as the speed of the wind changes. Commonly, a builder of a machine that produces wild AC will install a bank of semiconductor 'recifiers' which will convert the output to a sort of dirty DC power, which can only be used to run brush-type DC motors and (more commonly) charge batteries. . . . other equipment (electronics, for instance) don't do well with the 'lumpy', variable voltage DC coming directly from the windmachine.
the myth of DC power not traveling well long distance is due to the fact that, until recently, there was no easy, efficient way to 'transform' DC power from high voltage and low current to low-voltage, high current. (as is commonly done with AC power) the reason this difference is important, is that all practical wire for transmitting power has resistance, and the losses due to resistance are proportional to the current squared times that resistance, while the power transmitted is only voltage times current.
That means (leaving the math for the reader) that you want to transmit your power at as high a voltage and as low a current as you can possibly achieve, in order to reduce losses. this is easy (and fairly cheap) to do with AC. Hence AC is used almost universally to transmit power long distances.
(interestingly enough, transmitting AC long distances suffers from other losses that DC does not have. again, that's getting pretty deep into side avenues that don't really affect fieldlines-style installations.)
the other way to achieve low losses is to use REALLY LARGE wire. that gets expensive when you're talking about transmitting 1000's of kilowatts many miles.
for the size and complexity of most house-hold scale power systems, there is no practical difference in the 'transmittability' of AC and DC.
what this means, is if you're charging a battery bank, then using the power in the bank as DC at the bank voltage will yield the lowest losses. However, if you have machines that will not run on DC power like grid-powered refrigerators and microwaves, you will need an inverter to convert your power to something resembling grid AC power (at least as far as voltage and frequency are concerned).
it is possible to modify most things to operate off DC power. in fact, depending on the voltage of your battery bank, some of them will operate unmodified, and with more power. (most vacuum cleaners and power hand tools fit this category, but require a 120VDC power source for full power) Many others operate from internal batteries, and the only change they require is to get a 'car charger' of the appropriate voltage input rating for your battery bank.
while it's true that aircraft propellors operate at (nearly constant speeds) up to 3000 rpm, they are also inspected before every flight,(usually a flight lasts less than 12 hours) and have a useful life of only a few thousand flight hours before they must be retired or very carefully rebuilt and recertified. they are also made of high-strength alloys (or single pieces of VERY carefully chosen wood,) carefully balanced and not usually flown in really bad weather (like hail or sleet)
But there's more to the concern over high winds than the structural integrity of the turbine disc itself. the most pressing comes from the waste heat generated in the alternator at high outputs, and sometimes, the ability of a battery bank to absorb the power generated. (this last can he combated with dumploads)
As a theoretical question, yes, it is possible to make two alternators of equal efficiency and output but differing RPM. As a practical matter, however, parasitic losses (bearing drag, blade tip drag, vibration and eddy currents, to name a few) will be higher with a faster-turning machine.
which brings us to: there are two sorts of drag on a wind machine. . . the parasitic sorts I just mentioned, and the loaded 'drag' which is really a case of the input power being converted to electrical power. . . and so the machine that you ask about will be fairly easy to turn by hand until it sees an electrical load, whereupon it will feel quite stiff.
-Dan
