Date: Tue, 30 Apr 2002 21:03:11 -0700

Author: Martin Simon

Subject: Re: Bicycle Generator 5K40.83


Dear Jerry and others,

I asked a similar set of questions a couple years ago and got some good
advice which I mostly ignored. I had picked up an old exercise bike at
a thrift store and I wanted to power light bulbs in order to talk about
horsepower, kilowatt hours. etc., in units which students could understand.
I had seen something similar at the Toronto science center with a narrative
which went something like, "You come home, turn on a light in the living
room, then you go to the kitchen and turn on another light," etc. By the
time you switch in a few hundred watts, it's pretty hard to petal. I thought
it would also be funny to plug in various kitchen appliances like a blender.

To make a long story short, I never could find a permanent magnet motor
that could be run as a generator to produce 120 V AC. Instead, I also
looked for a permanent magnet DC generator. I wanted something in the
1 HP (746 W) range, because a human can burst up to half
HP briefly. A human can only sustain about 125 Watts though.

I bought a low RPM 1 kW 12 V permanent magnet generator from
They also have a smaller Wattage model which would probably also
work fine. The 1 kW is overkill. I had looked into old car generators
but they seemed to deliver only 200-300 Watts and I was afraid that
was too small. They would probably work fine. With this generator I
use a bank of eight 12 V 50 W bulbs which are sometimes called
Marine or RV bulbs. I also have a small B&W 12 V TV which can
be attached.

It works, but there are some problems that I would like to solve to
make it better.

1. It would be nice to start with one bulb, then switch in the second
bulb, then the third, etc, so that the pedalling is easy and then gets
progressively harder. However, with too small a load, the voltage
spikes up and burns out the connected bulbs. I have to have a
minimum of three or four bulbs in to keep the voltage around 12 V
or else I lose bulbs. It depends on how hard the student pedals
under light load. I would like to find a way to clamp the voltage to
12V without complicating the circuit much. One idea is to add a
discharged 12 V car battery but that complicates things conceptually.

2. Connecting more bulbs makes it very hard to pedal and makes
the illumination very uneven. It is very hard to deliver power from
the pedals continuously under heavy load. The exercise bike has
a flywheel but no gearing, and the flywheel is not enough to even
out the stroke.

3. I wanted to be able to connect an inverter so that I could go
up to 120V AC. I connected a cheap 1 KW inverter but it is
so lossy that I couldn't get any useful power out. I need to try
with a more efficient smaller inverter. The uneven stroke
and voltage swings may be a problem though.

Despite these limitations it does get used a lot and is liked
by the instructors.

Martin Simon

>I am seeking recommendations for a good permanent magnet, low voltage DC
>motor to be used as a generator with an exercise bike.

>Does any body do a Bicycle generator?
>Can you tell me anything about the motor you use, the specifications,
>maybe the make and the model?
>Do you have any gearing on the motor or the bike?