Hand-cranked stepper motor as a generator

[conradelektro]

By chance I found this web page depicting a hand-cranked stepper motor http://bjblaster.homedns.org/projects/renewable/hand_power/index.html and I decided to try it myself.

My video http://www.youtube.com/watch?v=qec_qsDseCY shows a hand-cranked stepper motor, a rectifier circuit and a Joule Thief circuit.

The stepper-motor is more than 20 years old and I bought it some time ago for very little money from a surplus dealer in Germany. The face plate of the stepper motor has a side length of 80 mm. A heavy and sturdy stepper motor with six wires. The axle has a diameter of 10 mm.

The hand-cranked stepper motor first drives eight white LEDs which seem to get too much power. A rectifier circuit allows to measure the output power (at about 60 rpm or one crank turn per second) which amounts to approximately 0.8 Watt. It does not help to turn the crank much faster as shown in the video, the output will not rise much.

The hand cranked-stepper motor and the rectifier circuit first drive a radio and then a Joule Thief which lights a 200 Volt / 1 Watt Led-lamp and alternatively a gutted CFL (compact fluorescent lamp, spiral shaped gas discharge tube, ignites at ~800 Volt).

Unless you find a person who is willing to crank the stepper-motor, this idea will not be very useful. But it is an interesting experiment.

Greetings, Conrad

[gyulasun]

Hi Conrad,

Nice and simple setup, thanks for showing.  Did I notice correctly that the cranking force needs to be a little bit higher for the LED lamp than for the gutted CFL?  Or just the small difference between left and right hand cranking?

rgds, Gyula

[conradelektro]

Did I notice correctly that the cranking force needs to be a little bit higher for the LED lamp than for the gutted CFL?  Or just the small difference between left and right hand cranking?

Yes, the Voltage over the 47000 µF cap rises to about 3 Volt when lighting the CFL (not so much power consumed) and only to about 2 Volt when lighting the 200V / 1W LED lamp (more power is consumed, more effort is needed to crank).

The stepper motor produces about 0.8 Watt when cranked at about 1 turn per second. It is possible (although not comfortable) to turn it by hand at up to 3 turns per second, but the output does not rise significantly. Why is that? May be it has to be turned much faster to see a difference? Or is some sort of satturation reached soon?

On the back of the motor where the specifications are imprinted on a aluminium foil, one reads "@ RPM 60 CPS" (on the left side, beneath "STEP ANGLE 1.8°", see the photo attached to my firts post in this thread). What does this "@ RPM 60 CPS" mean?

This person also made a video about a stepper motors as generators http://www.youtube.com/watch?v=cPMkBfuo824 (more tests, stepper motor on a bicycle to light some LEDs).

It seems that I have to spin the stepper moto much faster to get more power out of it:
http://www.solarheatengines.com/2011/01/28/generating-power-with-a-stepper-motor/

If you think about it, there could be useful applications, e.g. on a bicycle or a very simple wind turbine. Conversion efficiency is not such a big issue in a quick and dirty hack, costs and simplicity of build are more important.

The advantage of a stepper motor as a generator is that one gets some power out of it even at a low turn rate, which allows to drive it without gears (directly driving the axle with a slowly turning source like a wind wheel or water wheel).

Greetings, Conrad

[gyulasun]

Hi Conrad,

Yes I think it is a kind of saturation, both the LED and the CFL are non-linear loads (like Zener diodes) and able to limit voltage across themselves.
IF you also found a limit in output power when you hooked up the 10 to 100 Ohm resistors and no lamps, and you found the 0.8-0.9 Watt output limit,  this may come from core saturation due to load current I suppose or rather the effect of the "impedance protection" (see it also imprinted in the Alu foil) which may mean a provision to compensate for inductive reactance increase when the rpm increases. This is a special feature for the better, more precise stepper motors I think. 
I am not very good at stepper motor specs, what you ask I read as RPM @ 60 cps i.e. RPM at 60 Hz (cycle per second), if this makes sense but the real RPM data is missing? Should be in the rectangular box below the 60 cps but the box is empty...
Here is some definitions on AC and DC stepper motors but for slo-syn motor types (slo= slow syncron?) and not for rapid-syn ones, maybe still useful, see pdf page #18,  steps per second definition. etc: http://mcsupplyco.com/uploads/images/drawings/pdf1/SYNCGUIDE.pdf

Also, see this ebay offer where a 34H-600 type Rapid Syn stepper motor is shown and the step angle is defined as 72 RPM instead of the normal angle.  And below it, again the RPM @ 60 cps with an empty box:  http://www.ebay.com/itm/RAPID-SYN-34H-600-COMPUTER-DEICES-STEPPER-MOTOR-/271035025677?pt=LH_DefaultDomain_0&hash=item3f1af2490d
(A step angle of 1.8° obviously means 200 steps per revolution,  200 x 1.8°=360°  and this must be corresponded to rpm.)

rgds,  Gyula

[conradelektro]

@Gyula: you found nice infos about stepper motors, helps to understand better.

I found some stuff in German http://www.rafoeg.de/10,Forschungsprojekte/90,Schrittmotor/Schrittmotorversuch_RaFoeG.pdf, which contains some esoterics (Lenz force is lower at higher rpm) but also useful curves about output versus rpm (only useful for German speaking readers, the graphs may be useful for everybody).

I did some more measurements over the 10 Ohm and 100 Ohm resistor as a load: the voltage rises when I turn faster. And I will do some more tests with an electric drill to spin the stepper motor faster than by hand.

So it seems, the saturation was in the load (CFL or 220V/1WLED lamp, or even the 8 LEDs without rectification). There is of course a limit for output, but at very high rpm (several thousand revolutions per minute).

Greetings, Conrad