# Investigating the generative properties of a stepper motor

You probably know that if you spin a motor (mechanically) it generates electricity on what would normally be the inputs. This can be a problem when you shut off a spinning motor and is the reason that protection diodes are built into motor driver circuits. But [Dino] isn’t interested in driving a motor, he wanted to see what he could do with the electricity generated by spinning a stepper motor.

He built the test rig that you see above for this purpose. In the foreground a 12V DC motor is held in place with an electrical conduit clamp. This connects to the stepper motor being tested using a segment of rubber tube. The DC motor provides a reliable input for his experiments, but could be replaced in the future by a propeller to make it wind powered, or by a water wheel. Check out the video after the break to see what kind of juice [Dino] gets out of it, and how it can be used for powering LEDs, recharging batteries, or driving a motor.

## 40 thoughts on “Investigating the generative properties of a stepper motor”

1. Lupin says:

Do you only rectify one phase? As far as i understood the stepper motor has 4 phases and a common ground. Couldn’t you rectify all four phases? Something like described here: http://www.allaboutcircuits.com/vol_3/chpt_3/4.html

Would also be nice to investigate the output/input power ratio to get an idea of efficiency.

1. Dax says:

There’s two pairs of phases, so you’d actually get only two phases out of it.

2. Nick Short says:

Also, rectifying more than three phases is highly inefficient.

Three phase rectification gives you highest, most stable pulsed DC output.

1. Jeff says:

Couldn’t they be run through capacitors to put them in phase before rectifying?

2. Dax says:

If you have a stepper motor with four poles, you actually get two phases that are opposite to each other, so all you really need to do is flip the polarity of one phase and then wire them in parallel.

Then you get a single phase output that you can rectify.

2. Sonic says:

This is along the lines of what i want to do with my fish tank. I figure the water draining from my overflow should be able to generate enough power to at least power some lights. I would like to see more.

3. MrX says:

Brushless motors are also a good source for generators. Everybody has CDROM brushless motors laying around .. they are pretty simple to rewind (for better efficiency) too..

1. macona says:

I agree. A brushless motor is a much better candidate than a stepper for a generator. With a stepper’s high detent torque you will waste a lot of energy on input with no load.

A brushless motor will give you three phase out which can be rectified with a standard three phase bridge rectifier. I have a 1.5kw brushless servo motor I have thought about building a generator with.

1. bob says:

Detent torque nets out at 0. It will prevent motion for part of the rotation, but aid it in other parts. It does however create a minimum torque required at the input.

4. rick says:

you rectify each phase. ignore the center taps and ground on the stepper motor. with 2 rectified outputs you can either run them in series for more volts or parallel for more amps (ma in this case). i was getting 14 volts at 200ma out of a printer stepper wired in parallel spinning at 1500rpm through a drill to spin the stepper motor.

5. Old people and rap music. Lol.

1. I thought he was going to start with “Oh, I didn’t see you there.”

6. Chris says:

“is the reason that protection diodes are built into motor driver circuits”

I’m not sure that’s technically correct. The diodes are there to prevent damage from the huge voltage spike that occurs when you disconnect the inductive load that the motor provides. It’s true that the motor generates power that could damage your circuit as well, but if someone designed the diodes only to handle the voltage from the “generator” effect, their circuit could still let the smoke out from the inductive kickback.

1. Harry says:

Stepper drivers are notorious for letting the magic smoke out when you disconnect the motor while it is energized, despite the diodes. Any motor controller engineers out there that can weigh in?

1. jay says:

don’t think it needs a motor control engineer.

the stepper drivers are constant current sources, but they’re (obviously) not ideal. so when you raise the resistance from a few ohms to the resistance of an air gap, they’re hosed.

7. You could smooth out that DC profile with the capacitor. Just hook it in parallel to the bridge and you’ll have a virtually flat DC signal, with a little bit of ripple of course.

8. McGuiver says:

I liked the idea of this. The only thing that made me cringe was when he had the common ground on his bike frame. It was attached to his front forks. That means the back light is grounded through the bearings in the head tube of the bike. Running electricity thru bearings is always a no, no.

1. Out of sheer curiosity, why not? I’ve never heard that before.

1. danman1453 says:

Ever hear of a car battery grounding the starter through a drive shaft? The moving sealed environment makes for some ‘interesting’ corrosion. Long story short, drive shaft fails in VERY short order.

2. biozz says:

with this little current and voltage grounding threw a bearing will cause no trouble at all … suit caused by high current passing threw lubercant

less than a few amps at less than 30 volts you wont have such problems … atleast in your life time your kids kids might have to spray some WD-40 in to it but i think you will be fine

3. dattaway says:

We had to put brushes on large DC motors, because currents from static friction, corona discharge, insulator leakage, or whoever knows would slowly pit the bearings. The current may be small, but the bearings aren’t supposed to touch through the thin layer of oil. Something was breaking through that oil and the damage becomes total failure over time.

9. biozz says:

JUST the other day i took a large 4A stepper and connected all the wires to a 2A stepper and spinning the 4A spun the 2A just as precise … fun experiment!
(they were bipolar)

10. lotmi says:

I’ve often wondered what would make the best , most efficient, generator.

11. done some playing with small steppers i can say, that when powered from wind they at best can be used to light some leds. what i really wanted was to trickle charge car battery, but you need bigger stepper to get enougth voltage.

12. I guess his idea is a bit like those LED devices that you attach to your shower head.

When the water runs, it flows through a turbine in the device which powers the LEDs and colors the water as it comes out.

13. RBR says:

Just so you dont land yourself in a hospital you might wanna mount that stepper on the front side of the bicycle forks. Id hate to see that bracket fail when your hauling ass and it become a wedge locking the tire XD

1. Dino says:

The brake pad is in the way. There’s no way it could wedge in there anyway… not enough room. Worst case is it falls of, get’s the wires ripped out in the spokes and ends up on the road side. Chances of that are 1 in 1,386,283.42. but thanks for your concern! :)

2. K!P says:

Thats the normal way to mount a dynamo to a bike, we (the dutch) use em like that all the time. never heard of annyone who was hospitalized that way.

14. NATO says:

You probably know that if you spin a motor “(mechanically) it generates electricity on what would normally be the inputs. This can be a problem when you shut off a spinning motor and is the reason that protection diodes are built into motor driver circuits.”

Um, NO, INCORRECT.

The diodes are there to supress the transient voltage spikes that occur when you stop driving the motor winding. THIS IS A VERY FUNDAMENTAL CONCEPT!!!

Hackaday fail (no surprise). I am shocked that only one person has pointed this out… Does anyone here even know what they are doing, or are you all just copying someone elses projects??? (rhetorical question)

1. It’s really interesting that you chose one small part of much greater picture. You found a dirt spec on a bug in a garden, and you ignored the garden!
lmao
Thanks for joining in! :)

15. Laura Harris says:

If all you want to do is light LEDs, you can connect them in parallel but “back to back” (polarity swapped on one of the LEDs) and have light produced by the rising edge AND the falling edge of the current pulses. No other rectifiers needed. Here is a video I made a few years back that shows the effect: http://www.flickr.com/photos/imajilon/2610522040/in/set-72157605795103075.

BTW, the final build photos for the bike light are here:

16. Simon says:

lotmi says:
January 24, 2012 at 6:10 am

I’ve often wondered what would make the best , most efficient, generator.

When you’re dealing with power of large magnitude having efficiencies in the upper part of the 90% region is crucial to not have megawatts of waste heat destroy all your machinery. Go look up at all the technology involved in getting hydrogen cooled turbogenerators together. Engineering is awesome. :)

17. Bobikas says:

My friend has used a stepper mottor in place of rotary encoder. Used as a knob for an amplifier it can sense rotation by hand or could rotate it self when remote controlled. He also hacked the motor to support a pushbutton underneath. A few photos:
http://circuit.lt/?page=image&section=projektai/a03/project&img=encoder1&type=2
http://circuit.lt/?page=image&section=projektai/a03/project&img=encoder2&type=2
http://circuit.lt/?page=image&section=projektai/a03/project&img=volume_sch&type=1
http://circuit.lt/?page=image&section=projektai/a03/project&img=volume_pcb&type=1

18. Miroslav says:

I tried this before with bipolar stepper and it worked very well. At very low shaft speeds you get several volts. It requires some torque to spin and generate though. So re wind generation Savonius turbine or other VAWT would be best, imho.

Here is (shameless promotion follows) my page with asynchronous (induction) motor turned into generator: It doesn’t need any mods to produce good power.

http://www.arthropodsystems.com/AsynchronousGenerator1/AsynchronousGenerator1.html

19. kuhltwo says:

I used tape drive motors top generate voltage.
I am glad to see someone else using the same “antique” O-scope as me. have been using mine for over 20 years, both as a bench unit and a field unit. I just have a shade over the display.