Thursday, November 17, 2011

Specific aim 3: a dynamo wheel testing jig

In order to test my buck converter with a dynamo I need to be able to spin the wheel at different constant speeds.  This is not done practically by spinning the wheel by hand, so some kind of motorized system is required.  Fortunately, (DIY dynamo lighting legend) Martin over at pilom has already built one and he gave me some helpful advice on how to build my own. An abandoned bicycle fork recovered serendipitously from under a bridge and a washing machine motor salvaged from my in-laws' basement are the main ingredients.  Rudimentary speed control comes from a router speed controller, which I believe is TRIAC-based.  It took about 3 hours in my Dad's wood shop to put it all together:

hub dynamo testing jig with 700C wheel
1/3 hp washing machine motor. Routed slots allow motor adjustment to accommodate different sized wheels

The 6 inch drive wheel is based on achieving a maximum speed of 50 kph, or close to 400 RPM, from the motor's full speed of 1750 RPM.  Speed control is not great; the sweet spot cruising speed (25-35 kph) is very difficult to maintain.  In this region the motor's relay cycles on and off, resulting in a sinusoidal speed with a period of a few seconds.  Constant speeds in the 5 kph - 20 kph are easily achieved, then it cycles and finally takes off to a maximum speed.

So far so good. The contraption remains stable with minimum vibration at high speeds. I ordered a super cheap bicycle computer from DealExtreme to measure the wheel speed, but while I waited for it to arrive I figured how to calculate the speed based on the pulses coming out of the hub using a zero cross detector.  The cycle computer has arrived, so I'll be able to see how close my zero cross dynamo speedometer works (more on that later...).

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