I want to build a small air powered generator. it will be powered by a die grinder air motor, with a 22,000 rpm speed with no load, and about a 10,000 rpm speed with a heavy load. I want to produce 12 volts, and at least 100 watts, but no more than 300. I was thinking about using a small dc motor as a generator, but have no idea what type of motor to use. Permanent magnet maybe?

 

couple of things you'll want to consider. At higher speeds, a smaller motor is required, but then your wattage is limited. Air as a driving media is problematic in that you loose speed with load, again affecting your wattage. You may consider a larger motor as a generator, and either a larger air motor, or one geared down.

 

Yes, however, the load will not vary. That being said, the speed required at that load to produce the desired wattage and voltage, can be controlled by varying the pressure. This is completely hypothetical, but say I have a 100 watt, 12 volt load. If I need 10,000 rpm's to produce that, and it takes 100psi to accomplish this, then that can be accounted for. I thought about gearing it down, but this will be a tool. It may be dropped, thrown, or even have something heavy dropped on it. I think the fragile nature of small, clockwork like gears is a problem. I most certainly want NO plastic in any of the mechanics, and I have ran into plastic gears before. Not to mention, there will be no method of lubricating the gears, unlike the air motor, which can be oiled through the air inlet.

 

Out of curiosity, where are you getting the needed air pressure and volume to produce 12V/8A-25A, that can't be produced by some other means? Or is this an electrical isolation scheme?

Ken

 

I was going to recommend a geared down car alternator. but it sounds like its going into a handheld tool. not sure you're going to find something small enough and powerful enough, or something that works in tens of kilo-rpms.

as kmoffett touched on, what's the point of this? it would be much more efficient to do this electrically, and it is assumed you have electricity since you have compressed air. The compression & release of compressed air is very inefficient. This is why compressed air cars haven't replaced gasoline cars.

 

I work on a steam locomotive. I was using an air powered needle scaler, but I couldn't see a thing. A work light would have been a nice thing to have, but I don't want to have to drag around an air hose, as well as a cord. I had the idea to use the energy available to me, and strantor, do not assume I have electricity because I have compressed air. When the locomotive is running, the compressors are steam driven. It does not necessarily have to be handheld, but it needs to be able to fit in a 4 inch cube, without the light bulb or air fittings. It could also be up to 120 volts, if that would help. Practicality often trumps efficiency.

 

Ahhh! This is the kind of information that can help people here help you. It's useful when people tell not only what they want, but why they want it.

Ken

 

I have thought about size versus wattage, and I have reduced the requirements to 50 watts. I would prefer 12 volts, but it can be worked around as necessary. I have also been looking at drill motors and gear boxes, with all metal construction, to drive a small generator. This seems like a good size and power motor for use as my generator. http://www.robotmarketplace.com/products/BP389010-00.html

 

I have thought about size versus wattage, and I have reduced the requirements to 50 watts. I would prefer 12 volts, but it can be worked around as necessary. I have also been looking at drill motors and gear boxes, with all metal construction, to drive a small generator. This seems like a good size and power motor for use as my generator. http://www.robotmarketplace.com/products/BP389010-00.html

Thanks for the perspective. All drill motors I've dissected have been series wound motors, so I (safe) assumption is that the one you linked to is a series wound motor as well. Series wound motor will not work as a generator. If you want to use a motor as a generator then the short answer is that you are looking for a permanent magnet motor.

EDIT: confirmed, it is a brushed series motor.

 

this might be a perm mag motor. can't tell. It's a little big for your application.

 

If it can produce 100 watts, then it is even better. The size is almost perfect to tell the truth. Since I have a $199 motor possibility, I should select an air motor that can meet my demands, such as the Ingersoll-Rand SM1AM Air Motor, which can produce 1/3 hp @ 7300 rpm. Hopefully I can find a more affordable motor. By the way, I read your profile and noticed that you were in the Navy. Thank you very much for your service!


EDIT: I just realized I have an old scooter motor! It does work as a generator!

 

If it can produce 100 watts, then it is even better. The size is almost perfect to tell the truth.

Actually that motor I linkd to probably would produce much more than 100W. It's overkill. I'm not sure how the power rating works in reverse, but as a motor that thing runs 24V, 375A (stall) so at stall it's 9000W. If you only need 100W I would (someone confirm/deny this) choose something that's at least 200W motor. I *assume* that doubling the rating would be sufficient.

Since I have a $199 motor possibility, I should select an air motor that can meet my demands, such as the Ingersoll-Rand SM1AM Air Motor, which can produce 1/3 hp @ 7300 rpm.

I think that motor looks good. 1/3 hp = 250W, so figuring mechanical losses it should be right on the money if you're using a 200W motor(generator).

EDIT: I just realized I have an old scooter motor! It does work as a generator!

awesome! specs?

By the way, I read your profile and noticed that you were in the Navy. Thank you very much for your service!

No problem. Thank you for reminding me that I have a profile. I updated it, + 2years and + 1 kid.

 

From what I can gather, I have a 100 watt, 24 volt motor. If I can get close to 100 watts out of it; Great! If only 50; that will do. Unless I can find an inexpensive 200 watt motor. I may have one of those too, I just have to look.

 

From what I can gather, I have a 100 watt, 24 volt motor. If I can get close to 100 watts out of it; Great! If only 50; that will do. Unless I can find an inexpensive 200 watt motor. I may have one of those too, I just have to look.

I Really would feel more comfy if someone more knowledgeable chimed in. I'm really unsure how many watts motor takes to make a 100W generator. I would hate to tell you "you need a 100W motor" and you go buy it and can only get 15W out of it as a generator. If it were my project, my approach would be to get the largest motor that can be had affordably, and *hope* that it's sufficient. Buy big, buy once.

 

Considering that I have a 100watt motor already, there is no risk in trying. I also have a larger motor somewhere around here, so no risk there either. The only thing I hope to buy is the air motor.

 

Considering that I have a 100watt motor already, there is no risk in trying. I also have a larger motor somewhere around here, so no risk there either. The only thing I hope to buy is the air motor.

alright then, go for it. Let me know how it turns out. It would be a good idea though to find out what RPM your motor needs in order to produce the required voltage, before you buy an air motor. If your air motor spins 21,000RPM and your generator motor is only rated at 1200rpm, then you're likely to ruin the generator motor and be back at square 1. Also, I wouldn't test it with no load. I figure the air motor is probably going to spin super fast with no load, and as you apply load it will slow down. A free-spinning electric motor insn't much of a load, so it will shoot right up to the 21000rpm. So test it with the bulb in place so that the electric motor will provide a little mechanical resistance, then use a air regulator to slowly bring the RPMs up (from 0 psi) until the voltage reads 12V. Then you're "done" except for the fact that if the bulb ever goes out, you've lost your load and your RPMs are going to skyrocket. You might want to design in some kind of load loss detection.

 

Not to derail your ideas, but have you thought about something like this? http://starhuntinglights.com/catalog/product_info.php?cPath=1_28&products_id=95

If having it mounted on your head is a problem, it could be adapted to the needle scaler. to me the thought of having a second air tool(drill) operating in a confined space is a bad idea, hearing wise.

 

Strantor: the air motor I will use only goes to 7600rpm with a load, with no load, it can only achieve 10,000. The motor is rated for 6900. I will hook it to my drill hooked up to the bulb to see how it goes, but how do I know how many rpm it is spinning at when it produces the correct voltage? Shortbus: it is not technically a confined space, there is plenty of space for sound to dissipate. the only reason it is dark is because machinery surrounds the area, blocking the light. I also have planned an exhaust diffuser, so that the exhaust air will be quiet, and the air motor is designed to be low noise as well. I had thought about a light like that, but recharging batteries is such a hassle. I also want a flood light, not a directed one. Another thing is that it can be used as an auxiliary dc power source when the light is off.

 

Strantor: the air motor I will use only goes to 7600rpm with a load, with no load, it can only achieve 10,000. The motor is rated for 6900. I will hook it to my drill hooked up to the bulb to see how it goes, but how do I know how many rpm it is spinning at when it produces the correct voltage?

If you don't have a tach then the best you can do is an educated guess, based on how many volts it puts out.

 

I know the max rpm of the drill I'm hooking it to, so if it produces the correct voltage at the drills already established max rpm, then I know what it takes to produce the correct voltage. If not, i'll just have to make an educated guess.

EDIT: How lucky am I, that I got 12 volts at the drills max speed of 2500rpm. I know now what it takes to power a 50 watt load. If I want a 100watt load, then it should produce 6 volts at 2500rpm, and 12volts at 5000rpm right?

EDIT: After some testing, I have verified my above hypothesis. At 2500rpm, and 100watt load, I get 6 volts. Sadly I cannot test it at 5000rpm as I have nothing that spins that fast.