AC/DC Bike-Powered Generator System
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An exercise bike is fitted with a permanent DC magnet motor, which, charges a 12VDC lead-acid automotive battery. This
battery can be used to directly drive DC loads or can be fitted with an AC inverter to drive regular house-hold loads.
The system is interfaced through the PC to allow data-logging and monitoring of both
instantaneous current, voltage and power along with accumulated power.
First off I scored an exercise bike from the dumpster over on the Stanford campus over a year ago. I also got the automotive battery from the
Stanford campus, there seem to be a lot of them lying around in the parking lots. Apparently people think that is where they belong when they are done with them. Since then I have been on
the lookout for a suitable permanent magnet motor to use for the project but no luck. I finally ended up purchasing a pretty large
DC motor from ebay for $25. It's made by Ametek for running disc drive tapes or something like that. It's green, rated for 50VDC and
has a 1/2" shaft. I hooked it into the drill press at work and it cranks out 17 Volts at 400 RPM. I made the motor mounts out of
a sheet metal cover that came off an old BETA VCR. That thing was built like a tank, must have weighed 50 pounds! The motor
shaft is coupled to an additional length of 1/2" SS shaft (from a printer roller) with a rubber roller (also from a printer).
The extended shaft is run through a needle-bearing set to minimize the force perpendicular to its axis of rotation. The rubber
roller presses against the exercise bike wheel and the tension is controlled by a rubber tie-down strap.
The output of the motor is DC and first passes through a large diode (from SLAC's salvage yard) on its way to the battery. This diode prevents the
bike from being 'pedaled' backwards by the motor drawing current from the battery. When the voltage of the motor exceeds the battery
voltage, current begins to flow through the diode to the battery in a quite smooth transition. As you pedal faster, motor
generated voltage increases causing more current to flow to the battery, also making it harder to turn the motor. After the diode,
the current runs through a current shunt that outputs 50mV for 10 Amps of current. This mV signal is amplified with an operational
amplifier and fed to the data acquisition module (LABJACK) and on to the PC and Labview program (also got the monitor from the dumper).
The motor output voltage is monitored
at the battery. This voltage (12-20 Volts depending on how fast you pedal) is fed to a voltage divider where it is
reduced predictably below 10 VDC so it does not harm the LABJACK. Both current and voltage are recorded in the Labview program,
along with power (Power=IV). At the battery, a 400W inverter is installed to supply power to any house-hold device. Typical output
for the bike generator is about 50 Watts although the mechanical gear ratio is far from optimal.
Last updated: 02/20/05
Copyright 2005, Greg Miller