The stepper motor generator
This size 34 -14 v stepper motor was bought on eBay for £25.
Stepper motors are multi-pole alternators, but being more modern they have four phases while the Dynohub has only one. As with a DC permanent magnet motor, turning the motor's shaft makes it work backwards, causing
pulses of current to come out of the windings. However, the current is AC, going plus as a magnet pole approaches a coil and then minus as it moves away. Usually there are four phases at 90 degree intervals so when one comes down to zero, the next one has reached maximum. This is a benefit as it
means the output can be rectified to produce much smoother DC with hardly any gaps, but it means they have more wires coming out. It's quite easy to figure out which way around they are using a resistance meter. The six wire stepper is really two motors in one, so the six wires can immediately be separated into two groups of three. Each group will have some connection to each other, but no connection to any of the other group. In each group, one wire is the common
and the other two are the opposite ends of a winding which will give out oppositely phased AC.
In terms of resistance, the reading from the common to either end will be half the reading across the two ends. Having found the common on one set, you can use the same process to find the common in the other one. All four windings will have almost exactly the same resistance.
The simplest way to wire it up is to link the two commons to the minus terminal and then connect each of the four live phases through a diode (1n4001) to the plus output.
I found the simplest, and cheapest method of regulating the voltage was to buy a car cigarette voltage converter, the 1 amp version
was on sale at maplin for £6.99. It had the added advantage of a choice of output voltages, between 1.5v to 12v although the unit
might get a bit hot when operated at lower voltages.
I achieved some reasonable results with this stepper, getting 12volts at approximately 200rpm, and then rising to about 60 volts with increasing the shafts speed, using a cordless
drill to power the motors shaft. The current limited itself to 0.32 Amps. Therefore using ohms law W=I x V
12v x 0.32A = 3.84 watts @ 200 rpm.
60v x 0.32A = 19.2 watts @ 1000 rpm.
I need to find a way of regulating the voltage to approx 14 volts, for battery charging - and possibly increase the current, ideas welcomed.
Visitor Miles Hodkinson commented: As the voltage increases into the battery bank the current will increase proportionally and the voltage will stay reasonably constant.
With such a small generator I doubt a large leisure battery would ever be troubled (county batteries do a 110ah for £55). You could also use a solar charge controller.