Wind Power C

[kv1296]

So have you guys been finding three blades as most efficent?

[illusionist]

Actually, I have found two bladed designs to be the most efficient. JK, (i'm kv1296's partner):P

[ichaelm]

So have you guys been finding three blades as most efficent?

Last year, pretty much everyone found 2 blades to be most efficient, due to the high speeds. Since this year' setup will also involve high speeds, I think that 2 blades will still be most efficient, although I have not tested that theory yet. Have you guys tested both?

[illusionist]

Not yet with resistors... Tomorrow we will head to radioshack and grab a couple 10 ohm resistors.

[lllazar]

I'm having a hard time understanding this year's objective - exactly how is it different from last year, and what exactly is a load resistor (and how does IT effect this event).

Also, i have a list of stuff i need almost complete, anything else i should get?:

Box Fan (I have)
Mount (how can i make/obtain one?)
Multimeter (i'm going to get one, but how do you attach the load resistors to it?)
Building materials (wat would be the best glue of choice for the blade-cd attachment)

Also, i'm seeing a lot of people saying balsa is the best (obviously i don't mean best, but in general the better material out there), do you think itd be fine to sand a sheet to my liking, or is steaming better?

[fleet130]

For motors/generators, voltage is related to speed and torque is related to current. More speed = more voltage. More current = more torque.

Load = work. In mechanical systems, load is the mass x distance moved. In electrical systems, it is the amount of electrical energy converted to other forms. In this case, electrical energy (from the generator) is converted to heat by the load resistor.

The resistance connected to the generator determines the amount of current a given output voltage will produce. Lower resistance means more current. As current increases, more torque (turning force) is needed to turn the generator.

Last year, only a voltmeter (which typically has a resistance of several million ohms) was connected to the generator. The current consumed, and the effect it had on generator output voltage, was negligible.

This year a resistor is connected to the output of the generator(motor) to convert its electrical output to heat. With the low value for this year's load resistor, the current consumed should be several orders of magnitude greater and could greatly effect the voltage produced.

Internal resistance of the generator also plays a part. As current through the load increases, current through the generator also increases proportionally. This causes the voltage used by internal resistance of the generator to increase, reducing the voltage supplied to the load.

Theoretically, this year turbines must be designed to produce more torque than last year. Only time will tell how much this will effect their design.

[andrewwski]

I'm having a hard time understanding this year's objective - exactly how is it different from last year, and what exactly is a load resistor (and how does IT effect this event).

Last year, there was no load, the score was for open circuit voltage. This essentially meant only the speed of the blades mattered, not the power generated by the turbine.

This year there is a load, so you are concerned with current as well (and thus, power).

Also, i have a list of stuff i need almost complete, anything else i should get?:

Multimeter (i'm going to get one, but how do you attach the load resistors to it?)

You put the load resistor across the two leads of the generator, then measure the voltage across it. Attach one probe to one lead, the other probe to the other.

[lllazar]

Ahh, so i'll need a generator and some probes as well? Thanks

And 1 last question, in a general way could you briefly explain what kind of design/material would produce more power and not just a fast rpm, like for example:

Would a balsa turbine that has the same rpm as a turbine made of playing cards produce more power?

[ichaelm]

I'm having a hard time understanding this year's objective - exactly how is it different from last year, and what exactly is a load resistor (and how does IT effect this event).

Also, i have a list of stuff i need almost complete, anything else i should get?:

Box Fan (I have)
Mount (how can i make/obtain one?)
Multimeter (i'm going to get one, but how do you attach the load resistors to it?)
Building materials (wat would be the best glue of choice for the blade-cd attachment)

Also, i'm seeing a lot of people saying balsa is the best (obviously i don't mean best, but in general the better material out there), do you think itd be fine to sand a sheet to my liking, or is steaming better?

Sanding works fine, if you can get really good at it. I'd recommend using balsa around 1/16 inch thick. Personally I prefer steaming, but I've tried sanding too and it definitely works. If you ever want to try steaming, use thinner balsa.

In this year's event, a resistor is connected across the output of the generator (motor). This resistor allows the voltage produced by the generator to cause current to flow through the resistor. The torque required to turn the turbine depends on the current. More speed = more voltage = more current = more torque needed.

Theoretically, this year turbines must be designed to produce more torque than last year. Only time will tell how much this will effect this year's designs.

Correct me if my understanding of physics is wrong, but the way I understand it, the torque exerted by the generator onto the turbine should remain constant, as long as the blades are turning. The rest of the torque opposing the turbine's motion comes from friction and air resistance. The difference is that if the blades are moving faster, it needs to overcome that torque faster (Power = torque per second).

Ahh, so i'll need a generator and some probes as well? Thanks

And 1 last question, in a general way could you briefly explain what kind of design/material would produce more power and not just a fast rpm, like for example:

Would a balsa turbine that has the same rpm as a turbine made of playing cards produce more power?

The faster your blades turn, the more power you will generate. This is because there is no system for controlling mechanical advantage (i.e. a gearbox). So you still want the turbine to have as many RPMs as possible, but the blades have to push harder in order to get those RPMs this year.

[fleet130]

Simply stated: Voltage output is proportional to turbine speed. Lower resistance = more current = more torque = more work. This is true for any given speed/voltage. If more work is performed, the turbine will require more torque to produce a given speed/voltage.

Some torque is needed overcome the mechanical resistance of the generator/turbine. Torque to start rotation is greater than that necessary to keep the system turning. Once started torque is fairly constant. Additional torque is needed to generate the current output by the generator. This portion of the total torque varies with the current output, which varies depending on the load resistance.

Edit: Torque needed to overcome air resistance also varies with speed and increases with speed. It is non-linear. Since gearboxes are not allowed, this is a problem you will probably have to live with?

could you briefly explain what kind of design/material would produce more power and not just a fast rpm

Other than air resistance and rigidity and possibly weight, material is relatively unimportant. Use whatever you can get that's easy for you to work with. What's more important is the number of blades and their shape/curve/design. This is what the event is about and it's really is up to you to figure out what works best.