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Generator


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#1 SteamyTea

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Posted 18 November 2015 - 01:28 PM

I have been asked to do some work on an electrical generator. My side of it is just to select some suitable materials to encapsulate it.
But this does give me the opportunity to think a bit about this area as I do have an interest in micro CHP.

So first thing I don't under stand is Maxwell's Equations, or their derivatives.

Basically you have a coil and a magnet.
The stronger the magnet the better.
The smaller the gap between the magnet and coil the better.
The larger areas of the magnet and the coil the better (where I start to get confused)
The more turns in the coil the higher the voltage.
The thicker the wire the higher amperage it can handle.
The faster the coil and magnet meet and depart, the more power.

So this boils down to something like
V = N.A.(2Pi.RPM).B
or in English
Number of turns times the area of the coil, times the product of 2 Pi times the revs per minute, times the strength of the magnetic field.

Now this surface area.
Should the coil and the magnet match, or can one have a larger area than the other.
Does it matter if the turns overlap.
Should, when it spins, be a period when there is virtually no magnetic field affecting the coils.
Most generators I have seen have a steel or iron plate that the magnets are stuck to. What does this do.

Anyone know of some good websites that can help here. Had a look last night and got very confused.

#2 jsharris

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Posted 18 November 2015 - 01:48 PM

The coil only cares about changing flux density, so the closer the magnets to the coils and the higher the magnet flux density, the higher the efficiency. The other factor here is core loss, causes primarily by eddy currents set up in the core (reduced by using insulated thin laminations, thinner the better) and by losses cause by the permittivity of the core material (you want to core to accept large changes in flux density with negligible hysteresis, to reduce core losses). Finally the core has to be selected to avoid magnetic flux saturation at the highest induced flux density, or else you get excessive waveform distortion. Core losses dominate at low power generation, but remain fairly constant, being massively overtaken by I²R losses in the windings as soon as the generator starts to deliver significant power.

It doesn't matter if the coils overlap, BUT, the idea is to fill the coil space with conductor, so neatly winding the coil, or better still using square or rectangular windings, combined with thin and very effective insulation, improves efficiency, because it increases the copper volume within a given winding area. This reduces I²R losses for a given number of turns, by increasing the cross sectional area per turn, in effect.

There will be a period at the start gap where the magnetic flux in the windings is very small and near-zero, and this depends on the configuration, primarily the number of poles. More poles tends to equal a closer relationship between magnet pole gap width and stator pole gap width (for a PM machine).

The steel (or more probably iron) plate is to limit the magnetic flux from escaping from the unused poles of each magnet and contain it within the machine. This both improves effective flux density in the gap and prevents unwelcome magnetic interference to external equipment.

Edited by jsharris, 18 November 2015 - 01:49 PM.


#3 SteamyTea

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Posted 18 November 2015 - 02:54 PM

Much as I thought. It is a few years since I studied this.

What are the key equations?

#4 jsharris

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Posted 18 November 2015 - 04:41 PM

Primarily just Maxwells equations, plus a few simple ones like Ohms Law. It's not really rocket science and there is generally far more data available on PM brushless motors than their is on PM generators, but these are reciprocal machines, so what applies to a motor applies equally to a generator (strictly speaking an alternator)..

#5 SteamyTea

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Posted 18 November 2015 - 05:54 PM

Thanks.

Shall do some more digging.