Playing with wind power - RC-Monster Forums

J57ltr · **Playing with wind power -** 07.12.2009, 06:35 PM

So it's always windy here in Tomball at my house and I have been playing with the idea of a small wind generator. The type I have been looking at are the ones that look like a cylinder cut in half and then set up so it looks like an "S" so it picks up wind in any direction. I was thinking of something small scale just tinkering. But the ones on the Net the guys are assembling the stator and magnets separately which seems like a huge waste of time to me for efficiency that probably isn't that good.

So my idea is to use a brushless motor as the power device, my question is, what Kv? I have noticed that if I short the windings together that the back emf increases the amount of torque required to turn the shaft (obviously). I have also noticed that the lower the Kv the harder it is to turn, Which makes since because there are more windings turns in a lower Kv motor allowing for a higher voltage to be generated for a given RPM. So the motor would need to have enough voltage to be able to charge the battery (probably a 12V for now), but not stall the windmill. I know that my 7700 is super easy to turn with the leads shorted. I also notice that the braking is less as my overall gear ratio decreases, so I have to up the braking force, (I understand why).

I guess first I need to figure out what RPM the thing will spin and the voltage produced at that speed. I already have a 3phase bridge rectifier from another project (Alternator welder). So what do you guys see as a problem or suggestion?

Jeff

What's_nitro? · 07.12.2009, 06:53 PM

The only problem with using RC motors is that, since they have a small diameter rotor, you need to spin them up pretty fast to get usable power from them. This could be solved easily by using a planetary GB on the motor and attaching the windmill to that shaft. Maybe even two PGB in series if possible... I would guess that you would want 8-10K rpm on the motor to produce a decent current. Good luck with your project. I want to do something like that too since there is a lot of wind in my neighborhood but town laws are very prohibitive.

BrianG · 07.12.2009, 06:54 PM

A higher kv will generate more voltage for a given rpm, so I would go with that. And, I wouldn't worry about stalling the motor because charging a battery is not nearly as much load as shorting the motor leads. The battery charge current is "regulated" by the current battery voltage and any inline resistance or circuitry. And, you might have to gear up the motor because a windmill does not spin very fast and these motors will have more output at higher rpms. Maybe an outrunner would work better in direct drive?

J57ltr · 07.12.2009, 07:48 PM

Wasn't really thinking direct drive I was thinking 4:1 to the motor. I have a bunch of prototype parts from when I was developing a small coonveyor for our company so I have a bunch of .080" pitch belts and some 60 tooth 1/4" sheaves and belts and 15 tooth sheaves that are bored to 5mm (our motor uses a 5mm shaft too). I think the gearboxes would have too much drag, and the only ones I have are about 30:1 and another that's about 100:1.

I swear that at the same RPM the lower KV motor had a higher voltage out.

This is going to be a pretty small unit about 16" tall and about 12" in diameter. Just for testing too see what I can do just with what I have laying around.

Jeff

Keep the ideas going!

Metallover · 07.12.2009, 11:08 PM

It may be easier to use an automotive alternator or generator. Just an idea.

J57ltr · 07.13.2009, 12:58 AM

Ya that's been done and works, you can get about 500W out of one, but since you either have to drive the field coil (the magnet in an alternator) or make a new rotor with permanent magnets it’s a little more of a pain to do (Although I have been toying with making an alternator into a brushless motor). I want to keep this small since I have deed restrictions. I get enough crap about an oil spot on my driveway.

Jeff

_paralyzed_ · 07.13.2009, 01:23 AM

Quote:

Originally Posted by J57ltr

Wasn't really thinking direct drive I was thinking 4:1 to the motor. I have a bunch of prototype parts from when I was developing a small coonveyor for our company so I have a bunch of .080" pitch belts and some 60 tooth 1/4" sheaves and belts and 15 tooth sheaves that are bored to 5mm (our motor uses a 5mm shaft too). I think the gearboxes would have too much drag, and the only ones I have are about 30:1 and another that's about 100:1.

I swear that at the same RPM the lower KV motor had a higher voltage out.

This is going to be a pretty small unit about 16" tall and about 12" in diameter. Just for testing too see what I can do just with what I have laying around.

Jeff

Keep the ideas going!

I'm really curious about the small sheaves and belts you speak of. I've been toying with the idea of a belt drive rather than a traditional spur/pinion. Have any links or pics?

J57ltr · 07.13.2009, 02:53 AM

Stock Drive is a place that carries them. They are called Timing pulleys, I guess I just use sheave for anything that uses a belt. http://www.sdp-si.com/web/html/drivesystems.htm

I can’t pull up anything on their site right now but I use a 3/8” wide .080” pitch Kevlar reinforced belt. A 15 tooth drive pulley that we bore to 5mm and a 30 tooth that comes ¼” bore and, is just under 1” in diameter we bore it to 3/8”. The belts are strong but would probably wear out fast in a RC think belts run about $6 the larger pulley runs about $15 and the smaller ones about $9. Of course that’s just a 2:1 ratio they also have plastic pulleys and ones with larger teeth (that’s usually the first thing to go). There are some others but I’ll have to check at the office.

Jeff

Patrick · 07.13.2009, 06:26 AM

Quote:

Originally Posted by BrianG

A higher kv will generate more voltage for a given rpm

I thought some companies found the motors kv by finding the rpm needed for the motor to put out 1 volt. I don't remember who said that, but it was on rcm. Could take a while to find in a search, so I can't be bothered looking

J57ltr · 07.13.2009, 08:58 AM

Hmm, I am sure it's the other way around a motor will spin X rpm per given volt. Generating power and producing power seems to be 2 totally different results. I'll give it another try later tonight.

Jeff

BL_RV0 · 07.13.2009, 12:01 PM

Quote:

Originally Posted by BrianG

A higher kv will generate more voltage for a given rpm,

I think you got it backwards there Brian...

BrianG · 07.13.2009, 01:09 PM

Oops, yeah, I may have.

J57ltr · 08:31 PM

Not much info here but I used amy drill to spin the motors and took the voltage of of 2 leads. I used my Fluke 112 and set it to record ACV. They are all Delta winds but I spun them both ways to make sure and this is what I got.

7700Kv .045V
6000Kv? .059V
5300Kv 30mm can .062V
4436Kv .068V

This is only about 600rpm but I don't have anything faster at the moment. I am going to start gathering parts. I think I am going to make the blades out of fiberglass for weight savings. It's called a Savonius type.

This is a pic of the type I want to make:

What's_nitro? · 07.13.2009, 09:43 PM

Based on that geometry you could just get a thin tube (in the metarial of choice) and cut it down the middle for use as the vanes.

J57ltr · 07.13.2009, 10:14 PM

Thats what I had planned but the closest thing I have available (and light enough) is 2.5" X .0625 wall aluminum tube. I think I need something with a diameter of around 6" a Coffee can size would be perfect but it's too short. I'll look into some of that thin wall PVC. Any other suggestions?

Jeff