# Mechanical Question about project Urgent!

Say I was testing the efficiency of different turbines. What is the best way to measure the current generated by these blades in varying wind speeds?

Help is greatly appreciated.

Yikes! Your simple question is more complicated than it may seem. From your question, it sounds like you are referring to wind driven electric generators aka windmills. So do you already have a windmill that is driving a generator? If so you just get a multimeter, put it in current mode and measure the output of the generator.

If you are talking about design, then there are several factors to consider. But ultimately the amount of current generated is a factor of how fast the windmill turns.

Maybe you should explain the problem in a little more detail…

A little bit more info about the project is available here

although there still is no way we can answer the question without a LOT more information…

An important question is, do you have a little turbine, or a big turbine?

My initial suggestion is to use an inductance coil meter. It’s one of those claw looking things you put around wires, and it uses an inductance loop to measure various things. Typically a little more capable than a multi-meter, and you won’t hurt it by over-current.

But, if it’s small, a multi is fine.

Im sorry;

I am basically attaching a stack of little turbines (3 in diameter) on the top of the hood of a car. This is where an air scoop usually goes. This assembly can mount on say a Honda Civic. I want to see how much electricity I will generate with this setup at different wind speeds (20, 30, 40, 50, 60) mph. This is useful as a 100 buck aftermarket kit. I know its not as effective as running a generator to the wheels, but this can be sold for cheaper and installation will be easy.

Once again,

Thanks for the responses, and sorry for the lack of detail.

Brenn,
A few things came to mind when I saw your post. The output of any of these fan/generators will be pretty low as far as current is concerned but in order for you to use it effectively, you need to follow up the output with some form of voltage regulator. As the speed of the car increases so will the output voltage and current but the RPM of the fans is also determined by the electrical load. The greater the current demand the more electrical “drag” there will be. You might also want to control the amount of air flow as there will be a point where the fans may self destruct due to excessive speed. This can be a way to get charge current back to the battery but you will need some form of isolation to prevent the battery and alternator from back feeding the fans. The voltage regulator, if properly designed can handle this function.
Experimentation is key here. I would start by making a box with some lamps that you can switch in and out. Connect that to one fan and hold it out the window while someone else drives the car at specified speeds. Measure the current generated while you switch in additional lamps as a an increasing load and I think the data will be very revealing.
BTW, there are DC fans and there are brushless DC fans. The brushless fans will not act as generators.

Good Luck

It sounds as if Brenn is not even concerned with attaching any sort of load. So, Brenn, correct me if I am interpreting correctly. You already have the 3 wind turbine/generator assemblies. Your task is to determine the amount of current is possible at several different wind speeds, as simulated on a car. Measuring current implies a complete circuit such as you might see in a charging circuit as described by Al. If all you have are these wind driven generators, then you can only really measure voltage (again with a multimeter). You would expect that as you drive faster, the voltage will go up. Connect the outputs of your 3 generators in parallel (not series although this would be interesting), lead the + and - ends back to the passenger compartment and attach to the multimeter to get a reading as you drive. Al alluded to it, but do you know what sort of generator you are using? AC or DC?

So I get brush DC fans and measure the voltage between the two leads? Would that work?

I want to generate DC current.

Thanks for all the input.

Voltage is only part of what you need to measure. You must also measure current, to calculate POWER. There must be a small load on the generator - maybe a light bulb, maybe just a resistor - and by measuring voltage (V) and current (I), P=V*I (P is in Watts)

(Rememebr that Watts is power, and if your system can generate xx watts, the resistor must be able to handle xx watts or more, or it’ll overheat)

Don

PS: you DO realize of course that the amount of fuel being spent to overcome the drag of the wind turbines is far greater than the energy produced by the turbines, right? That’s called “efficiency”, always less than 100%.

Yeah I know that, but wouldnt this be useful in a city environment?

And does the DC brush fan include a generator. Is it itself a generator?

I can’t see how it could be any use at all for saving fuel…but it could help teach you about different forms of energy, so it’s a very useful project in that regard.

When youre braking wouldnt the fans help conserve some energy. So in a city environment where one is constantly braking…

Not really, no. Unless you’re traveling extremely fast (wind turbines generate power approximately proportional to the fourth power of wind speed), or unless your fan blades are huge (think helicopter), you will lose more energy by having to accelerate the weight of the fans than you can possibly recover by letting them spin as you slow down.

You can get a much greater overall benefit by carefully streamlining the vehicle to avoid wind resistance. That helps reduce energy needs the entire time you are moving, where the fans can only provide a theoretical benefit for the first few seconds of braking.

Bren
Could you be a little more specific about your idea? We are trying to understand your thoughts on braking as opposed to driving. Brush type, DC permanent magnet motor act as generators when they are driven from an external source. If you have ever pushed your robot on the floor when it is turned off you will see this effect as the fans on the Victors and the LEDs will light due to the voltage and current generated by the drive motors backfeeding the Victors.

First a bit of information about energy use in cars, at different speeds. There are three basic ways that typical cars use energy, all of the energy comes from the gasoline powered engine, but goes to different things.

The first is overcoming rolling resistance…it takes energy to overcome the tire’s resisance to roll. This load is mostly proportional to the speed of the car, so the faster you drive, the more power it takes to keep the car moving. To get a feel for this, try (safely!) pushing a car, it will be easy to push it slowly, but it becomes harder to push the faster it goes.

The second is overcoming air resistance, this is aerodynamic drag…it takes energy to move the air out of the way of the body of the car. This load is mostly proportional to the square of the speed of the car. That means that it is very small at low speeds, but very high at high speeds. To get a feel for this, see if a light breeze will push a car, and also see how it is different driving into a headwind at highway speed, vs. driving with a tailwind.

The third is accelerating the mass of the car. This energy is not wasted, unless you use the brakes to slow down (then your kinetic energy turns into heat energy, which is dissipated by the brakes). Hybrid cars get about the same mileage in town as they do on the highway, because they recover the kinetic energy, and store it in a battery for use in accelerating the car again.

I think that you are trying to recover a very small part of the energy that is being used to overcome aerodynamic drag. In city driving, there is not much energy lost to drag, most is lost to the brakes and overcoming rolling resistance. And still, the best way to reduce losses to air resistance, is to make the car “smoother” to reduce the coefficient of drag, and to reduce the frontal area (height and width).

If you don’t understand any part of this, please ask more questions!

If you are looking to recover energy lost by a car, you would be better off trying to recover heat from the engine and brakes.

Thanks a lot guys; this is really informative.

I understand your arguments, and I too believe that I will not recoup enough. But I think it is a useful experiment. At the ver yleast I can learn more on this subject.

So I just basically stacked the PAPST large CPU fans on the hood of the car, perpendicular to the ground. I have 3 of these large fans. They are wired in series and connected to a resistor. I will use hte multimeter to read voltages when driving hte car at a variety of speeds. After this I will calculate the power and then I can see how much I generate. Unfortuantely I will not be able to see the increase of my drag coefficient with this assembly.

Any thoughts?

Once again; I cant thank you guys enough for your input.

Brenn,
What is the part number of your fans. Most of the Pabst line is brushless.

I was able to use my fan as a generator.

I used a 1 k-ohm resistor and read the voltage across the two leads.

My data as follows:

15 mph = 1.20 V
20 mph = 2.06 V
25 mph = 2.68 V
30 mph = 2.83 V
35 mph = 2.77 V
40 mph = 2.82 V
45 mph = 2.70 V
50 mph = 2.52 V
55 mph = 2.26 V
65 mph = 2.21 V

The fan probably was not designed to handle such high rpm’s past 40 mph.

Can I make any conclusions from this data taht will allow me to test the efficacy of using fans. Please note that I was not able to calculate how much the overall drag coefficient increased on my car.

Thanks so much for all your help.

You are making about one hundredth of one watt of power…that’s not much!

P = E^2 / R