Limit the Speed of Turbine Generator

Discussion in 'Renewable Energy' started by cloakinghalk, Feb 2, 2012.

1. cloakinghalkNew Member

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So I am working on a project to test turbine blades under water. I want to be able to limit the speed of the blade using a DC motor as a generator. The blade only rotates at about 25 RPM so I'll need to use some type of gear box to to increase the speed of the input shaft of the motor(this has been hard since it needs to be small and require low torque to turn). I have been looking at brushless motors because they're very easy to turn, specifically airplane motors. Like the motor below

http://www.hobby-lobby.com/450_size_brushless_outrunner_850kv_528687_prd1.htm

So next I am trying to figure out how to limit the speed of the motor. I have looked at ESC for airplanes but I am not sure if they work with a generator. I have also been wondering if there is a way to do this with a potentiometer? Any help any one could provide would be greatly appreciated.

2. duffyNew Member

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Not sure what you mean here, are you intending to limit the speed by using the motor as a brake? This is always a bad approach for wind generators, nature can throw more energy at it than it can handle. A better technique is to feather the blades, and allow the force of the moving fluid to pass through.

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The linked motor is rated ~ 11V 20A (= 220W) so I don't see why you can't run it as a generator and draw up to 20A from it to give the required braking effect, providing the turbine isn't producing more than 220W power. A power FET operating in PWM mode could easily handle that current and drive a suitable resistive load. The motor also inherently gives you an output voltage proportional to speed, so can be used in a feedback loop for speed control.
I'm pretty sure they could easily be adapted, using a resistive load instead of their usual motor. Are you planning to use an off-the-shelf ESC or build one?

Last edited: Feb 3, 2012
4. DaveNew Member

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You are aware the those little motors normal run at 10,000 - 20,000 RPM to put out their rated power.

Going from 25 RPM to 10,000+ RPM is not practical or likely even possible on a small scale home built design.

Your better solution would be to use a much larger and slower DC motor for your generator.

A laarger 1750 RPM 180 volt permanent magnet motor spinning at roughly 120 RPM would be a much more practical way to go if you are after a 12 volt output.

Although more costly on the motor end a common 10:1 gearbox or chain or belt drive is way easier and cheaper to deal with in the over all design.

6. cloakinghalkNew Member

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@Duffy Yes I would like to limit the speed using the motor as a break. I am not actually using this to generate power in a real life setting. I want to slow down the speed of the blade to optimal performance and then look at the vorticity of the flow behind the blade. Then use this set up to test other turbine blades. Also feathering the blades is also something on the list to do but that will be done later on.

@alec_t I am not that great with designing circuits. I can easily put them together if i have a schematic but I was hoping to just buy something off the shelf if it was possible.

@tcmtech I am not using this as a home generator. Its more to test different designs of small scale turbine blades. The turbines spinning at full speed produce a low amount of torque so they would not be able to even turn a larger motor. That is why I was hoping to use something small one like an airplane motor. I know I'll never be able to reach 10,000 RPM but I was hoping that I could still be able to spin fast enough to use the motor for the braking effect.

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If you are just looking to produce a braking torque effect I would recommend a common hydraulic pump or motor as the driven load source.

They are compact, sealed to the level of being submersible and water proof, capable of working at high torque loads at very low RPM's, and are easy to work with having only one line in and one line out.

8. duffyNew Member

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cloakinghalk - for that you could just connect the windings across an adjustable resistor. Try going from open to shorted first, and see if this gives you the range you need for your vorticity experiments.

9. cloakinghalkNew Member

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Duffy- So I tested shorting out and using resistors across a brushless motor and it did nothing. I couldn't feel any difference. I was going at a slow speed though. Do I need to go as fast as the operating speed of the motor?

Last edited: Feb 7, 2012
10. duffyNew Member

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Yes, nothing happens at slow speeds with (almost) any generator.

See that idiotic "kv" rating on the spec sheet? I HATE the term. HATE IT!!!! "Kv" means "killivolts"!!!! Some jerk decided to use this instead of RPM/Volt, which is what the spec really means. So, for your motor's 850 RPM/Volt rating, at 9.6V, it would be running around 8,160 RPM.

I don't know what the pitch of your prop or your flow rate is, but you aren't going to notice much spinning this or almost any other motor by hand without gearing. Get it spinning a few hundred RPM and THEN short the windings.

Last edited: Feb 7, 2012
11. duffyNew Member

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Here - this is a brushless ESC I built that you could feel the braking action on spinning it by hand. See how it's geared?
View attachment 60916

Edit - gone now? Who keeps killing my pictures on this board?

So, I'll try again - to help OP see how big the gear ratio needs to be:

View attachment 60925

Edit: Gone again. Why?

Last edited: Feb 11, 2012
12. cloakinghalkNew Member

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So I tried to drive the motor with a drill and short out the motor or use a resistor but it still didn't seem to do anything. I haven't actually bought a motor yet. I am using a standard dc motor I found laying around to test before I went out and bought a motor. Since I can only get 25RPM out of the turbine should I be looking for a different motor? Ideally I would want a motor with the lowest Kv(RPM/Volt) with the least amount of torque required to turn it. The attachment you provided does not seem to be working.

13. cloakinghalkNew Member

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I was also wondering what if I wired the motor to a potentiometer and a rechargeable battery. Then let the turbine turn the motor while the resistance in the potentiometer is high then to slow down the motor, lower the resistance. Would this work or is it bad to do this?

Last edited: Feb 7, 2012

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The more important question is how big is this turbine? Physical size and driving method that sort of stuff.

As far as low speed high torque while maintaining high relative efficiencys its going to be difficult to beat simple hydraulic pumps or motors that will work in direct drive applications.

15. cloakinghalkNew Member

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It is about 15 inches in diameter and an average chord length of about 1.7 inches. Shaft diameter is 1/2 inch. It is being driven by a water tunnel. I do not remember the max speed of the water but if you need that I can check. The turbine is very easy to stop with your finger while its spinning so I don't believe it could drive a hydraulic motor.

16. duffyNew Member

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That's pretty big. Your motor might be undersized.

Here's that image again, hope it works this time. I'm just trying to show you the kind of gear ratio you will need:

So, you need to get it spinning pretty fast. But you CAN feel the drag, even with just ONE of the 3 windings shorted!

Maybe what you need is a "powdered-core" brake or clutch. These have iron filings between rotary poles of a magnet. Turn the magnet on a little, some filings start to stick, you get some drag. Turn it on full, and the whole thing locks up solid.

Here's a link to some big ones, so you can see -

http://www.oocities.org/hankfu/indexEN.htm

Last edited: Feb 7, 2012
17. duffyNew Member

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This place has several sold as surplus, maybe one is small and inexpensive enough for what you need. Type "clutch" into the search box -
http://www.electronicsurplus.com

If not, maybe what you need to do is drive a permanent motor backwards. But I think maybe that "outrunner" brushless will do what you want, if you just gear it up. They sell gear assemblies like the one above on hobby sites - these motors actually spin too fast for most props.

Last edited: Feb 7, 2012
18. cloakinghalkNew Member

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With that large gear ratio I think it might slow down the blade to the point that it would be slower than the critical speed I am looking for. I might try it just to see how well it works. It wouldn't cost too much.

I have looked at clutches/brakes before and couldn't find many in the size range I was looking for but the link you gave me provides me with some that may work. I will have to look at them a little more.

Powering a permanent motor backwards is the same thing I was trying to say above? Just drive the motor in the opposite direction and tune it using a pot to get the right speed?

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Being that small I would recomend just using a common stepper motor for the generator. They com in all sorts of sizes and can usualy be found for next to nothing.

20. cloakinghalkNew Member

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How would you go about wiring the stepper motor to get the braking effect?

Last edited: Feb 7, 2012

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Most stepper motors are either two phase or three phase so you have to either run each phase through rectifiers or set up each winding with its own load.

Either way its not hard to work with.