From: jobst.brandt@stanfordalumni.org
Subject: Re: How does a Mag resistance work?
Newsgroups: rec.bicycles.tech
Date: Thu, 22 Nov 2001 18:01:41 GMT

Jay Hill writes:

>>> I'm curious how the resistance unit on my Mag trainer works.  I've
>>> opened it up and found 9 magnets on both sides of a non-ferrous
>>> spinning aluminum disk.

>> This is the same effect that generates current in a generator
>> (copper wire) except that the aluminum disk has no output and
>> recirculated the current in the disk.

> Does anyone know where there are plans for a bicycle-based
> generator?

This is an age-old problem.  At what speed do you want the light to
work?  In order to get this to work at low speeds, some hub generators
(the only ones that don't waste energy rubbing on a tire) use gears to
speed up the generator.  This in turn requires voltage limiting
circuitry and makes the generator wasteful at high speeds.  I can
imagine a processor based system that optimizes this but don't hold
your breath.

One problem is that any system that has field windings has even
greater losses and this would be a way of controlling voltage.  Most
generators use permanent magnets and these cause an increase in
voltage as speed increases.  The Sturmey Archer Dynohub was designed
to magnetically saturate at 3.5 watts, but then it was a terrible dog
and 3.5 is not enough to make good light.

Jobst Brandt    <jobst.brandt@stanfordalumni.org>