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Efficient Generator

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Njguy

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I was wondering if a generator could be made more efficient by just using high voltage low amperage coils. For example lets say we have a generator made from permanent magnets on the rotor and coils on the stator. As a load is drawn from a generator it will have to work harder or slow down because a voltage is being generated in the coils which produces an opposing magnetic field. Since the strength of electromagnets is derived from amps/turns of wire, wouldn't it be more efficient to have the generator produce high voltage AC by using very thin wire with many turns, then using a transformer to turn that high voltage AC into lower voltage higher amperage output? The conversion into higher current is done away from the generator so that the generator will not experience the heavy loads. I am assuming that many generators do this already, but I am looking for confirmation that this is correct.
 
There is an optimum output voltage for generators that is determined by the current a given wire size can handle versus the thickness of the insulation required to isolate the high voltage. Here's a short discussion of that.
 
One proble is that a generator generally requires control of both frequency and aamplidude.

Frequency is controlled by speed. The output is controlled by the magnetic field on the rotor. Some residual magnets are required to get the generator to "start".
 
One proble is that a generator generally requires control of both frequency and aamplidude.
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True. But that's not related to the design of a generator for high output voltage.
 
The conversion into higher current is done away from the generator so that the generator will not experience the heavy loads.
It will experience the same load as before. You are just trading volts for amps. Power to load (plus losses) = power from generator = volts x amps. Any efficiency improvement would be due to any reduced losses.
 
It will experience the same load as before. You are just trading volts for amps. Power to load (plus losses) = power from generator = volts x amps. Any efficiency improvement would be due to any reduced losses.

Are you sure because inductor strength is dictated by amps/turns. Not voltage. I once did an experiment making inductors and found that in order to double the strength of an inductor you only had to double the amperage. If you tried to do it by increasing the voltage you ended up using like 4X the power.
 
By 'strength of an inductor' I take it you mean strength of the magnetic field produced by the inductor?

Consider a generator 1 with a coil resistance R and which produces V volts at current I amps at revs X rpm. The coil is wound with N turns of wire of cross-sectional area A and wire length L.
The power produced is V x I Watts and the losses in the coil are I^2 x R Watts.
Now consider a generator 2 with a coil made from thinner wire of half the diameter. The wire cross-section is now A/4, so you can fit 4 x N turns into the same winding space. The length of wire is now 4 x L. So the coil resistance will be 4 x 4 x R = 16 x R. The generated voltage will be 4 x V but as the power remains V x I the new current will be I/4. The coil losses will be (I/4)^2 x (16 x R) = I^2 x R Watts, the same as for generator 1.
 
Maximum energy transfer from a generator to a load occurs when load resistance = generator resistance; so if you know the load resistance will be constant you could perhaps wind the generator coils to have a matching resistance.
 
Maximum energy transfer from a generator to a load occurs when load resistance = generator resistance; so if you know the load resistance will be constant you could perhaps wind the generator coils to have a matching resistance.
But then half the energy is wasted in the generator coils as heat. That's counter to the OPs original question about improving generator efficiency.
 
I guess it depends how you define efficiency :). How would you get more than 50% energy transfer, crutschow?
 
What does resistive energy transfer have to do with efficiency of a generator? The most efficient generator will have the highest ratio of electrical output power to input shaft power. For coil resistance, that situation will occur when the generator coil resistance is zero, or as close to zero as possible.

So the "most efficient" generator occurs when the difference between generator resistance and load resistance is very large, ie gen resistance is zero (or close to zero) and load resistance is X.

Or maybe we are talking about different things?
 
I guess it depends how you define efficiency :). How would you get more than 50% energy transfer, crutschow?
Alec_T, I don't define it as the maximum power that can be extracted from a generator (which would blow up a normal power generator). I define it as the ratio of power out to (mechanical) power in. That's definitely much larger than 50% for most generators. ;)

The only power source I'm aware of, (besides RF amplifiers) that you use the matching impedance theorem is for solar cells, where you are interested in extracting maximum power, not power with the least losses. For that purpose they use MPPT converters.
 
So the "most efficient" generator occurs when the difference between generator resistance and load resistance is very large, ie gen resistance is zero (or close to zero) and load resistance is X.
Point taken. I was considering overall system efficiency in terms of powering a load, rather than efficiency of the generator per se. In that case increasing winding turns to generate a higher voltage would increase resistance and hence reduce generator efficiency.
Or maybe we are talking about different things?
I think we were :).
 
What you describe is close to an inverter/generator. Look up Honda or Yamaha that make the best. There are quite a few Chinese copies of ranging quality.

In their energy efficient mode they modulate the engine speed to meet power demand. They use permanent magnet three phase alternator that produces up to 300 vdc through three phase full wave rectifier. A PWM inverter makes the AC. The minimum DC voltage must be slightly higher then the peak sinewave voltage.

In the energy efficient mode where engine speed is modulated there is some lag time to spin up engine speed in response to a surge load like starting an electric motor.
 
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