[Peripherals] Charging the XO off a bicycle dynamo
Richard A. Smith
richard at laptop.org
Wed Apr 30 18:18:30 EDT 2008
Arjun Sarwal wrote:
> Also, we were told that dynamos don't generate any output, if they are
> run below the rated rpm. Alternators however do generate an output
> below rated rpm and the output(number of amps) is proportional to the
Strictly speaking I don't think this is correct. Unless the device in
question has specific circuitry designed to prevent output @ < rated
rpm. Like some sort of DC/DC converter or linear voltage regulator perhpas?
If you pass a coil of wire through a magnetic field you get a current.
It might be small but its not optional.
I think what they were trying to describe is that dynamos or permanent
magnet generators are designed around a much smaller target rpm range.
When you operate them outside that range they are not very efficient.
This has to do with the strength of the field. In a permanent magnet
setup you can't adjust this strength.
In an automobile setup where you have lots of rpms they are designed to
operate at a high rpm. The reasons alternators can achieve so much
higher efficiency is because they have control of the field strength.
This also allows them to operate across a wide rpm range. But even then
there's still a design range. If you look at the output of most
automobile alternators you will see that they don't produce rated output
until higher rpms.
Richard Smith <richard at laptop.org>
One Laptop Per Child
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