On a generator, you have a prime mover (say, an engine) connected to the actual generator, which consists of either rotating coils of wire within a magnetic field, or rotating magnets surrounded by coils of wire.
The number of poles (magnetic poles) and the rotational speed determine the output frequency: Freq = Engine_RPM * Number_Of_Poles / 120.
Typically, a United States portable generator runs at 3600 RPM, with 2 poles, for a design frequency of 60Hz. Larger portable generators run at 1800 RPM with 4 poles here.
That is how frequency is determined. The number of turns and the magnetic structure determine how many volts are produced at the design frequency, voltage and frequency aren't related in any fashion except for design. Again, in the States, most portable generators are wound to have a 240VAC single phase output, which is center tapped and delivered as two 120VAC hots with one neutral, but virtually any voltage can be delivered.
The current output of a generator is determined by its load, as long as the load doesn't exceed the maximum capacity of the generator's prime mover (engine) plus the conversion losses of the actual generator. Prime mover power is often rated in horsepower (US) or kilowatts (everywhere else). With no losses, a 10 horsepower engine could deliver 7457 watts (actually VA for non-resistive loads) continuously, or 62.1 amps at 120VAC continuously. Try to take more, and the engine will slow down (reducing both the frequency and the voltage, which will also drop the current) until you reach a point that the engine actually stalls.
You get fluctuation of frequency and voltage as the load changes because the engine cannot respond immediately to the actual load change. There are regulators controlling the engine throttle that attempt to keep the engine at a fixed (design) speed, but it takes time for the engine to respond to new commands as it has to deal with varying fuel/air mixtures and combustion which aren't instantaneous.
As a clarification to other discussions here:
For a purely resistive load, halving the voltage would halve the current, and result in one quarter the power consumed. You can't say that just cutting the voltage in half cuts the power consumed in half. With some devices, that may be true, but it entirely depends on the load.
Since this turns out to be an anemometer - oh, the torture we go through to extract relevant details from the questioners! - and it's probably related to your Schmitt trigger question I suggest you try a different approach. I presume you will feed this into a micro-controller to calculate the wind speed.
If so, you could use an op-amp based precision rectifier to convert the signal to DC and use a potential divider to feed it to the micro ADC. The PMG p-p voltage should have a linear relationship with speed.
Best Answer
The synchronous speed for an electric induction motor is determined by the power supply frequency and the number of poles in the motor winding and can be expressed as:
$$ n = f \frac {2}{p} 60 $$
where
Rearranging for f we get
$$ f = \frac {n \cdot p}{2 \cdot 60} $$
$$ f = \frac {2800 \cdot 12}{2 \cdot 60} = 280~Hz$$
Your calculations are correct. Your frequency is rather unusual.
If the heating elements are just resistive they will behave normally at these low frequencies. If they exhibit significant inductance their impedance to AC will rise with frequency. It's unlikely to be a problem for water heaters as they use straightish elements rather than coils of wire.
Be careful not to run transformers or relay coils from this 280 Hz supply unless you have confirmed that they are suitable.