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Dec 14, 2014, 08:25 PM
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The dynamo

Building a dynamo was long dreaded, but necessary to have any hope of regulating the speed.

Out of sheer luck, the Tamiya came with a tool which perfectly coupled 1 shaft end with a spare motor. A simple jig could hold everything together. The bench supply couldn't provide stable voltage because the current & voltage were out of phase. The old LM317 system with giant capacitor could provide stable voltage.

The load could be adjusted by shorting out different motor leads with different lengths of wire. An op-amp measured the current between 2 motor leads to give RPM. It was surprising that the op-amp was so sensitive, it could detect the current in a short piece of wire.

An input had to be grounded for this to work. Letting the motor leads float above ground only showed 0V. There was a lot of ringing when the motor leads weren't shorted, too.

Relying on the phone app to show voltage & the oscilloscope to show RPM. It had 1 more problem: the speed fluctuated. If it started slow & ramped up, the speed would be higher than if it started fast. Slight variations in wheel pressure also affected speed.

A software program would have to step though the entire PWM range for each voltage to build a table of RPMs. Then, given a target PWM value & target voltage, it could look up a modified PWM value for the real voltage that would give the same power. There are still dreams of measuring the current accurately enough to make constant power feedback.
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