How do stepper motors perform as generators?

[LMF5000]

I come from an RC background so I'm much more familiar with brushless DC motors. I've recently acquired a desktop 3D printer and it got me interested in stepper motors. I've spent the last month studying up on their construction and operation.

One thing that impressed me was that simply moving printer axes by hand generates enough voltage to illuminate the printer's display if the supply is off. It got me thinking it would would be perfect for a small scale wind turbine since it generates useful voltages without having to be geared-up.

So, how would a small hybrid stepper motor would perform as a generator (similar to the one in the attached pic)? How does it compare to a DC brushless motor of the same size? The output of the motor would be rectified, and the DC would be fed to a buck/boost converter to make a constant 5V for charging a phone. I have a supercapacitor bank I could throw in somewhere as well.

That's the main question, I also have some other questions if anyone would be kind enough to answer:
1. How does a stepper compare to other types of motor at driving a regular rotating load like a fan or a pump? I know this is like using a stapler to hammer a nail but I'm curious about their characteristics.
2. If I wanted to rotate a stepper at constant speed as cheaply as possible, could I use a 555 Timer IC to repeatedly trigger the "Step" pin of an A4988 or similar stepper-driver IC?
3. I hear that disassembling stepper motors destroys much of the magnetism - why is this? I've heard theories ranging from the base acting as magnetic "keeper" to the fact that letting the rotor touch the stator generates fields strong enough to demagnetise the rotor...

 

[Sparweb]

Little steppers like the one you're using can't generate much current. Maybe enough to light up some LED's. The waveform will be a mess and probably put a lot of ripple on the DC unless you take heroic efforts to suppress it.

If you want to generate power, go big or go home:
This guy built a wind turbine from a 400V servo that he bought off e-bay - taken from a CNC lathe being scrapped.



STF

 

[LMF5000]

Very interesting thread, sparweb. I thought of 3d-printing a vertical-axis wind turbine for my attempt. Unfortunately the costs of getting such a massive motor to my address in Malta would be prohibitive.

I'm not looking for much power, 10 watts would be plenty enough to charge a phone at 5V,2A. My buck/boost DC-DC converter would probably handle a lot of the filtering and conversion.

Would I be better off with a brushless DC motor instead of a stepper motor?

 

[Sparweb]

Yes. You'll need to rectify the output of the BLDC. Full wave rectifiers will do, but if that's all you do, the voltage will fluctuate wildly with speed.
You either need a way to regulate the speed or charge a battery to keep the voltage constant.

STF

 

[LMF5000]

The voltage conversion isn't such an issue, my DC-DC converter will do it. Why would a BLDC be better in this case?

 

[cswilson]

Both hybrid stepper motors and brushless servo motors are permanent-magnet AC synchronous motors. The biggest difference is likely to be the "pole count". A typical 100-pole (1.8 degree) hybrid stepper motor will produce 25 times the AC frequency than a common 4-pole servo motor for the same RPM. This may well be an advantage in what you are trying to do.

Another issue is the motor back-EMF (generator) constant Ke, the ratio of output voltage to speed. While this is usually quoted for servo motors, I don't see it in stepper motor specs. But fundamentally, you want to see that the AC output voltage in the expected RPM range is useful to you. And you must make sure that the higher voltages produced at the top RPMs you ever expect do not fry your electronics.

You will need to rectify the AC outputs to DC before your buck/boost converter. Probably simple diode bridges will be good enough. Stepper motors typically have two electrically independent phases 90 degrees apart. You will want to combine these somehow before your B/B converter.

The cogging torque of a hybrid stepper motor will cause velocity fluctuations and non-sinusoidal output waveforms, but I don't see these as a real problem for what you are trying to do.

 

[LMF5000]

Thank you, that was my reasoning as well. I would probably use two full-wave rectifier chips, one on each coil's terminals, then connect their DC outputs in series or in parallel once I know what sort of output I get in typical wind conditions.

I was attracted to steppers because of the high voltage relative to low input rpm. Typically brushless DC motors don't go far below a kv of 1000 rpm per volt so it's hard to drive one from a wind turbine without gearing. The DC-DC buck boost converters typically accept 3-30VDC as input. I thought I might let the capacitor on the input side of the DC converter board do the job of smoothing the output of the rectifiers.