# Howto calculate the torque

1. Jun 17, 2009

### tingting

Hello:
My one is Darrieus wind turbine with 2 blades. Diamerter is about 1 meter and also know the average speed is 2.5m/s.So,how to calculate the torque ?

2. Jun 17, 2009

### ank_gl

What is it connected to? Can you measure thw work output of the turbine? If yes, work is the product of torque & the angular velocity of the turbine. If no, measure it somehow

3. Jun 18, 2009

### tingting

The wind turbine is connected to DC generator to charge a battary. And i can calculate the input of the wind turbine.

4. Jun 18, 2009

### ank_gl

Input of the wind turbine? Do you mean the air speed?

Either way, what would the torque represent? You should be worried about getting the power of the unit. Measure the rpm & calculate the torque as power divided by angular velocity(I messed up the formula in the last post). The torque will vary as the load on the turbine varies. If the rotor senses lesser resistance(lesser load), it will speed up & hence torque will be low, & vice versa. The turbine will produce the maximum torque when its speed is zero, & this torque is called stalling torque. YOu can calculate it simply by holding the blade via a force scale.

5. Jun 18, 2009

### Bob S

I will first estimate the power in the wind at 15 meters per sec, then the power you should be able to get out at 2.5 m/sec.
Air has a density ρ of about 1.29 Kg per cubic meter. at 15 m/s the wind power density is

Pdens = (1/2) ρ v3 = 2200 watts per meter2.
Note that power density scales as velocity cubed. The theoretical max efficiency of a horizontal axis wind turbine is about 59.3% (Betz limit). Actual is about 40%. For a vertical axis Darrieus (egg-beater) machine, I will guess 30%. So your machine with an area of 0.78 meter2 might give 510 watts at 15m/sec wind velocity. The Darrieus blade-tip speed should be about 4 x wind velocity.

In a 2.5 m/s wind speed, your max power is about (2.5/15)3 510 =2.4 watts. Your rotor speed should be about 3.2 rps or ω = 20 radians per sec. Your optimum torque for a wind speed of 2.5 m/s and rotor speed of 3.2 rps is then 2.4 watt/ω = 0.12 Newton-meters.

See http://www.windturbine-analysis.netfirms.com/index-analyse.htm [Broken]

 updated area, efficiency, and torque estimates.

α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ ς σ τ υ φ χ ψ ω

Last edited by a moderator: May 4, 2017
6. Jun 22, 2009

### tingting

The torque can use this formular T= Flift * r right?
and how to find Flift

7. Jun 22, 2009

### ank_gl

8. Jun 22, 2009

### tingting

In a 2.5 m/s wind speed, your max power is about (2.5/15)3 510 =2.4 watts. Your rotor speed should be about 3.2 rps or ω = 20 radians per sec. Your optimum torque for a wind speed of 2.5 m/s and rotor speed of 3.2 rps is then 2.4 watt/ω = 0.12 Newton-meters.

how do you know the rotor speed of 3.2rpm? From which graph?

9. Jun 22, 2009

### Bob S

For Darrieus machines, the speed of the rotor blade tip should be about 4 times the wind speed for optimum power output. See
https://netfiles.uiuc.edu/mragheb/www/NPRE%20498WP%20Wind%20Power%20Systems/Optimal%20Rotor%20Tip%20Speed%20Ratio.pdf [Broken]
So if the wind speed is 2.5 m/sec, then the oprimum rotor blade tip speed should be 10 m/sec. Dividing by the radius (0.5 m) gives the angular speed w = 20 radians per sec. dividing by 2 pi gives rps = 3.2 revolutions per sec.

Last edited by a moderator: May 4, 2017
10. Aug 19, 2009

### mitchel6

I am trying to determine the stall torque on a 90 DC 1/8 hp motor. we have a 10in diameter propeller attached. my goal is to create a power curve with a max RPM of 185. I dont really know much about motors or power so any tips would be appreciated! Thanks