# Energy harvesting using a brushless generator

#### shortbus

Joined Sep 30, 2009
9,053
I thought of another dumb question about what you're doing here. What is the source of power for this, the rotation? If it in itself is only a small force, will loading it down, by generating a voltage, be a problem? You do remember the old,"there is no free lunch" saying? And if there is an electric motor diving the shaft, wouldn't a convertor/ invertor from the motor supply make better sense?

And another thing, from what I've seen documented online, this device will produce an AC waveform regardless of the magnet's orientation. This because it appears that a current is induced in one direction when the gear's tooth approaches the magnet and then it's induced in the opposite direction when the gear's tooth moves away from it.
While they do make a 'sine wave' type output it really isn't AC, especially if all of the magnets would be the same polarity. It would/should look more like "half wave rectified AC" than AC.

#### cmartinez

Joined Jan 17, 2007
7,552
I thought of another dumb question about what you're doing here. What is the source of power for this, the rotation? If it in itself is only a small force, will loading it down, by generating a voltage, be a problem? You do remember the old,"there is no free lunch" saying? And if there is an electric motor diving the shaft, wouldn't a convertor/ invertor from the motor supply make better sense?

While they do make a 'sine wave' type output it really isn't AC, especially if all of the magnets would be the same polarity. It would/should look more like "half wave rectified AC" than AC.
The magnets won't be the same polarity, but rather alternate. And maybe probably for the best, since the rotor could end up being magentized itself if that weren't the case, and that's a bad thing. But that latter observation of mine is just a guess.

As for their generating AC, I'm almost sure that they will, according to the small amount of research I've done. Also, I'm very much aware that a drag will be created when power is being generated. But I'm not too worried about that since very little power will be demanded from this thing.

What's really bothering me is if I'm using the best design for this purpose. That is, wrapping coils around stationary magnets, and making a ferrous rotor. This other design, for instance, rotates a magnet and the coils are wrapped around a ferrous stator:

#### Alec_t

Joined Sep 17, 2013
12,456
Yebbut, that's not really a variable reluctance generator, which is your stated aim.

#### cmartinez

Joined Jan 17, 2007
7,552
Yebbut, that's not really a variable reluctance generator, which is your stated aim.
My aim is to build a miniature brushless generator... which could be of the variable reluctance, switched reluctance, or whatever ... but right now I'm having a serious case of intellectual reluctance ... ...

#### cmartinez

Joined Jan 17, 2007
7,552
There... I've adjusted the thread's title to better fit what's being discussed here.

Joined Jul 18, 2013
24,206
This other design, for instance, rotates a magnet and the coils are wrapped around a ferrous stator:
That is a simple 3phase AC generator with 2 electrical cycles per rotation.
Max.

#### cmartinez

Joined Jan 17, 2007
7,552
That is a simple 3phase AC generator with 2 electrical cycles per rotation.
Max.
Thanks for joining in, Max. What I want is the simplest, most efficient and miniature passive (as in not needing additional external excitation) brushless generator I can build. I'm currently considering the configuration shown in post #36. I'm just wondering if that is the best option for what I want.

Joined Jul 18, 2013
24,206
Personally I would go with #42.
What do you call 'miniature'.?
Possibly make the armature from button magnets of the right size?
They drive this Faraday motor pretty good! Ebay 112729895384
Max.

#### cmartinez

Joined Jan 17, 2007
7,552
Personally I would go with #42.
What do you call 'miniature'.?
Possibly make the armature from button magnets of the right size?
They drive this Faraday motor pretty good! Ebay 112729895384
Max.
You mean that magic bolt and nut toy thingy? ... I don't need that small. I'd like it to have an outer diameter of between 1-1/2" to 2", if possible. I was thinking about using six neodymium magnets of 1/4" diam x 1/4" long.

Joined Jul 18, 2013
24,206
I just used the 'Magic' bolt as an example of the small magnets's power.
I picked up a selection of button magnets off ebay, surprising the power of these.
Although China does have the market on rare earth magnetic material.
What revs do you hope to operate at? if not really high you could epoxy them into a machined rotor.
Max.

#### cmartinez

Joined Jan 17, 2007
7,552
I just used the 'Magic' bolt as an example of the small magnets's power.
I picked up a selection of button magnets off ebay, surprising the power of these.
Although China does have the market on rare earth magnetic material.
What revs do you hope to operate at? if not really high you could epoxy them into a machined rotor.
Max.
Not high at all. I'm thinking about 100 rpm tops. So yes, I have the option of rotating the magnets and having coils on the stator is quite open. I'm just wondering what would be best in a power to size ratio.

Joined Jul 18, 2013
24,206
What values are you intending to generate?
Max.

#### cmartinez

Joined Jan 17, 2007
7,552
What values are you intending to generate?
Max.
I'd be very happy if I could generate at least an output of 20 mW peaking at 0.5V with this thing. An average of 0.1V would be ideal. But the more the merrier... I have a feeling this thing can perform better than that. You have no idea of how much I appreciate your help, btw. Many thanks.

Joined Jul 18, 2013
24,206
Those requirements appear rather trivial, I would expect you to be able to reach that easily.
Some empirical testing/experimentation is needed usually.
Max.

#### cmartinez

Joined Jan 17, 2007
7,552
Some empirical testing/experimentation is needed usually.
Yeah, I've guessed that much... I'm just trying to use the best possible approach on the best try ... you haven't told me which configuration you like best, though... coils around stator magnets, or a rotating magnet with coiled stator teeth?

Joined Jul 18, 2013
24,206
I did, in post #48 favoring post #42.
As per simple BLDC motor back-driven.
Max.

#### shortbus

Joined Sep 30, 2009
9,053
That is a simple 3phase AC generator with 2 electrical cycles per rotation.
Max.
Which also gets us back to the stepper motor mentioned back a page or two, and those are cheap and very available. And close to the wanted size.

#### cmartinez

Joined Jan 17, 2007
7,552
I did, in post #48 favoring post #42.
As per simple BLDC motor back-driven.
Max.
Unfortunately, I cannot use either a BLDC nor a Step motor for this project, for reasons that I'm unable to disclose.

Joined Jul 18, 2013
24,206
Unfortunately, I cannot use either a BLDC nor a Step motor for this project, for reasons that I'm unable to disclose.
So I am assuming you cannot use a P.M. rotor as in post #42?
Max.