# Electronic Circuits and Projects Forum

## Limit the Speed of Turbine Generator

1. 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.

2. 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?

3. 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.

4. 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.

5. 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:

brushless 003b.jpg

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

6. 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.

7. 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?

8. It is about 15 inches in diameter and an average chord length of about 1.7 inches. Shaft diameter is 1/2 inch.
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.

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

10. 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.

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